Vascular System

The movements of the heart consist of a series of contractions which succeed each other with a certain rhythm. The period of contraction is called the systole and that of relaxation the diastole. The two auricles contract and relax synchronously, and these movements are followed by the synchronous contraction and relaxation of the ventricles. the Finally, there is a short period when the whole heart is in heart. diastole. The whole series of movements is known as the cardiac _ Right posterior cusp of aortic valve _Anterior (infundibular) cusp of tricuspid valve _Right (marginal) cusp of tricuspid valve _Posterior (septal) cusp of tricuspid valve Right ventricle riculo-ventricular, Right anterior cusp of pulmonary valve Right coronary artery Anterior cusp of aortic cycle. Taking 75 as the average number of heart-beats per minute, each cardiac cycle will occupy. 8 seconds. Of this period auricular systole occupies .1 second auricular diastole occupies. 7 „ ventricular systole occupies 3 „ ventricular diastole occupies. 5 „ In 1861 Chauveau and Marey obtained direct records of the heart of a horse, and determined the sequence and duration of the events happening in the heart, and measured the endo-cardiac pressure by an instrument termed the cardiac sound. The sound - a twoway tube - was pushed down the jugular vein until the orifice of one tube lay in the right ventricle and of the other in the right auricle. The tubes were connected with recording tambours which wrote on a moving drum covered with smoked paper.

Another tambour was used to record the cardiac impulse. The tracings so obtained (fig. to) teach us the following facts: (1) The auricular contraction is less sudden than the ventricular, and lasts only a very short time, as indicated by the line ab. The ventricle, on the other hand, contracts suddenly and forcibly and remains contracted a considerable time, as shown by the line c' d' and by the flat top to the curve which succeeds d'. (2) The auricular movement precedes the ventricular, and the latter coincides with the impulse of the apex against the wall of the chest. (3) The contraction of the auricle influences the pressure in the ventricle as shown by the small rise a'b', and that of the ventricle influences the pressure in the auricle somewhat as shown by the waves cd. Much labour has been spent in the contrivance of rapidly acting spring pressure gauges, freed as far as possible from inertia, in order to investigate more exactly the changes of intracardiac pressure, which were first described by Chauveau and Marey. As the intraventricular pressure FIG. 10. - Tracings from the Heart of a Horse, by Chauveau and Marey. The upper tracing is from the right auricle, the middle from the right ventricle, and the lowest from the apex of the heart. The horizontal lines represent time, and the vertical amount of pressure. The vertical dotted lines mark coincident points in the three movements. The breadth of one of the small squares represents one-tenth of a second.

may rise 150 mm. of mercury in one-tenth of a second, it is no easy matter to contrive an instrument which will respond as rapidly and yet yield an accurate result without overshooting the mark. The final result of a most careful inquiry is the confirmation in almost every point of Chauveau and Marey's pressure curves. Karl Hi.irthle's differential manometer has proved to be an instrument of great value and precision. A double-bored tube cannula is introduced so that one tube reaches the right auricle and the other the right ventricle. In observations on the left side of the heart, one tube is placed in the left ventricle and the other in the aorta, and each of these tubes is brought into connexion with a tambour. The two tambours are placed one on either side of the fulcrum of a lever. This lever works against a light spring, which in its turn sets in motion a writing-style. The style records the pressure changes on a drum covered with smoked paper. By this means there can be recorded the exact moment at which the auricular pressure exceeds that in the ventricle, that is to say, the moment when the auriculo-ventricular valves open; likewise the moment when the ventricular pressure becomes greater than that in the auricles, and the auriculo-ventricular valves shut. Similarly, there can be recorded the moment when the intraventricular pressure exceeds that in the aorta and the semilunar valves open, and the moment at which the diastole of the ventricle begins, when the aortic pressure becomes the greater, and the semilunar valves shut. The smoothness with which the heart works is shown by the fact that neither the opening nor the closing of the valves is marked by any peak or point on the pressure curves.

The absence of a mechanism for preventing regurgitation of blood from the auricles of birds and mammals is remarkable, for in fishes, amphibia and reptiles this is effected by valves guarding the sino-auricular junction. In the warm-blooded vertebrata with the appearance of the diaphragm the sinus becomes merged into the right auricle, and the venous cistern formed by the superior and inferior venae cavae, the innominate, iliac, hepatic and renal veins takes the place of the sinus.

Six pairs of valves prevent regurgitation from this cistern, viz. those placed in thee common. femoral, the sub-clavian and jugular veins. The cistern when filled holds some 400 c.c. of blood; in the liver there is some 500 c.c. of blood, and this can be expressed into the cistern by abdominal pressure; in the portal venous system, when distended, another 500 c.c. may be held, which can be expressed through the liver into the cistern. A large volume of blood is thus at the disposal rena cave of the heart for it to draw on during diastole. Respiration by the aspirating action of the thorax sucks this blood into the heart, while the inspiratory descent of the diaphragm squeezes the abdominal contents and forces blood from the liver and cistern into the heart. These forces take the place of the sinus and are far more efficient. The intra-abdominal pressure may be raised on bending or straining till it becomes equivalent to the pressure of a column of mercury 80-10o mm. high (Keith). Under such conditions the pericardium prevents the right side of the heart being over-distended with venous blood.

With these facts in view, we can now describe the complete course of a cardiac cycle. We will start at the moment when the blood is pouring from the venae cavae and pulmonary veins into the two auricles. The auricles are relaxed and their cavities open into the ventricles by the funnelshaped apertures formed by the dependent segments of the tricuspid and mitral valves. The blood passes freely through these apertures into the ventricles. The small positive pressure which is always present in the venous cistern (aided by the respiratory forces) From Diseases of the Heart, by James Mackenzie, M.D., by permission.

FIG. 12. - Tracings of the Jugular Pulse Apex Beat, Carotid and Radial Pulses. The perpendicular lines represent the time of the following events. 1, the beginning of the auricular systole; 2, the beginning of ventricular systole; 3, the appearance of the pulse in the carotid; 4, the appearance of the pulse in the radial; 5, the closing of the semilunar valves; 6, the opening of the tricuspid valves.

is at this time filling the right heart, while the positive pressure in the pulmonary veins is filling the left heart. The auricular systole now takes place. The circular muscle bands compress the blood out of the auricles into the ventricles, while the longitudinal bands aid in this and pull up the base of the ventricles to meet the load of blood. As the contraction starts from the mouths of the venae cavae, and sweeps towards the ventricles, there can NHN?aiN? ' 'MEW  ?[?

Nnnn((/O¦. ??

'NM MN?r¦?1 EMU 'A ' 'Wwii I¦mNill ' 'ME M .t?._?N A. Keith, in Journal of Anatomy and Physiology. 'FIG. 11. - Diagram of the Venous Cistern from which the Heart is filled. The abdominal or infradiaphragmatic part of the cistern is indicated in black; the thoracic or supra-diaphragmatic is stippled.

occur but little regurgitation of blood into the venous cistern, but the cessation of flow into the auricle during its systole does produce a slight rise of pressure in the cistern, as is shown by tracings taken from the jugular pulse. The function of the auricles is to rapidly complete the filling of the ventricles.

The auriculo-ventricular valves are floated up and brought into apposition by eddies set up in the blood which streams into the ventricles, and close without noise or jar at the moment when the intra-ventricular pressure exceeds in the least that in the auricles. The systole of the ventricles immediately following that of the auricles closes the auriculo-ventricular valves, and as the intra-ventricular pressure rises above that in the pulmonary artery and aorta respectively the semilunar valves open and the blood is expelled; these elastic vessels are in their turn expanded by the expulsive force of the heart so as to receive the blood. The papillary muscles, by contracting synchronously with the muscular wall of the ventricles, pull down and flatten the dome-like diaphragm formed by the closed auriculo-ventricular valves, thus shortening the longitudinal diameter of the ventricles, while at the same time they enlarge the auricles and so help to fill these cavities. The outflow of blood from the ventricles is rapid at first. It becomes slower as the big arteries become distended and the pressure of blood rises within them, and ceases finally when the pressure becomes equal to that in the ventricles. As the outflow diminishes the semilunar pockets are filled by eddies of blood, and their thin edges are brought nearer and nearer, until finally they come into apposition. The closure is effected without jar or noise at the moment when the outflow ceases and the ventricles begin to expand. The heart, as a good pump should, works with the least possible jar. During the contraction of the ventricles blood has been pouring from the veins into the auricles, and directly the ventricular systole ceases the auriculo-ventricular valves open, and the blood begins to fill the expanding ventricular cavities. For a brief moment the ventricles remain dilated and at rest, then the auricles contract again, and the cycle of changes, once more, is repeated. During the first period of ventricular systole - the period of rising tension - all the valves are closed and the ventricle is getting up pressure. This period has been measured and is found to occupy 02 "--04". The second period is that of systolic output, and lasts about .2", that is, from the moment when the semilunar valves open to the moment when they close.

The upstroke of the pulse curve taken in the aorta, or in the carotid artery in man, can be taken as marking the moment so that either only a rise or a fall of pressure is recorded. In the right ventricle of the dog the maximal pressures recorded equalled 35mm. of mercury, in the left ventricle 114-135 mm., in the auricles 2-20 mm. (Michael Jdger, 1795-1838). A negative pressure, of considerable amount but of very fleeting duration, sometimes occurs in the ventricles at the beginning of diastole. This is produced by the elastic rebound of the fleshy columns of the inner wall of the heart, which become pressed together as the blood is wrung out of the ventricular cavities. The entry of the first few drops of blood from the auricles abolishes this negative pressure, and it has no important influence on the filling of the heart.

When the ear is applied over the cardiac region of the chest, or a stethoscope is employed, two sounds are heard, the first, heard most intensely over the apex, is a duller and longer sound than the second, which is shorter and sharper and is heard best over the base of the heart. The syllables lub, dupe express fairly well the characters of the two sounds, and the accent is on lub when the stethoscope is over the apex, thuslub-dupp - lub-dupp - lub-dupp, and on the second sound when over the base, thus - tub-dupp - lub-dupp - lub-dupp. The sounds of the heart have been successfully recorded by means of the microphone. Hi rthle inserted the microphone in the primary circuit of an E. Du Bois-Reymond induction coil, and placed the nerve of a frog-muscle preparation in the secondary circuit. The muscle, being attached to a lever, recorded its contraction on a revolving drum at the moment when the sound of the heart reached the microphone and closed the primary circuit. A capillary electrometer can be inserted in place of the frog-muscle indicator, and the movements of the electrometer photographed on a sensitized plate moved by clockwork (Willem Einthoven). Each sound gives rise to a succession of vibrations of the mercury meniscus of the capillary electrometer. The first sound is formed of many component tones derived from the sudden tension, and consequent vibration, of the ventricular muscle, and of the auriculo-ventricular valves with their chordae tendineae. The first sound can be resolved by a trained musical ear into two tones, one deep and the other high. The deeper tone alone is heard on the contraction of the excised and bloodless heart, while the higher tone is produced by throwing the auriculo-ventricular valves into tension (John Berry Haycraft). In the cold-blooded animal, such as the turtle, the heart muscle does not become tense rapidly enough to produce a sound (Allen). This sound is not produced by fluid friction as the blood rushes through the arterial orifices, for the velocity of outflow is too small to produce in this way any noise. Nor is it produced by sudden opening of the semilunar valves, for these open quietly and without jar at the moment when the intra-ventricular pressure rises above that in the aorta.

The second sound of the heart is produced by the tension of the semilunar valves in the aorta and pulmonary artery at the moment when the ventricles pass into diastole. These valves close without any jar or shock so soon as the arterial pressures rise to the slightest degree above that in the ventricles. In the next moment the ventricles dilate, and the valves, no longer supported on one side, become taut. The elastic vibrations of the walls of the distended arteries probably share in the production of this sound.

When the sounds and the impulse are recorded together the record shows that the first sound begins about ooi sec. before the cardiagram marks the beginning of systole, and for the first o06 sec. of its duration this sound is heard only over the apex. Over the base of the heart the first sound is heard just at the time when the semilunar valves open and the output begins. The first sound ceases before the ventricular contraction is over, for it is the sudden tension, not the continuance of contraction, that causes it. The beginning of the second sound marks the sudden tension of the semilunar valves which immediately follows their closure.

For practical purposes it is important to bear in mind what is happening in the heart whilst one listens to its sounds. During the first sound we have (I) contraction of the ventricles, closure of the auriculo-ventricular valves and impulse of the apex against the chest; (2) rushing of the blood into the aortic and pulmonary artery, and filling of the auricles. With the second sound we have closure of the semilunar valves from the elastic recoil of the aorta and pulmonary artery, relaxation of the ventricular walls, opening of the auriculo-ventricular valves so as to allow the passage of blood from auricle to ventricle, and diminished pressure of apex against chest wall. With the long pause there are (1) gradual refilling of the ventricle from the auricle, and (2) contraction of the auricle so as to entirely fill the ventricle. The sound of the tricuspid valve is heard loudest at the junction of the lower right costal cartilages with the sternum, of the mitral over the apex beat, of the aortic semilunar valves in the direction of the aorta where it comes nearest to the surface at the second right costal cartilage, and of the valves of the pulmonary orifice over the third left costal cartilage, to the left and external to the margin of the sternum. The sounds are changed in character by valvular lesion or muscular weakness of the heart, and afford important signs to the physician. Murmurs are produced by eddies setting some part of the membranous walls or valve flaps in vibration.

If a stethoscope be placed over a large artery, a murmur will be From Further Advances in Physiology, by permission.

FIG 13. - Diagrammatic representation of the Cardiac Cycle and of the Carotid and Jugular Pulses in relation to standard movements. The scale of abscissae is I mm. to i oa sec.

S.C. =semilunar valve closure' when the semilunar valves open, O. = auriculo - ventricular pen, valves open. The broken lines while the dicrotic notch on the indicate those portions of the pulse curve marks their closure. respective curves over which The second sound of the heart there is doubt or controversy. occurs immediately after their closure, and can be used to mark the time of this event on the impulse curve.

The intra-ventricular pressure curve may rise or fall during the output period according to the state of the peripheral resistance. If the carotid pulse be recorded synchronously with the impulse curve, the time relations can be determined for the human heart. The beginning of the upstroke of the impulse curve marks the beginning of systole, that of the pulse curve marks the opening of the semilunar valves, and the dicrotic notch, which precedes the dicrotic wave, marks the closure of these valves and the end of the output. The first sound of the heart is synchronous with the upstroke of the impulse curve. The maximal systolic pressure exerted by the heart varies with the degree of diastolic filling and with the obstruction to outflow. The heart responds to the latter by a greater output of energy, and this it does with little loss in rapidity of action. The total fluid pressure to which the wall of the heart is submitted rapidly increases as the radii of curvature become greater. Hence the greater energy required of a dilated heart, its tendency to hypertrophy and liability to fail. By its reserve power the heart may throw out three or even six times the volume of the normal output per minute, and may maintain its output when the aortic pressure is twice its normal value.

The maximal and minimal pressures have been accurately recorded in the heart by a manometer fitted with a valve arranged heard, caused by the blood rushing through the vessel narrowed by the pressure of the instrument. The fluid escapes into a wider portion of the vessel beyond the point of pressure, and the sound is caused by the eddies set up there throwing the membranous wall of the vessel into vibration. Such a sound is heard over an aneurism. The placental bruit heard during pregnancy is a sound of this kind, arising from pressure on the uterine arteries. In cases of insufficient aortic valves a double blowing murmur may be heard, the first being due to the rush of blood into the vessel, and the second to the regurgitation of the blood back into the ventricle. These murFIG. 14. - Scheme of a Cardiac Cycle. murs are produced by eddies The inner circle shows what events of branous blood setti into the memb occur in the heart, and the outer r parts n a o vibration.

the relation of the sounds and Occasionally a murmur is silences to relation of events. produced by the displacement of air in the bronchial vessels by the beat of the heart, and may simulate the murmur of aortic incompetence. By placing a stethoscope over the jugular vein on the right above the collar bone a murmur is heard, the bruit de diable, particularly if the subject turn his head to the left. This is held to be due to the vibration of the blood in the jugular vein rushing from the dilated to the contracted part. It is more marked during auricular diastole and during inspiration.

In the lower vertebrates, as the frog, the heart is directly nourished by the blood which fills the cavities in its sponge-like structure. In the warm-blooded vertebrates there is a special arrange ment of coronary vessels. The two coronary arteries (right and left) originate at the root of the aorta from the sinuses of of Valsalva. Their branches penetrate the muscular sub stance and end in a rich plexus of capillaries. From these arise the radicles of the coronary veins which open into the right auricle by the coronary sinus and other small veins. These openings are valved. The heart in contracting exerts a greater pressure than that of the coronary arteries, and so arrests the flow in these during the height of systole, and squeezes the blood within the coronary capillaries and veins on into the right auricle. On diastole the coronary system fills again. Sudden occlusion of any large part of the coronary arteries produces irregular and inco-ordinate contractions, followed by death of the heart. Gradual occlusion of the coronary arteries by degenerative changes in advanced life is one of the causes of the distressing form of cardiac distress known as angina pectoris. The work of the left ventricle is calculated by the formula W = VP +mv 2 , where V =volume of blood in c.c. expelled of per beat, P = mean pressure in aorta, m = mass of the blood expelled on systole, and v=the velocity imparted to it.

The volume of the output has been determined directly by inserting the stromuhr in the ascending aorta (Robert Adolf Tigerstedt), and indirectly by determining (1) how much oxygen is absorbed per minute, (2) the difference in the oxygen content of the arterial and venous blood, (3) the number of heart beats. If woo c.c. of oxygen are absorbed from the air breathed in a minute, and the arterial blood contains to % more oxygen than the venous, it is clear that loo X loo c.c. of blood must have passed through the lungs in that time, and if the heart beat too times, the output for each beat would be 100 c.c. From the determinations made on animals the output is calculated for man to be 60-100 c.c. The velocity of the output can be calculated if the volume of the output is known, the duration of the period of output, and the diameter of the aorta. The pressure is measured with a manometer. The velocity is much greater at the orifice than in the aorta, for the blood can flow from the aorta during the whole cardiac cycle, while the whole of it must escape through the orifice into the aorta during the period of output. The work spent on maintaining the velocity is not, however, more than 4 1 0 of the whole and is generally neglected in the calculation. The output is not greater than 60-10o c.c. (3 oz.) (Tigerstedt, Nathan Zuntz), and the mean arterial pressure in a healthy man, determined by the sphygmometer, is not more than 110 mm. of mercury (L. Hill). The work of the right heart can be reckoned to be a that of the left, for the pressure in the pulmonary artery does not exceed 30 mm. The total work of the heart during the day may be taken as equal to 20,000 kilogr.- metres, and this would be equivalent to 50 calories out of the total 2500 calories which a man takes in as food. A labourer does about 150,000 kilogrm.-metres of external work a day. The work of the heart is increased two or three times over during severe muscular labour. It has been estimated that the heart requires per diem, to maintain its energy, an amount of solid food (water-free) equal to the weight of solids in the heart itself, i.e. about 60 grms. of sugar or proteid. 30 c.c. of blood must be circulated per minute through the coronary arteries of a dog to maintain the vigour of the heart.

The use of oxygen per grm. of weight per minute is high for the heart. Thus for the whole body of the dog there was used 017 c.c. per grm. per min., for the heart 045-083, The and for the active secretory glands 07-1.0 (Barcroft artificial and Dixon). It has long been known that the heart of circuiafrog or tortoise can be kept beating normally for hours tion of after removal from the body, if it is provided with an artificial circulation of blood or a suitable solution of salts. Sydney, _Ringer worked out the necessary ingredients of this solution to be Sodium chloride o '7 Potassium „. 0.03% Calcium „.. 0.025% The excised mammalian heart can be kept beating in the same way provided the nutritive fluid is oxygenated and the heart kept at body temperature. A solution containing one-third defibrinated blood and two-thirds Ringer's salt solution is most suitable. .A mammalian heart thus was restored to activity 7 days after death. The beat of the heart of a child was restored 20 hours after death from pneumonia. The excised heart of a cat was kept beating for 4 days. The heart of a monkey was restored after freezing the body of the animal. The nerves of the excised heart retain their action for some time if the nutritive fluid is immediately circulated through the coronary arteries. Thus the heart's action can be conveniently studied when taken from the body of a mammal.

The cause of the heart beat has naturally been one of the most continued objects of inquiry, and the point of view shifts with each advance of our experimental methods, and the wider extension of the inquiry throughout the animal world. H. Allen in 1757 was the first to announce that the activity of the heart is not dependent on its connexion with the nervous system. The excised heart, properly fed, con tinues to beat. The heart of a dog continued to work effectively and the animal to keep in health for months after division of all the nerves passing to the heart. The heart, it is true, is controlled and influenced constantly by the nervous system - attuned to the general needs of the body - but this control is not essential to life. The above dog, when exercised, became fatigued quickly, owing to the lack of the nervous control of the heart. When in 1848 Robert Remak discovered that groups of nerve cells are contained in the heart of the frog, the causation of the beat was attributed to the activity of these ganglia.

Confirmation of this view was found in the experiment of Hermann Stannius which demonstrates that the apex of the heart ceases to beat rhythmically if physiologically separated from the rest of the heart by ligature or momentary application of a clamp. The sinus, on the other hand, which contains ganglion cells, continues its beat as before when separated. Further experiment has shown that the beat of the heart cannot be ascribed to the rhythmic activity of the ganglion cells, which in the mammalian heart lie scattered in the base of the heart, in the neighbourhood of the venous opening and in the auriculo-ventricular groove. That this is so is shown by the fact that every strip of heart muscle, whether free of ganglion cell or not, is capable of rhythmic activity under suitable conditions (Walter Gaskell, 1847-, Theodor Wilhelm Engelmann, Alfred Wm. Porter). The inherent power of rhythmic contraction is most clearly seen in the embryonic heart, for the pulsation of the chick's heart became visible by the 24th to 48th hour of incubation, while the migration of the ganglion cells into the heart from the sympathetic system does not take place until the sixth day (His.). The heart muscle is pervaded by a network of nerve fibrils, and the supporters of the neurogenic theory have had to fall back upon this network as the cause of the beat. The " myogenic " theorists place the causation in the muscle itself.

The pulsating " umbrella " of the jelly-fish is formed of a network of nerve fibril and contractile elements, and this can be excited to contract by irritating any one of the sensory endings of the nervous network which are situated on the edge of the " umbrella." In the manifestation of a " refractory period " the " umbrella " behaves like the heart. Against this view we may cite the experiment of Julius Bernstein (1839), who clamped off the apex of the frog's heart to destroy the physiological continuity, kept the animal alive till the nerve network had degenerated and then found the apex could be mechanically excited to contract. Moreover, skeletal muscle-fibres can be thrown into rhythmic contraction by the application of a suitable solution of salts (Wilhelm Biedermann, 1854), and it is probable that heart muscle is excited to rhythmic activity by such means. At any rate the beat is profoundly affected by varying slightly the nature and percentage of salts supplied in the nutritive fluid. Carlson has recorded experiments upon the heart of the horseshoe crab (Limulus) which show that its beat at any rate depends on the integrity of the median nerve (and its ganglion cells) which runs down the heart. On the other hand, Gaskell has shown that any small bridge of heart muscle left connecting the auricle and ventricle of the tortoise heart will transmit the wave of contraction, while if the nerve passing from of sinus to ventricle be left, and the muscular connexions entirely severed, no wave passes. In contradistinction to cross-striated muscle, the structural unit of the heart is not also a functional unit, for the heart-cells are, from the earliest stage of development, joined together by branches into networks and bands so as to form one functional whole, and hence excitation of any one part leads to the contraction of the whole. The first part to begin to functionate in the embryo is the venous end, and the waves of contraction passing thence .spread over the developing ventricular segment. The muscle-cells of the ventricles are thicker, less sarcoplasmic and more clearly striated than the auricular muscle, which is more embryonic in structure. The contraction lasts longer in the ventricular than in the auricular muscle, while the automatic rhythm not only persists longer in the auricles, but is of greater frequency, as is clearly seen when the cavities of the heart are divided from each other. The venous orifices of the heart are least sensitive to injury, beat longest after death, and are the first to recover after arrest. Owing to the more powerful automatism of the venous extremity, the contraction normally proceeds thence, and, passing as a peristaltic wave over the auricles and ventricles, finally reached the arterial orifices. This peristaltic form of contraction is invariable in all periods of development and in all hearts, both of invertebrate and vertebrate animals. The peristalsis may, with difficulty, be artificially reversed by the application of a powerful rhythmic stimulus to the ventricular end. Antiperistalsis does not, however, take place easily, because the comparatively slow excitatory process in the ventricle has little effect on the auricular muscle. The latter, by initiating more rapid contraction-waves, over-dominates the former. The frequency of the whole heart is accelerated by warming the auricles, while the period of systole is alone shortened on warming the ventricles.

The sequence in the beat of the three chambers of the heart is attributed by Gaskell to the delay that occurs in the excitatory wave passing through the muscular connexions in the sino-auricular and auriculo-ventricular junctions. He showed that such delay could be imitated by moderately clamping a strip of heart muscle; the compressed part transmitted the wave less readily, so that the part above and below the clamp contracted in sequence.

In the mammalian heart there has recently been discovered a remarkable remnant of primitive fibres persisting in the neighbourhood of the venous orifices (representing the sinus). These fibres are in close connexion with the vagus and sympathetic nerves, and form the sino-auricular node of A. Keith and Martin Flack. If this l3 ' node is squeezed by a clamp, it prevents the effect of excitation of j 4 3 .,'? the vagus reaching th ? r. n = 5heart. The auricle and ventricles of the mam 6 malign heart are connected ?, -' i through the septum by a remarkable bundle of muscle fibres which is believed to convey the ex citatory wave from the FIG.15. - The Right Auricle andVentricle one cavity to the other. of a Calf's Heart, exposed to show the The root of this auriculocourse and connexions of the auriculoventricular bundle lies in ventricular bundle. I, central cartilthe right auricle, the main age exposed by dissection; 2, the part is buried in the intermain bundle; 3, auricular fibres from ventricular septum; its which the main bundle arises; 4, branches and twigs are right septal division; 5, moderator distributed to all parts of band; 6, a cusp of the tricuspid valve; either ventricle; the papil7, posterior group of the musculi lary muscles and fleshy papillaries; 8, orifice of the coronary columns, in particular, sinus; 9, above orifice of the inferior receive a direct supply. vena cava (Io); II, orifice of the The muscle fibres are of superior vena cava; 12, septal wall a peculiar type, known of the right auricle; 13, appendix of as the cells of Purthe right auricle; 14, septal wall of kinje. By this bundle it the inf undibulum; 15, beginning of is believed every part of the pulmonary artery; 16, apex of the ventricle is brought the right ventricle. (After A. Keith, in into synchronous contrac- Journal of Anatomy and Physiology.) tion. To its degeneration has been ascribed certain cases of disturbed cardiac rhythm, when the ventricle no longer follows the sequence of auricle.

The evidence of such degeneration is, at present, not convincing. The contraction of the heart, like that of other muscle, is accom panied by an electrical change. The part in contraction is at different potential to the part at rest. Thus an electrical wave accompanies the wave of contraction. This has been studied by means of the capillary, or the string, electrometer (Sir John Scott Burdon-Sanderson and Page, Einthoven, Gotch). The photographic records obtained with these instruments afford us a most beautiful method of recording the rhythm of normal and abnormal hearts in man, for they can be obtained by connecting the right hand and left foot of a patient with the instrument. Einthoven, by making A R - V A R - V A R - V FIG. 16. - Electrical Changes of Heart. A, diphasic variation of auricle; R - V, diphasic variation of ventricle. R = base negative; V = apex negative to base. After auricular contraction the ventricular is delayed - an example of arhythmia. (Einthoven.) The string galvanometer is the best method for elucidating disorders of cardiac rhythm.

use of the telephone wires, recorded in his laboratory the electrical changes of the hearts of patients seated in a hospital 2 m. away.

The heart during the period of systole is refractive to artificial excitation, but its susceptibility returns with diastole. The force and amplitude of any cardiac contraction depend on the previous activity of the heart and on such physical conditions as the degree of diastolic filling, the resistance to systolic outflow, temperature, &c., but are independent of the strength of the artificial stimulus so long as the latter is efficient. Owing to the refractory period, the slow rate of contraction and the independence of the amplitude of contraction on the strength of stimulus, the heart under ordinary conditions cannot be thrown, by rapidly repeated excitation, into a complete state of tetanic spasm. The refractory period can be shortened by heat (40° C.), or by calcium and sodium salts until tetanus is obtainable. The cardiac muscle is rich in sarcoplasm, and on this depends its power of slow, sustained contraction. The heart-muscle, besides rhythmically contracting, possesses " tone," and this tone varies with the conditions of metabolism, temperature, &c. Chloroform, for example, produces a soft dilated, strychnine, adrenalin or ammonia a tonically contracted heart. The mammalian heart ceases to beat at temperatures below 7° C. and above 44° C., and passes into " heat rigor " at 45° C.

In 1845 the brothers Weber made the astonishing discovery that the vagus nerve, when excited, slowed or even arrested the action of the heart. This was the first proof of the existence of inhibitory nerves. The cardiac inhibitory nerves have since been found in all classes of vertebrates and in many invertebrates. Some years later v. Beyeld (1862) and Moses and Il'ya Cyon (1843) discovered the existence of nerve fibres which, when excited, augmented and accelerated the beat of the heart. These nerves arise from 1-5 thoracic anterior spinal nerve roots and have their " cell stations " in the first thoracic and inferior cervical ganglia, whence they pass to the heart partly in company with the cardiac branches of the vagus, and partly as separate twigs. The vagus cardiac fibres arise by the middle of the lowermost group of vagus roots, and have their cellstations in the ganglion cells of the heart. These ganglion cells lie chiefly in the sub-pericardial tissue in the posterior wall of the auricles between and around the orifices of the venae cavae and pulmonary veins and between the aorta and pulmonary artery. The minute structure of these ganglia and the terminations of the nerves have been studied particularly by Dogiel. The inhibitory fibres arise from a centre in the spinal bulb which is in tonic action and constantly bridles the heart's action. When the vagi are divided the frequency of the heart increases and the blood pressure rises. The vagus centre is reflexly excited by the inhalation of chloroform, ammonia or other vapour irritant to the air passages, also by the want of oxygen in the blood in asphyxia. It may be excited by irritation of the abdominal nerves, e.g. a blow on the abdomen, and by increased pressure in the cerebral vessels. The acceleratory and augmenting fibres OWNIRIAINIII/Mlaf Medulla 'r' x Heart FIG. 17. - The origins of pneumogastric and vasomotor systems are in medulla, that of the sympathetic in upper portion of cord. The arrows indicate direction of nerve currents. In the heart R represents a reflex centre, I an inhibitory centre and A an accelerating centre.

likewise have their centre in the spinal bulb, and are in tonic action, the vagal centre. The vagus nerve works directly on the cardiac muscle, and produces some change (signalized by a positive variation in the electrical state of the heart) which results in a depression of the x x excitability, the con ductivity, the force and the frequency of the heart. After the vagal arrest the heart beats more forcibly, owing, it is thought, to the greater accumulation of contractile material dur ing the period of rest.

The converse of all these effects occurs on stimu lation of the accelerator nerves. Excitation of 20 these nerves may excite to renewed efforts an excised heart which has just ceased to beat after withdrawal of the supply of nutritive solution. Hence it is thought by some that the accelerator nerves tonically exert a sustaining influence on the heart.

The alkaloid atropin paralyses the vagal nerve endings in the heart, while nicotine paralyses the ganglion cells. Muscarin obtained from poisonous fungi slows and finally arrests the heart. Adrenalin, the active principle of the medulla of the supra-renal glands, augments its power. Chloroform depresses it and in poisonous dose throws the heart into paralytic dilatation. A great many of the cardiac vagal fibres convey impulse to the spinal bulb (centripetal), and reflexly influence the heart frequency, the breathing and the tonus of the blood vessels. In particular certain fibres, termed depressor (discovered by Ludwig and Cyon, 1866), cause dilatation of the arterioles and a fall of arterial pressure by inhibiting the tonic action of the vaso-motor centre in the spinal bulb. The depressor fibres arise from the root of the aorta, and overdistension of this part excites them, as evidenced not only by the above effect, but also by the electrical variation (action current) which has been observed passing up the depressor nerve. Sensory impressions originating in the heart do not as a rule enter into consciousness. They are carried by the cardiac nerves to the sympathetic ganglia, and thence to the upper thoracic region of the spinal cord, where they come into relation with the sensory nerves from the pectoral region, upper limb, shoulder, neck and head. The impressions are not felt in the heart, but referred to these sensory cutaneous nerves. Thus cardiac pain is felt in the chest wall and upper limbs and particularly on the left side. The function of the cardiac nerves is to co-ordinate the beat of the heart with the needs of the body and to co-ordinate the functions of other organs with the needs of the heart. For example, an undue rise of arterial pressure, induced, let us say, by compression of the abdomen, excites the centre of the vagus and produces slowing of the heart and a consequent lowering of arterial pressure. The heart of a mammal, however, continues to functionate after a section of all the branches of the cardiac plexus has been made, so that the nervous control and co-ordination of the heart are not absolutely essential to the continuance of life.

Water flowing through a tube from a constant head of pressure encounters a resistance occasioned by the friction of the moving water particles against each other and against the stationary layer that wets the wall of the tube. Part of factors the potential energy of the head of pressure is spent in endowing the fluid with kinetic energy, g gy, the greater part in circula- overcoming this resistance is rubbed down into heat. The narrower the tube is made, the greater the friction, until finally the flow ceases, the total energy being then insufficient to overcome the resistance.

The resistance may be measured at any point in the tube by inserting a side tube in the vertical position. The water rises to a certain height in the side tube, indicating the head of pressure spent in overcoming the resistance between the point of measurement and the orifice. If the lower end of the side tube is bent thus and inserted so that its orifice faces the stream, the water will rise higher than it did in the first case. The extra rise indicates the head of pressure spent in maintaining the velocity of flow. Such a method has been used to measure the velocity of flow in the vascular system (Napoleon Cybulski). When a stream of water is transmitted intermittently by the frequent strckes of a pump through XXVII. 30 a a long elastic rubber tube, the fluid does not issue in jets as it would in the case of a rigid tube, but flows out continuously. The elastic tube is distended by the force of the pump, and its elasticity maintains the outflow between the strokes. The continuous outflow here depends on the elasticity of the tube and the resistance to flow.

In the vascular system an area of vessels of capillary size is placed between the large arteries and veins. This area opposes a great resistance to flow. The arteries also are extensile elastic tubes. The effect of the peripheral resistance, as it is called, is to raise the pressure on the arterial side and lower it on the venous. The resistance to flow is situated chiefly, not in the capillaries, but in the small arteries, where the velocity is high; for " skin friction "- that is, the friction of the moving concentric layers of blood against one another and against the layer which wets the wall of these blood vessels is proportional to the surface area and to the viscosity of the blood - is nearly proportional to the square of the velocity of flow, and is inversely proportional to the sectional area of the vessels. Owing to the resistance to the capillary outflow, the large arteries are expanded by each systolic output of the heart, and the elasticity of their walls comes into play, causing the outflow to continue during the succeeding diastole of the heart. The conditions are such that the intermittent flow from the heart is converted into a continuous flow through the capillaries. If the arteries were rigid tubes, it would be necessary for the heart to force on the whole column of blood at one and the same time; but, owing to the elasticity of these vessels, the heart is saved from such a prolonged and jarring strain, and can pass into diastolic rest, leaving the elasticity of the distended arteries to maintain the flow. As a result of disease, the elastic tissue may degenerate and the arteries become rigid. Besides the saving of heart-strain, there are other advantages in the elasticity of the arteries. It has been found that an intermittently acting pump maintains a greater outflow through an elastic than through a rigid tube; that is to say, if the tubes be of equal bore. The four chief factors which co-operate in producing the conditions of pressure and velocity in the vascular system are - (I) the heart-beat, (2) the peripheral resistance, (3) the elasticity of the arteries, (4) the quantity of blood in the system. Suppose the body to be in the horizontal position and the vascular system to be brought to rest by, say, excitation of the vagus nerve and arrest of the heart. A sufficiency of blood to distend it collects within the venous cistern. The arterial system, owing to its elasticity and contractility, empties. If the heart now begin to beat, blood is taken from the venous system and is driven into the arterial system. The arteries receive more blood than can escape through the capillary vessels, and the arterial side of the system becomes distended, until equilibrium is reached, and as much blood escapes into the venous side per unit of time as is delivered by the heart. The flow in the capillaries and veins has now become a constant one and if the side pressure be measured it will be found to fall from the arteries to the capillaries, and from the capillaries to the venae cavae. In the large arteries there is a large side pressure which rises and falls with the pulses of the heart. The pulse waves spread out over a wider and wider area as the arteries branch. They finally die away in the arterioles. An increase or decrease in the energy of the heart-beat will increase or decrease respectively the velocity of flow and pressure of the blood. An increase or decrease in the total width of the arterioles respectively will lessen or raise the resistance; increase or decrease the velocity; lower or raise the blood pressure. A loss of blood, other conditions remaining the same, would cause a decrease in pressure and velocity. As a matter of fact, such a loss is compensated for by the adjustability of the vascular system. Tissue lymph passes from the tissues into the blood, and the blood vessels of the limbs and abdomen constrict, and thus the pressure is kept up, and an efficient circulation maintained through the brain, lungs and coronary vessels of the heart.

The whole vascular system is lined within by a layer of flattened cells, the endothelium; each cell is exceedingly thin and cemented to its fellows by a wavy border of an interstitial protoplasmic substance. The endothelium affords a smooth surface along which the blood can flow with of ease. Outside it there exists in the arteries and veins blood a middle and an external coat. The middle coat varies l greatly in thickness and contains most of the non-striated muscle-cells, which in the smaller arteries and arterioles form a particularly well developed band. In the larger arteries (fig. 19) a great deal of yellow elastic tissue, together with some white, fibrous tissue, pervades the middle coat. At the inner and outer border of this coat the elastic fibres fuse to form an internal and external fenestrated membrane. This coat endows the arteries with extensibility, elasticity and contractility. The outside coat consists mostly of white fibrous tissue and not only protects the arteries, but by its rigidity prevents over-distension. In the veins (fig. 20), where the middle coat is somewhat thinner and contains less elastic tissue, the outer coat consists mostly of muscle-fibres. The valves of the veins are formed of fibrous and elastic tissue covered with endothelium. As the arterioles branch into capillaries the muscular and elastic elements become less and less, until. in the capillaries themselves there is left only the layer of endothelium, supported by some stellate connective tissue cells. The.

antagonizing more or less the action of tArVACPVIVV K B 140 120 no ' '80 ' '60 ' 'Stfm.of Vagus peripheral end From Howell's Text-Book of Physiology, by permission of W. B. Saunders Co.

FIG. 18. - B, arterial blood pressure. K, record of volume of kidney. Inhibition of heart on faradizing vagus nerve.

capillaries form networks which accommodate themselves to the structure of the organs, e.g. longitudinal networks in muscle, loops in the papillae of the skin, close-meshed networks round the alveoli of glands, cells of liver, &c. In the liver the blood penetrates into the substance of the livercells. As the capillaries join together to form the vennules, musclefibres again appear and continuously. To stop the haemorrhage the ligature must be applied between the wound and the heart in the case of the artery, and between the peripheral parts and the wound in the case of the vein. The pulse travels about 20 times as fast as the blood flows in the arteries (7-8 metres per second). By feeling the pulse we can tell whether the heart-beat is frequent, quick, strong, regular, &c., and whether the wall of the artery is normal and the pressure in the arteries high or low. Frequency expresses the number per minute, quickness the duration of a single beat. The pulse is a most important guide to the physician. The pulse can be registered graphically by means of a sphygmograph. A lever rests on the radial artery and transmits the pulse to a system of levers which magnifies the movement and records it on a smoked surface moved by clockwork.

In such a record, or sphygmograph, the upstroke corresponds to systolic output of the left ventricle, marking the opening of the aortic valves, and the pouring of the blood into the arteries.

The downstroke represents the time during which the blood is flowing out of the arteries into the capillaries. There are subsidiary waves on the downstroke. The chief of these is called the dicrotic wave, the notch preceding which marks the closure of the semilunar valves. The dicrotic wave is caused by the jerk back of the blood towards the heart when the outflow ceases, and is most manifest when the systole is short and sharp and the output of blood from the arterioles rapid, in other words when the heartbeat is strong, the systolic pressure high and the diastolic pressure low. A smaller wave, predicrotic, preceding this occurs during the period of output and sometimes is placed on the ascending limb of the pulse curve. This occurs when the peripheral resistance is great, and the pulse is then termed anacrotic.

From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 19. - Transverse Section through the Wall of a Large Artery. A, tunica intima; B, tunica media; C, tunica externa.

From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 20. - Transverse Section of the Wall of a Vein. A, tunica intima; B, tunica media; C, tunica externa.

coat the walls of the latter. The veins have a greater capacity than the arteries. Blood vessels, the vasa vasorum, supply the walls of the large vessels with nutrition.

B C B Ai From Young and Robinson, Cunningham's Text-Book of Anatomy. FIG. 21. - Structure of Blood Vessels (diagrammatic). A l, capillary - with simple endothelial walls. A 2, larger capillary - with connective tissue sheath, " adventitia capillaris "; B, capillary arteriole - showing muscle cells of middle coat, few and scattered; C, artery - muscular elements of the tunica media forming a continuous layer.

The vaso-motor nerves end in a plexus of fibrils among the musclefibres. Ganglion cells occupy the larger nodes of the nerve plexus. The ends of a torn artery retract, coil up within the external coat and prevent haemorrhage. The arteries contract when mechanically irritated and remain contracted for a long time after excision. They tend to contract when submitted to increased blood pressure. The capillaries cannot contract of themselves, but their lumen can be widened or narrowed by the varying contractility or turgidity of the tissues in which they run.

The arteries successfully withstand elastic strain of the pulse 7 0 times a minute throughout the years of a long life. It has proved possible to stitch divided arteries and veins together so perfectly that the circulation can continue through them. A kidney has thus been successfully transplanted from one dog to another, and has continued to functionate normally.

The elastic coefficients of the several layers of the coat of an artery increase from within out, and thus great strength is obtained with the use of a small amount of material. Over-expansion of the arteries is checked by an external coat of inextensible connective tissue. The elasticity of a healthy artery is almost perfect, while the breaking strain is very great and far above that exerted by the blood pressure. The small arteries and arterioles are essentially muscular tubes, and can, under the influence of the central nervous system, vary considerably in diameter.

By the expulsion of the blood at each systole the walls of the aorta are suddenly distended. From the aorta a wave of distension ripples down the walls of the arteries. This wave of distension is called the pulse. As the pulse is distributed over an ever-widening field its energy is expended and it disappears finally in the arterioles. From a wounded artery the blood flows out in pulses, from a wounded vein FIG. 24. - Normal Pulse, and Time Tracing in 1 1 6 sec.

A, Primary wave. C, Dicrotic wave.

B, Predicrotic wave. D, Post-dicrotic wave.

The form of these waves is modified by the pressure of application of the sphygmograph, and by instrumental errors; and we have no scale by which we can measure the blood pressure in sphygmograph tracings. To do this another instrument, the sphygomanometer, is employed.

The pulse may pass through the arterioles and reach the capillaries when the arterioles are dilated or when the capillaries are only filled at each systole, as may be seen in the pink of the nail when the arm is held above the head, and in cases of aortic regurgitation.

A venous pulse may be recorded in the jugular vein; it exhibits oscillations synchronous with auricular and ventricular systole, and affords us important information in certain cases of heart disease. The normal average pulse rate is 72 per minute, in woman about 80; but individual variations from 40-100 have been observed consistent with health. In the newborn the pulse beats on the average 130-140 times a minute; in a one-year-old child 120-130; three years 1 00; ten years 90; fifteen years 70-75. Active muscular exercise may increase the pulse rate to 130. Nervous excitement, extreme debility and rise of body temperature also increase it markedly. The pulse is more frequent when one stands than when one sits, or lies down, and this is especially so in states of debility. The taking of food, especially hot food, increases it. By placing tambours on, say, the carotid and radial arteries and recording the two pulses synchronously, it has been found that the pulse occurs later, the further the seat of observation is from the heart. The velocity with which the pulse wave travels down the arteries has been determined thus. It is about 7-8 metres per second. The wave length of the pulse is obtained by multiplying the duration of the inflow of blood into the aorta by the velocity of the pulse wave. It is about 3 metres. As the return of venous blood and pulmonary circulation is favoured during inspiration so that the output of the left ventricle during the first part of inspiration is lessened and subsequently increased, D Fies. 22, 23 and 2 4 from Allchin's Manual of Medicine, by permission of Macmillan & Co. Ltd.

T. externa T. media T. intima A1 the sphygmograph reveals respiratory oscillations; the whole line of the tracing falls during the first part of inspiration and rises subsequently.

The circulation in the capillaries may be studied by placing under the microscope a transparent membrane such as the web of the frog's foot, tail of tadpole, wing of bat, &c. By a special illumination one may see the shadow of the blood cor crcula- puscles moving through the retinal vessels of one's own eye,. and even calculate the velocity of flow. The diameter of the smaller capillaries is such as to permit the passage of the red blood corpuscles in single file only; their length is about nth of an inch. The endothelial cells confine the blood from direct contact with the tissue lymph and so prevent its coagulation, but allow and regulate the exchange of material between the blood and lymph. This exchange is regulated by the vital activity of the cells, and does not follow such laws as pertain to filtration and diffusion through dead membranes. There is evidence to show that the cells of the hepatic capillaries are capable of protoplasmic movement and of phagocytosis. The pressure in the capillaries stands in closer relationship to that in the veins than to that in the arteries; for example, a rise of pressure in the venae cavae, other things remaining the same, raises the pressure in the hepatic capillaries to a like amount, while a rise of pressure in the aorta does not, for most of the arterial pressure is spent in overcoming the peripheral resistance. The filling of the capillaries in the skin varies greatly with temperature, posture, &c. When the hand is cold the arterioles are so constricted that blood only passes through the wider and more direct capillaries. As the skin becomes warm it flushes, the arterioles dilating and all the capillary networks becoming filled with blood. Muscular movements express the blood out of the capillaries, as may be seen by the blanching of the skin which occurs on clenching the hand. Raising the hand blanches, and lowering it congests the capillaries. The pressure and velocity in the capillaries thus constantly vary, owing to alterations in hydrostatic pressure, the pressure of the body against external objects, the contraction of the muscles, and the contraction of the arterioles. It is not possible therefore to set any definite figure to the capillary pressure or velocity. In the frog's web, with the foot confined and at rest, the velocity is about i mm. per second. We continually make slight movements to counteract the hydrostatic effect and prevent the congestion of blood in the capillaries of lower parts of the body. It is this tendency to congestion which makes it so difficult to stand absolutely motionless for any length of time. The red corpuscles, being the heavier, occupy the axis, and the white corpuscles the peripheral layer of the capillary stream. If an irritant is placed on the membrane it will be observed that the capillaries become wider and crowded with corpuscles, the flow slackening and finally becoming arrested owing to the passing out of the plasma through the damaged capillary wall. The white corpuscles creep out between the endothelial cells into the tissues. Such are the first phenomena of inflammation. After obstruction of an artery collateral pathways are in most parts rapidly formed, for the anastomatic capillaries, stimulated by the increased blood flow, develop into arterioles and arteries.

Numerous anastomoses exist between the veins, so that if the flow of blood be obstructed in one direction it readily finds a passage flow, in another. Muscular movement, alterations of posture and respiratory movements particularly forward the venous circulation. The barber's pole of the barber surgeon was grasped to increase the flow in the old blood-letting days. The valves in the veins allow the blood to be forced only towards the heart. The pressure in the veins varies according to the hydrostatic pressure of the blood column above the point of measurement. In the horizontal position, when this factor is almost eliminated, the pressure in the large veins is about equal to 5 - Io mm. of mercury, and even may become negative on taking a deep inspiration. There thus arises the danger of air being sucked into a wounded jugular vein. If air does thus gain entry it may fatally obstruct the circulation.

The venous circulation is impeded by (I) a lessening of heart power, (2) valvular defects, such as incompetence or narrowing of the orifice which they guard, (3) obstruction to the filling of the heart, as in cases of pericardial effusion, (4) obstruction of the pulmonary circulation as in coughing, by pleuritic effusion, &c. The results of venous congestion are a less efficient arterial circulation, a dusky appearance of the skin, a fall of cutaneous temperature, and an effusion of fluid into the tissue spaces producing oedema and dropsy. This last effect is not due to increased capillary pressure producing increased transudation as has been supposed, for no such increase in venous and capillary pressure persists under the conditions. It is due to the altered nutrition of the capillary endothelium and the tissues, which results from the deficient circulation.

If for any reason the left ventricle fail to maintain its full systolic output, it ceases to receive the full auricular input, and in consequence the pulmonary vessels congest. This tells back on the right heart, and the right ventricle is unable to empty itself into the congested pulmonary vessels, and this in its turn leads to venous congestion. The final result of any obstruction thus is a pooling of the blood in the venous cistern. Dyspnoea results from cardiac insufficiency.

It is excited by the increased venosity of the blood acting on the respiratory centre. Both excess of carbon dioxide and deficiency of oxygen excite this centre. The increased respiratory movements aid the circulation.

The venous side of the vascular system, owing to the great size of the veins, has a large potential capacity, while man y of the capillaries in each organ are empty and collapsed, except at those periods of vaso-dilatation and hyperaemia which accompany extreme activity of function. The vascular system cannot be regarded as a closed system, for the blood-plasma,; whenever the capillary pressure is increased, transudes through the capillary wall into the tissuespaces and enters the lymphatics. Thus, if fluid be transfused into the circulatory system, it not only collects in the capacious reservoirs of the veins and capillaries - especially in the lungs, liver and abdominal organs - but leaks into the tissue-spaces. Hence the pressure in the vascular system cannot be raised above the normal for any length of time by the injection of even enormous quantities of fluid. The lymphatics of tissue-spaces must be regarded as part of the vascular system. There is a constant give and take between the blood-plasma and the tissue lymph. If the fluid part of the blood be increased, then the capillary transudation becomes greater, and the excess of fluid is excreted from the kidneys and glands of the alimentary canal. If the fluid part of the blood diminish, then fluid passes from the tissue-spaces into the blood, and the sensation of thirst arises, and more drink is taken. The circulation may be greatly aided by the trans fusion of salt solution (6.8%) or blood after severe hemorrhage, or in states of surgical shock. Only the blood of man must be used. The direct giving of blood by connecting the radial artery of a relation to the median vein of a patient has been used as a means of effecting restoration. Blood may be withdrawn from the system slowly to the extent of 4%, rapidly to the extent of 2% of the bodyweight, without lowering the arterial pressure, owing to the compensatory contraction of the arterioles and the rapid absorption of fluid from the tissues into the blood. The withdrawal of the tissue-lymph excites extreme thirst and the great need for water which occurs after severe hemorrhage. About 75% by weight of the tissues, excluding fat and bone, consists of water. The quantity of blood in the body is about 2 oth of the body weight. That of tissue-lymph is unknown, but it must be considerable, probably greater than that of the blood. The lymphatics drain off the excess of fluid which transudes from the capillaries, and finally return it to the vascular system. The interchange between tissue, blood and lymph depends on the forces of the living cells, which are as yet far from complete elucidation.

We may define the velocity of the blood at any point in a vessel as the length of the column of blood flowing by that point in a second. In the case of a tube, supplied by a constant The head of pressure, we can divide the tube and measure T hiec;ty the outflow per second; knowing the volume of this, of and the cross area of the artery, we can determine the ow. length of the column. This kind of experiment cannot be done on the living animal, because the opening of the vessel alters the resistance to flow, and the loss of blood also changes the physiological conditions. To determine the velocity other means must be devised. Ludwig invented an instrument called the stromuhr, consisting of two bulbs mounted on a rotating platform pierced with two holes. One bulb is filled with oil - the other with blood. The bulbs are connected together by a tube at their upper end, and the lower end of the one full of oil is brought over the hole in the platform. The central end of the artery is connected to the same hole and the peripheral end to the other, over which stands the bulb full of blood. The blood being allowed to flow displaces the oil out of the one bulb into the other; directly this happens, the bulbs are rotated and the one full of oil is again brought over the central end of the artery. The number of rotations per minute is counted, and the volume of the bulb being known we obtain the volume of blood that passes through the instrument per a minute. In another instrument, the haemoc dromograph of Chauveau, there is inserted FIG. 25. - Ludwig's into the artery a 1 tube in which hangs a small Stromuhr. pendulum; the stem of the pendulum passing through a rubber dam which closes the vertical limb of the tube. The pendulum is deflected by the flow, and the greater the velocity the greater the deflection. The deflection can be recorded by connecting the free end of the pendulum to a tambour arrangement. This instrument allows us to record and measure the variations of velocity during systole and diastole of the heart, but it can only be used in the vessels of large animals. Still other methods have been employed by Cybulski and Stewart. The general relations of the velocity of the blood in the arteries, capillaries and veins is expressed by the curve shown in fig. 26. The velocity in the large arteries may reach 500 mm. per second in systole and fall to 150 mm. in diastole. The smaller the artery the less is this difference and the more uniform the rate of flow.

FIG. 26. - Diagram showing General Relations of the Velocity of the Blood in the Arteries, Capillaries and Veins.

The flow in the large veins is approximately equal to that in the large arteries. In the jugular vein of a dog the mean velocity was foun,-1 to be 225 mm. and in the carotid 260 mm. per second. The velocity in the capillaries has been measured by direct observation with the microscope. It is very small, e.g. o5 - I mm. per second. The variation of velocity in different parts of the vascular system is explained by the difference in width of bed through which the stream flows. The vascular system may be compared to a stream which on entering a field is led into a multitude of irrigation channels, the sum of the cross sections of all the channels being far greater than that of the stream. The channels unite together again and leave the field as one stream. If the flow proceeds uniformly for any given unit of time, the same volume must flow through any cross section of the system. Thus the greatest velocity is where the total bed is narrowest, and slowest where the bed widens to the dimensions of a lake. _ The blood in leaving the heart may take a short circuit through the coronary system of the heart and so back to the right heart, or it may take a long and devious course to the toes and back, or through the intestinal capillaries, portal system and hepatic capillaries. It is obvious, then, that the time any two particles of blood take to complete the circuit may be widely different. Experiments have been made to determine how rapidly any substance, like a poison,which enters the blood may be distributed over the body. A salt such as potassium ferrocyanide is injected into the jugular vein, and the blood collected in successive samples at seconds of time from the opposite jugular vein. These samples are tested for the presence of the salt, or a strong solution of methylene blue is injected into the jugular vein, and the moment determined with a stopwatch when the blue colour appears in the carotid artery.

The velocity of flow also can be determined in any organ by injecting salt solution into an artery, and observing, with the aid of a Wheatstone's bridge arrangement, the galvanometric change in electrical resistance which occurs in the corresponding vein when the salt solution reaches it. The moment of injection and that of the alteration in resistance are observed with a stop-watch (Stewart).

It has been determined that the blood travelling fastest can complete the circuit in about the time occupied by 25 to 30 heartbeats, say in 20 to 30 seconds; a result which shows how rapidly methods must be taken to prevent the absorption of poisons - for example, snake-poison. The blood travelling fastest in the pulmonary circuit occupies only about one-fifth of the time spent by that in the systemic circuit. That some of the blood takes a very long time to return to the heart is shown by the long time it takes to wash the vascular system free of blood by the injection of salt solution.

That the blood is under different pressure in the various parts of the The system has long been known. From a divided artery the blood flows out in forcible spurts, while from a vein it flows out continuously and with little force. It takes very little pressure of the fingers to blanch the capillaries of the skin, but an appreciable amount to obliterate the system. radial artery.

Stephen Hales (1733) was the first to measure the blood pressure. He inserted a brass tube into the femoral vein of a horse and connected it to a long glass tube held vertically, using the trachea of a goose as a flexible tube, and found the blood rose to the height of 8 ft., oscillated there with each heart-beat, and rose and fell somewhat with inspiration and expiration. In the vein he found the pressure to be only about 12 in. Poiseuille (1828) adapted to the same purpose the mercurial manometer, a U-shaped tube containing mercury, which, being 13.5 times heavier than blood, allowed the manometer to be brought to a convenient height.

The introduction of rubber tubing for the connexions made the method of inquiry comparatively simple. The tubing connecting the arterial cannula and the manometer was filled with a suitable fluid to prevent coagulation of the blood; also to prevent more than a trace of blood entering the connexions. A saturated solution of sodium sulphate, or a 1% solution of sodium citrate, may be employed for this purpose. Ludwig (1847) added a float provided with a writing style to the mercurial manometer, and brought the style to write on a drum covered with smoked paper and driven slowly round by clockwork - a kymograph. By this means tracings of the arterial blood pressure are obtained, and the influence upon the blood pressure of various agents recorded and studied. For the veins a manometer filled with salt solution is used, as mercury is too heavy a fluid to record the far slighter changes of venous pressure. The manometer may be connected with a recording tambour.

The arterial blood-pressure record obtained with the mercurial manometer exhibits cardiac and respiratory oscillations as shown in fig. 18. The method gives us a fairly accurate record of the mean pressure, but the mass of the mercury causes such inertia that the instrument is quite unable to faithfully record the systolic and diastolic variations of pressure. To effect this record, delicate spring manometers of rapid action and small inertia have been inFrom Howell's Text-Book of Physiology, by permission of vented. A mercury W. B. Saunders Co.

manometer provided FIG. 27. - Diagram showing Systolic, Mean with maximum and and Diastolic Pressure. minimum valves has also been employed to indicate the maximal systolic and minimal diastolic pressure. To determine the blood pressure in man, an instrument called the sphygmometer is used. The writer's sphygmometer consists of a rubber bag covered with silk which is filled with air, and con nected by a short length of tube to a manometer. This manometer con sists of a graduated glass tube, open at one end. A small hole is in the side of the tube near this end. A meniscus of water is introduced up to the side hole - the zero mark on the scale - by placing the open end of the tube in water. The bag is now con nected to the gauge so that the side hole is closed by the rubber tube.

Covering the rubber bag with the hand and pressing it on the radial artery until the pulse (felt beyond) is obliter ated, one reads the height to which the meniscus rises in the manometer, and this gives us the systolic pressure in the artery. The air above the me niscus acts as a spring, converting the instrument into a spring manometer.

It is empirically graduated in mm. Hg.

It is very necessary to remember that the blood pressures, taken in differ ent vessels and postures, vary with the hydrostatic pressure of the column of blood above the point of measure ment. Thus in the standing posture the arterial pressure in the arteries of the leg is higher than in the arm by the height of the column of blood that separates the two points of measure ment. In the horizontal posture the pressure is practically the same in all the big arteries. The pressure in the ascending aorta is kept about the same in all postures, while that of the leg arteries varies widely. The effect of gravity is compensated there by active changes in heart force, splanchnic dilatation, &c. (L. Hill). The systolic pressure of young men, taken in the radial artery with the arm at the same level as the heart, may be taken to be about II() mm. of Hg. In men of 40-60 years the systolic pressure is often about 140 mm., but in some robust men it is no higher than in youth.

The venous pressure in man may be measured by finding the pressure just required to prevent a cutaneous vein refilling after it has been emptied beyond a valve. There is no accurate method Arteries Capillaries Veins From Allchin's Manual of Medicine, by permission of Macmillan & Co. Ltd.

jr4 ?luo to FIG. 28. - Hill's Sphygmometer.

of measuring the capillary pressure. It and the venous pressure constantly vary from nothing to a positive amount with rest or movement of muscles, change of posture, &c.

The arterial pressure is raised during exertion by the more forcible beat of the heart - e.g. pressures of 140-190 mm. Hg have been observed immediately after a 3-mile race. It rapidly sinks to a lower level than usual after the exertion is over, e.g. 90 mm. Hg, owing to the quieter action of the heart and the persistence of the cutaneous dilatation of the blood vessels which is evoked by the rise of body temperature. The writer has observed in athletes rectal temperatures of 102-105° F. after long races. After meals there is an increase in cardiac force to maintain the flow through the dilated splanchnic vessels. Mental excitement raises the pressure - e.g. the writer's pressure may be 110 mm. before and 125 mm. Hg after giving a lecture. The origin of the blood pressure in the arteries is the energy of the heart. The pressure gradient depends on the peripheral resistance. In the arterials the pressure is spent, and little of it reaches the capillaries. The return of the capillary blood to the veins and the pressure in the veins is due partly to the remainder of the cardiac force, but more largely to the contraction of the skeletal muscles and the viscera, to the action of gravity in changes of posture and to the respiratory pump.

The pulmonary artery, carrying venous blood, divides and subdivides, and the smallest branches end in a plexus of capillaries The pul- on the walls of the air-cells of the lung. From this plexus the blood is drained by the radicles of the four pulmonary monary veins which open into the left auricle. The pressure in arcula- the pulmonary artery is less than one-third the aortic Elan. pressure, and the blood takes only one-third of the time to complete the pulmonary cir,cuit that it takes to make the systemic. The four chief factors which influence the pulmonary circulation are: (1) the force and output of the right ventricle; (2) the diastolic filling action of the left auricle and ventricle; (3) the diameter of the pulmonary capillaries, which varies with the respiratory expansion of the lungs; (4) the intrathoracic pressure.

In inspiration the lungs are distended in consequence of the greater positive pressure on the inner surfaces being greater than the negative pressure on their outer pleural surfaces. The negative pressure in the intrathoracic cavity results from the enlargement of the thorax by the inspiratory muscles. When the elastic lungs are distended by a full inspiration they exert an elastic traction amounting to about 15 mm. Hg. The heart and vessels within the thorax are submitted to this traction - that is, to the pressure of the atmosphere minus 15 mm. Hg - while the vascular system of the rest of the body bears the full atmospheric pressure. The thinwalled auricles and veins yield more to this elastic traction than the thick-walled ventricles and arteries. Thus inspiration exerts a suction action, which furthers the filling of the veins and auricles. This action is assisted by the positive pressure exerted by the descending diaphragm on the contents of the abdomen. Blood is thus both pushed and sucked into the heart in increased amount during inspiration.

Experiment has shown that the blood vessels of the lungs when distended are wider than those of collapsed lungs. Suppose an elastic bag having minute tubes in its walls be dilated by blowing into it, the lumina of the tubes will be lessened, and the same occurs in the lungs if they are artificially inflated with air; but if the bag be placed in a glass bottle, and the pressure on its outer surface be diminished by removing air from the space between the bag and the side of the bottle, the bag will distend and the lumina of the tubes be increased. Thus it is evident that inspiration, by increasing the calibre of the pulmonary vessels, draws blood into the lungs, and the movements of the lungs become an effective force in carrying on the pulmonary circulation. It has been estimated that there is about one-twelfth of the whole blood quantum in the lungs during inspiration, and one-fifteenth during expiration. The great degree of distensibility of the pulmonary vessels allows of frequent adjustments being made, so that within wide limits as much blood in a given time will pass through the pulmonary as through the systemic system. The limits of their adjustment may, however, be exceeded during violent muscular exertion. The compressive action of the skeletal muscles returns the blood to the venous cistern, and if more arrives than can be transmitted through the lungs in a given time, the right heart becomes engorged, breathlessness occurs, and signs of venous congestion appear in the flushed face and turgid veins. The weaker the musculature of the heart the more likely is this to occur; hence the breathlessness on exertion which characterizes cardiac affections. The training of an athlete consists largely in developing and adjusting his heart to meet this strain. Similarly the weak heart may be trained and improved by carefully adjusted exercise. Rhythmic compression of the thorax is the proper method of resuscitation from suffocation, for this not only aerates the lungs, but produces a circulation of blood. By compressing the abdomen to fill the heart, and then compressing the thorax to empty it, the valves meanwhile directing the flow, a pressure of blood can be maintained in the aorta even when the heart has ceased to beat, and this if patiently continued may lead to renewal of the heart-beat. There is no certain evidence that the pulmonary arteries are controlled by vaso-motor nerves. In the intact animal it is difficult to determine whether a rise of pressure in the pulmonary artery is induced really by constriction of the pulmonary system, or by changes in the output of the heart; hence different observers have reached conflicting conclusions. In the case of lungs which have been supplied with an artificial circulation and a constant head of pressure to eliminate the action of the heart, no diminution in outflow has been observed in exciting the branches of the vagus or sympathetic nerves which supply the lungs, or by the injection of adrenalin (Sir Benjamin C. Brodie (1783-1862), and Dixon, Burton-Spitz).

The portal circulation is peculiar in that the blood passes through two sets of capillaries. Arterial blood is conveyed to the capillary networks of the stomach, spleen, pancreas and intestines The by branches of the abdominal aorta. The portal vein is formed by the confluence of the mesenteric veins with the ortal splenic vein, which together drain these capillaries. The tion la- portal blood breaks up into a second plexus of capillaries within the substance of the liver. The hepatic veins carry the blood from this plexus into the inferior vena cava. Ligation of the portal vein causes intense congestion of the abdominal vessels, and so distensile are these that they can hold nearly all the blood in the body: thus the arterial pressure quickly falls, and the animal dies just as if it had been bled to death. The portal circulation is largely maintained by the action of the respiratory pump, the peristaltic movements of the intestine and the rhythmic contractions of the spleen; these agencies help to drive the blood through the second set of capillaries in the liver. The systole of the heart may tell back on the liver and cause it to swell, for there are no valves between it and the inferior vena cava. Obstruction in the right heart or pulmonary circulation at once tells back on the liver. The increased respiration which results from muscular exercise greatly furthers the hepatic circulation, while it increases the consumption of food material. Thus exercise relieves the over-fed man. The liver is so vascular and extensile that it may hold one-quarter of the blood in the body.

The circulation of the brain is somewhat peculiar, since this organ is enclosed in a rigid bony covering. The limbs, glands and viscera can expand considerably when the blood pressure The rises, but the expansion of the brain is confined. By the cerebral c expression of venous blood from the veins and sinuses the r- brain can receive a larger supply of arterial blood at e each pulse. Increase in arterial pressure increases the velocity of flow through the brain, the whole cerebral vascular system behaving like a system of rigid tubes when the limits of expansion have been reached. For as the pressure transmitted directly through the arteries to the capillary veins must always be greater than that transmitted through the elastic wall of the arteries to the brain tissue, the expansion of the arteries cannot obliterate the lumina of the veins. The pressure of the brain against the skull wall is circulatory in origin: in the infant's fontanelle the brain can be felt to pulse with each heart-beat and to expand with expiration. The expiratory impediment to the venous flow produces this expansion. A blood clot; on the brain or depressed piece of bone raise the brain pressure by obliterating the capillaries in the compressed area and raising the pressure therein to the arterial pressure. The arterial supply to the brain by the two carotid and two vertebral arteries is so abundant, and so assured by the anastomosis of these vessels in the circle of Willis, that at least two of the arteries in the monkey can be tied without grave effect. Sudden compression of both carotids may render a man unconscious, but will not destroy life, for the centres of respiration, &c., are supplied by the vertebral arteries. The vertebral arteries in their passage to the brain are protected from compression by the cervical vertebrae.

Whether the muscular coat of the cerebral arteries is supplied with vaso-motor nerves is uncertain. Hurthle and others observed a rise of pressure in the peripheral end of the carotid artery on stimulating the cervical sympathetic nerve. The writer found this to be so only when the cervical sympathetic nerve was excited on the same side as the carotid pressure was recorded. If the circle of Willis was constricted, excitation of either nerve ought to have the effect; it is possible that the effect was produced by the vasoconstriction of the extra-cranial branches of the carotid. After establishing an artificial circulation of the brain Wiggins found that adding adrenalin to the nutritive fluid reduced the outflow, and it is supposed that adrenalin acts by stimulating the ends of the vasomotor nerves, rather than by stimulating the muscular coats of the arteries. The veins of the pia and dura mater have no middle muscular coat and no valves. The venous blood emerges from the skull in man mainly through the opening of the lateral sinuses into the internal jugular vein; there are communications between the cavernous sinuses and the ophthalmic veins of the facial system, and with the venous plexuses of the spinal cord. The points of emergence of the veins are well protected from closure by compression. The brain can regulate its own blood supply by means of the cardiac and vaso-motor centres. Deficient supply to these centres excites increased frequency of the heart and constriction of the arteries, especially those of the great splanchnic area. Cerebral excitement has the same effect, so that the active brain is assured of a greater blood supply (Bayliss and L. Hill).

In each unit of time the same quantity of blood must, on the average, flow through the lesser and greater circuit, for otherwise the circulation would not continue. Likewise, the average cir velocity at any part of the vascular system must be in versely proportional to the total cross-section at that part.

In other words, where the bed is wider, the stream is slower; activity. the total sectional area of the capillaries is roughly estimated to be 700 times greater than that of the aorta or venae cavae. Any general change in velocity at any section of this circuit tells both backwards and forwards on the velocity in all other sections, for the average velocity in the arteries, veins and capillaries, these vessels being taken respectively as a whole, depends always on the relative areas of their total cross-sections.

The vascular system is especially constructed so that considerable changes of pressure may be brought about in the arterial section, without any (or scarcely any) alteration of the pressures in the venous or pulmonary sections of the circulatory system. A highpressure main (the arteries) runs to all the organs, and this is supplied with taps; for by means of the vaso-motor nerves which control the diameter of the arterioles, the stream can be turned on here or there, and any part flushed with the blood, while the supply to the remaining parts is kept under control. Normally, the sum of the resistances which at any moment opposes the outflow through the capillaries is maintained at the same value, for the vascular system is so coordinated by the nervous system that dilatation of the arterioles in any one organ is compensated for by constriction in another. Thus the arterial pressure remains constant, except at times of great activity. The great splanchnic area of arterioles acts as " the resistance box " of the arterial system. By the constriction of these arterioles during mental or muscular activity the blood current is switched off the abdominal organs on to the brain and muscles, while by dilating during rest and digestion they produce the contrary effect. The constriction of the splanchnic vessels does not sensibly diminish the capacity of the total vascular system, for the veins possess little elasticity. Thus variations of arterial pressure, brought about by constriction or dilatation of the arterial system, produce little or no effect on the pressure in the great veins or pulmonary circuit. The contraction of the abdominal muscles, on the other hand, greatly influences the diastolic or filling pressure of the heart. It is obviously of the utmost importance that the heart should not be over-dilated by an increased filling pressure during the period of diastole.

When a man strains to lift a heavy weight he closes the glottis, and by contracting the muscles which are attached to the thorax raises the intrathoracic pressure. The rise of intrathoracic pressure aids the pericardium in supporting the heart, and prevents overdilatation by resisting the increase in venous blood pressure. This increase results from the powerful and sustained contraction of the abdominal and other skeletal muscles. In the diagram already given it is clear that the contraction of T will counteract the contraction of A. At the same time the rise of intrathoracic pressure supports the lungs, and prevents the blood, driven out from the veins, from congesting within the pulmonary vessels. Over-dilatation both of the heart and lungs being thus prevented, the blood expressed from the abdomen is driven through the lungs into the left ventricle, and so into the arteries. So long as the general and intense muscular spasms continue, there is increased resistance to the outflow of the blood through the capillaries both of the abdominal viscera and the limbs. The arterial pressure rises, therefore, and the flow of blood to the central nervous system is increased. The rise of the intrathoracic and intra-abdominal pressures, and the sustained contraction of the skeletal muscles, alike hinder the return of venous blood from the capillaries to the heart, and, owing to this, the face and limbs become congested until the veins stand out as knotted cords. It is obvious that at this stage the total capacity of the vascular system is greatly diminished, and the pressure in all parts of the system is raised. It is during such a muscular effort that a degenerated vessel in the brain is prone to rupture and occasion apoplexy. The venous obstruction quickly leads to diminished diastolic filling of the heart, and to such a decreased velocity of blood flow that the effort is terminated by the lack of oxygen in the brain. During any violent exercise, such as running, the skeletal muscles alternately contract and expand, and the full flood of the circulation flows through the locomotor organs. The stroke of the heart is then both more energetic and more frequent, and the blood circulates with increased velocity. Under these conditions the filling of the heart is maintained by the pumping action of the skeletal and respiratory muscles. The abdominal wall is tonically contracted, and the reserve of blood is driven from the splanchnic vessels to fill the dilated vessels of the locomotor organs. The thorax is tonically elevated and the thoracic cavity enlarged, so that the pulmonary vessels are dilated. At each respiration the pressure within the thoracic cavity becomes less than that of the atmosphere, and the blood is aspirated from the veins into the right side of the heart and lungs; conversely, at each expiration the thoracic pressure increases, and the blood is expressed from the lungs into the left side of the heart. While the respiratory pump at all times renders important aid to the circulation of the blood, its action becomes of supreme importance during such an exercise as running. The runner pants for breath, and this not only increases the intake of oxygen, but maintains the diastolic filling of the heart. It is of the utmost importance that man should grasp the fact that the circulation of the blood depends not only on the heart, but on the vigour of the respiration and the activity of the skeletal muscles. Muscular exercise is for this reason a sine qua non for the maintenance of vigorous mental and bodily health. Under the influence of the muscular system comes not only the blood but the lymph. The lymphatics form a subsidiary system of small valved vessels, and drain the tissues of the excess of lymph, which transudes from the capillaries of the organs during functional activity, or in consequence of venous obstruction. The larger lymphatics open into the veins at the root of the neck. It is chiefly by the compressive action of the skeletal and visceral muscles, and the aspirating action of the respiratory pump, that the lymph is propelled onwards. It must be borne in mind that the descent of the diaphragm during inspiration compresses the abdominal organs, and thus aids the aspirating action of the thorax in furthering the return to the heart both of venous blood and of lymph.

The circulation remains efficient not only in the horizontal but also in the erect position, and just as much so when a man, like a gymnast, is ceaselessly shifting the position of his body. Yet in a man standing six feet the hydrostatic pressure of a column of blood reaching from the vertex to the soles of the feet is equal to 14 cm. of mercury. The blood, owing the on to its weight, continually presses downwards, and under the , cf.r.. influence of gravity would sink if the veins and capillaries of the lower parts were sufficiently extensile to contain it. Such is actually the case in the snake or eel, for the heart empties so soon as one of these animals is immobilized in the vertical posture. This does not occur in an eel or snake immersed in water, for the hydrostatic pressure of the column of water outside balances that of the blood within. During the evolution of man there have been developed special mechanisms by which the determination of the blood to the lower parts is prevented, and the assumption of the erect posture rendered possible. The pericardium is suspended above by the deep cervical fascia, while below it is attached to the central tendon of the diaphragm. Almost all displacement of the heart is thus prevented. The pericardium supports the right heart when the weight of a long column of venous blood suddenly bears upon it, as, for example, when a man stands on his head. The abdominal viscera are slung upwards to the spine, while below they are supported by the pelvic basin and the wall of the abdomen, the muscles of which are arranged so as to act as a natural waist-band. In tame hutch rabbits, with large patulous abdomens, death may result in from 15 to 30 minutes if the animals are suspended and immobilized in the erect posture, for the circulation through the brain ceases and the heart soon becomes emptied of blood. If, however, the capacious veins of the abdomen be confined by an abdominal bandage, no such result occurs. Man is naturally provided with an efficient abdominal belt, although this in many is rendered toneless by neglect of exercise and gross or indolent living. The splanchnic arterioles are maintained in tonic contraction by the vaso-motor centre, and thus the flow of blood to the abdominal viscera is confined within due limits. The veins of the limbs are broken into short segments by valves, and these support the weight of the blood in the erect posture. The brain is confined within the rigid wall of the skull, and by this wall are the cerebral vessels supported and confined when the pressure is increased by the head-down posture. Every contraction of the skeletal muscles compresses the veins of the body and limbs, for these are confined beneath the taut and elastic skin. The pressure of the body against external objects has a like result. Guided by the valves of the veins, the blood is by such means continually driven upwards into the venae cavae. If the reader hangs one arm motionless, until the veins at the back of the hand become congested, and then either elevates the limb or forcibly clenches the fist, he will recognize the enormous influence which muscular exercise, and continual change of posture, has on the return of blood to the heart. It becomes wearisome and soon impossible for a man to stand motionless. When a man is crucified - that is to say, immobilized in the erect posture - the blood slowly sinks to the most dependent parts, oedema and thirst result, and finally death from cerebral anaemia ensues. In man, standing erect, the heart is situated above its chief reservoir - the abdominal veins. The blood is raised by the action of the respiratory movements, which act both as a suction and as a force pump, for the blood is not only aspirated into the right ventricle by the expansion of the thoracic cavity, but is expressed from the abdomen by the descent of the diaphragm. When a man faints from fear, his muscular system is relaxed and respiration inhibited. The blood in consequence sinks into the abdomen, the face blanches and the heart fails to fill. He is resuscitated either by compression of the abdomen, or by being placed in the head-down posture. To prevent faintness and drive the blood-stream to his brain and muscles, a soldier tightens his belt before entering into action. Similarly, men and women with lax abdominal wall and toneless muscles take refuge in the wearing of abdominal belts, and find comfort in prolonged immersion in baths. It would be more rational if they practised rope-hauling, and, like fishermen, hardened their abdominal muscles.

In the mature foetus the fluid brought from the placenta by the umbilical vein is partly conveyed at once to the vena cava ascendens. by means of the ductus venosus and partly flows through two trunks that unite with the portal vein, returning the blood from the intestines into the substance of the liver, thence to be carried back to the vena cava by the hepatic vein. Having thus been transmitted through the placenta and the liver, the blood that enters the vena cava is purely arterial in character; but, being mixed in the vessels with the venous blood returned from the trunk and lower extremities, it loses this character in some degree by the time that it reaches the heart. In the right auricle, which it then enters, it would also be mixed with the venous blood brought down from the head and upper extremities by the descending vena cava were it not that a provision exists to impede (if it does not entirely prevent) any further admixture. This consists in the arrangement of the Eustachian valve, which directs the arterial current (that flows upwards through the ascending vena cava) into the left side of the heart, through the foramen ovale - an opening in the septum between the auricles - whilst it directs the venous current (that is being returned by the superior vena cava) into the right ventricle. When the ventricles contract, the arterial blood contained in the left is propelled into the ascending aorta, and supplies the branches that proceed to the head and upper extremities before it undergoes any further admixture, whilst the venous blood contained in the right ventricle is forced into the pulmonary artery, and thence through the ductus arteriosus - branching off from the pulmonary artery before it passes to the two lungs - into the descending aorta, mingling with the arterial currents which that vessel previously conveyed, and thus supplying the trunk and lower extremities with a mixed fluid. A portion of this is conveyed by the umbilical arteries to the placenta, in which it undergoes the renovating influence of the maternal blood, and from which it is returned in a state of purity. In consequence of this arrangement the head and upper extremities are supplied with pure blood returning from the placenta, whilst the rest of the body receives blood which is partly venous. This is probably the explanation of the fact that the head and upper extremities are most developed, and from their weight occupy the inferior position in the uterus. At birth the course of the circulation undergoes changes. As soon as the lungs are distended by the first inspiration, a portion of the blood of the pulmonary artery is diverted into them and undergoes aeration; and, as this portion increases with the full activity of the lungs, the ductus arteriosus gradually shrinks, and its cavity finally becomes obliterated. At the same time the foramen ovale is closed by a valvular fold, and thus the direct communication between the two auricles is cut off. When these changes have been accomplished, the circulation, which was before carried on upon the plan of that of the higher reptiles, becomes that of the complete warm-blooded animal, all the blood which has been returned in a venous state to the right side of the heart being transmitted through the lungs before it can reach the left side or be propelled from its arterial trunks. After birth the umbilical arteries shrink and close up and become the lateral ligaments of the bladder, while their upper parts remain as the superior vesical arteries. The umbilical vein becomes the ligamentum teres. The ductus venosus also shrinks and finally is closed. The foramen ovale is also closed, and the ductus arteriosus shrivels and becomes the ligamentum arteriosum.

The blood vessels are supplied with constrictor and dilator nerve fibres which regulate the size of the vascular bed and the distribution of the blood to the various organs. The arteries may be compared to a high pressure main supplying a town. By m means of the vaso-motor nerves the arterioles (the house taps) can be opened or closed and the current switched on to or off any organ according to its functional needs. If all the arterioles be dilated at one and the same time, the aortic pressure falls, and the blood taking the pathways of least resistance, gravitates to the most dependent parts of the vascular system, just as if all the taps in a town were opened at once the pressure in the main would fail, and only the taps in the lower parts of the town would receive a supply. The discovery of the vaso-motor nerves is due to Claude Bernard (1851). He discovered that by section of the cervical sympathetic nerve he could make the ear of a rabbit flush, while by stimulation of this nerve he could make it blanch. Claude Bernard had the good fortune to make the further discovery that stimulation of certain nerves, such as the chorda tympani supplying the salivary gland, produces an active dilatation of the blood vessels. The vaso-constrictor fibres issue in the anterior spinal roots, from the second thoracic to the second lumbar root, and pass to the sympathetic chain of ganglia. The fibres are of small diameter, and probably arise from cells situated in the lateral horn of the grey matter of the spinal cord. They each have a cell station in one other ganglion and proceed as post-ganglionic fibres to the cervical sympathetic, to the mesenteric nerves and to the nerves of the limbs. Nicotine paralyses ganglion cells, and by applying this test to the various ganglia the cell stations of the vaso-constrictor fibres supplying each organ have been mapped out. The vaso-dilator fibres have not so restricted an origin, for they issue in the efferent roots in all parts of the neural axis. The two kinds of nerves, although antagonistic in action, end in the same terminal plexus which surrounds the vessels. The presence of vaso-dilator fibres in the common nerve trunks is masked, on excitation, by the overpowering action of the vaso-constrictor nerves. The latter are, however, more rapidly fatigued than the former, and by this and other means the presence of vaso-dilator fibres can be demonstrated in almost all parts of the body. The nervi-erigentes to the penis and the chorda tympani supplying the salivary glands are the most striking examples of vaso-dilator nerves. The vaso-dilator nerves for the limbs issue in the posterior spinal roots (Bayliss). The posterior roots contain the afferent nerves (touch, pain, &c.). Excitation of these fibres causes reflexly a rise of blood pressure directly, a vaso-dilatation of the part the nerves supply. Thus it is assured that the irritated or injured part receives immediately a greater supply of blood. The vaso-motor centre exerts a tonic influence over the calibre of the arterial and portal systems.

Much labour has been done since to determine the origin and exact distribution of the vaso-motor nerves to the various organs, and the reflex conditions under which they come normally into action, and, as the fruit, our knowledge of these inquiries has come to a condition of considerable exactness. This knowledge is of great practical importance to the physician, and it is worth noting that it has been obtained entirely by experiment on living but anaesthetized animals. No dissections of the dead animal could have informed us of the vaso-motor nerves. Vaso-motor effects can be studied by (I) inspection of the flushing or blanching of an organ; (2) measuring the venous outflow; (3) recording the pressure in the artery going to and the vein leaving the organ; (4) observations on the volume of an organ. To make these observations, the organ is enclosed in a suitable air-tight box or plethysmograph, an opening being contrived for the vessels of the organ to pass through so that the circulation may continue. The box is filled with air or water and is connected with a recording tambour (see fig. 18).

The chief effects of vaso-constriction are an increased resistance and lessened flow through the organ, diminished volume and tension of the organ, the venous blood issues from it darker in colour and the pressure rises in the artery and falls in the vein of the organ, and its temperature sinks. Lastly, if a large area be constricted the general arterial pressure rises.

The centre is situated in the spinal bulb beneath the middle of the floor of the fourth ventricle. The tone of the vascular system is not disturbed when the great brain and mid brain is destroyed as far as the region of the pons Varolii, but as soon as the spinal bulb is injured or destroyed the arterial pressure falls very greatly, and the animal passes into the condition of surgical shock if kept alive by artificial respiration. Painting the floor of the fourth ventricle with a local anaesthetic, e.g. cocaine, has the same lowering effect on the blood pressure. Division of the cervical spinal cord or of the splanchnic nerves lowers the blood pressure greatly. The one lesion cuts off the whole body, the other the abdominal organs from the tonic influence of the centre. The fall of pressure is due almost entirely to the pooling of the blood in the portal veins and vena cava inferior. On the other hand, electrical excitation of the lower end of the divided cord or splanchnic nerves raises the pressure by restoring the vascular tone. If an animal be kept alive after division of the spinal cord in the lower cervical region, as it may be, for the phrenics, the chief motor nerves of respiration, come off above this region, it is found that the vascular tone after a time becomes restored and the condition of shock passes away. By no second section of the spinal cord can the general condition of shock be reproduced, but a total obstruction of the cord once more causes a general loss of the vascular tone. From the experimental result, so obtained, it is argued that subsidiary vaso-motor centres exist in the spinal cord, and there is evidence to show that these centres may be excited reflexly. After the lumbar cord has been destroyed the tone of the vessels of the lower limbs is recovered in the course of a few days. In this case the recovery is attributed to the ganglionic and nervous structures which are intercalated between the spinal cord and the muscular walls of the blood vessels. There are thus three mechanisms of control, the bulbar centre influenced particularly by the visual, auditory and vestibular nerves, the spinal centres and the peripheral ganglionic structures.

The vaso-motor centre is reflexly excited by the afferent nerves, and its ever-varying tonic action is made up of the balance of the " pressor " and " depressor " influences which thus reach it, and from the quality of the blood which circulates through it. Pressor effects, i.e. those causing increased constriction and rise of arterial pressure, may be produced by stimulating the central end of almost any afferent nerve, and especially that of a cutaneous nerve. Depressor effects are always obtained by stimulating the depressor nerve, and may be obtained by stimulating the afferent nerves under special conditions. That these reflex vaso-motor effects frequently occur is shown by the blush of shame, the blanching of the face by fear, the blanching of the skin by exposure to cold and the flushing which is produced by heat. The rabbit's ear blanches if its feet are put into cold water. The vaso-motor mechanism is one of the most important of those mechanisms which control the body heat. Stimulation of the nasal mucous membrane causes flushing of the vessels of the head, constriction elsewhere and a rise of arterial pressure. Food in the mouth, or even the sight or smell of food, cause dilatation of the vessels of the salivary gland. The mucous membrane of the air passages flush and secrete more actively when a draught of cold air strikes the skin. Ice placed on the abdomen constricts not only the vessels in the skin but those in the kidney. Many other examples might be given of the control which the vaso-motor system exerts, but the above are sufficient to suggest the influence which the physician can bring to bear on the blood supply of the various organs.

Discussion has taken place as to whether depressor reflexes are brought about by lessening of the vaso-constrictor tone or by excitation of vaso-dilator nerves. Proof of an undoubtable character seems to have been produced that after division of the vaso-constrictor nerves dilatation of a limb can be brought about reflexly by stimulating the depressor nerve, and in this case the effect must be produced by active excitation of the vaso-dilator nerves.

Under certain unusual conditions, e.g. deficient supply of oxygen, the vaso-motor centre exhibits rhythmical variations in tonicity which make themselves visible as rhythmical rises and falls of arterial pressure of slow tempo. A waxing and waning of respiration (Cheyne-Stokes breathing) frequently accompanies these waves. Such are observed in sleep, especially in children and in hibernating animals.

Bibliography. - References to all the authoritative papers up to 1892 on the circulation of the blood will be found in Tigerstedt's Lehrbuch der Physiologie des Kreislaufs, and up to1905-1908in the articles on the circulation published in Nagel's Handbuch der Physiologie des Menschen, viz. "Allgemeine Physiologie des Herzens, Die Innervation der Kreislaufsorgane," by F. B. Hofmann, " Die Mechanik der Kreislaufsorgane," by O. Frank. An elementary introduction to the subject will be found in Leonard Hill's Manual of Physiology, and a more extensive treatment of it in the same author's article on the " Mechanism of the Circulation," and Gaskell's article on the " Heart " in Schafer's Text-Book of Physiology, or in one of the larger text-books of physiology, such as that of Howell, Stewart Halliburton or Starling. (L. E. H.) IV. Pathology Of The Vascular System On account of its intimate relations with every part of the body, the circulation is prone to disturbances arising from a great series of causes. Some of these produce effects which may be regarded as functional - mere changes in metabolism, whose disturbances react upon the rest of the body; others give rise to definite structural alterations. In considering the pathology of the circulation, it is useful to divide it into that of the heart, that of the blood vessels and that of the blood.

The heart is liable to changes in the pericardium, malforma tions, changes in the myocardium, changes in the endocardium, valvular lesions and functional dis orders.

(I) The pericardium may become the seat of morbid changes in various cardiac enlargements, it may become stretched or distended; but the most common and important of the changes is an inflammatory one, i.e. pericarditis. This may arise by way of the blood stream, as in rheumatism, scarlatina and other infective diseases, or by way of the lymph stream. The micro-organisms chiefly responsible for the production of pericarditis are the pneumococcus, the different varieties of streptococci and staphylococci, the bacillus tuberculosis, the bacillus coli, and sometimes the gonococcus. In the acute form of the disease the shining serous membrane becomes first dull and lustreless, the blood vessels engorged and an exudation of serum takes place; then fibrin is deposited both on the visceral and parietal layers. When the fluid is insufficient to keep the surfaces apart, the separation at each diastole gives rise to the well-known " friction rub." Sometimes the amount of exudation pent up in the pericardial sac is so great as to necessitate its being drawn off. The fluid may be serous or sero-fibrinous, or may be haemorrhagic, or have undergone a putrefactive change. An effusion of serous fluid into the pericardial sac causes considerable embarrassment to the course of the blood, by rendering the negative pressure, normally present in the sac, positive. The reason for the interference with the circulation brought about by this alteration of pressure is that the auricles are by compression rendered incapable of accommodating the blood-return from the veins. Analogous effects are produced by pressure upon the heart from without, whether by aneurysm or tumour, and pleural effusion or pneumothorax, affecting the viscera from without. In pericarditis it has further to be remembered that the effect of the process itself upon the muscle fibres lying beneath the membrane is to cause a softening of texture and weakening of function, whereby the driving power of the heart is diminished. In obliteration of the pericardium, again, the presence of the adhesions between these two layers leads to interference with the contraction of the myocardium, whereby its functions are interfered with. Acute ventricular dilatation may be associated with pericarditis particularly when the latter is of rheumatic origin and is the result of the myocardial softening referred to. Pericardial effusions usually undergo absorption, but various adhesions, and thickenings known as " white spots," may remain. Effusions other than inflammatory are found in the pericardium, i.e. hydropericardium, a dropsical accumulation, may be mistaken for an inflammatory one. It occurs in scarlatina, Bright's disease, as part of a general dropsy, or occasionally from some mechanical difficulty interfering with the local circulation. When the fluid is abundant, it may produce the effects noticed under the inflammatory effusion, and the pericardium may become soddened and its endothelium degenerated. Haemopericardium, or blood in the pericardium, may occur apart from the amount that may be mixed with inflammatory effusions. It is associated with foreign bodies penetrating from the oesophagus, rupture of an aneurysm, or occasionally associated with scurvy and purpura. Gas and air may sometimes distend the pericardium. It is also liable to new growths, which are usually secondary in character, and tuberculosis and hydatids are sometimes found.

We are ignorant of the causes which lead to imperfect development of the heart. Many of its malformations are of purely pathological interest, but others, such as deficiencies of the intraventricular septum, non-closure of the foramen ovale, patency of the ductus arteriosus, or malformations of the valves, produce a series of secondary effects resultant on the deficient aeration of the blood and sluggishness of the circulation and of venous congestion. The train of symptoms is similar to those mentioned below under acquired valvular lesions, but dropsy is very rare.

The coverings of the heart muscle cannot long be diseased without affecting the contractile substance itself. Any morbid changes in the lung tissues which impede the circulation through them, and more particularly emphysema, lead to change in the substance of the right ventricle, while morbid changes in the systemic arteries lead to changes in the left ventricle. In hypertrophy we have an increase of substance. Tangl found by direct measurement that the muscle cells are increased in diameter. The hypertrophy may be due to increased work thrown upon the muscle, as in athletics (idiopathic hypertrophy), or may. be compensatory, when the muscle is trying to overcome a circulatory defect, as in valvular stenosis or regurgitation. Hypertrophy, when within physiological limits, is to be considered as a means of adaptation. When occurring in pathological circumstances, it must be regarded as a method of compensation. Every structure and every function in a healthy body has greater or lesser reserve of energy. In healthy conditions the ordinary demands made upon various organs are far below their possible responses, and if these be excessive in extent or duration, the organs adapt themselves to the conditions imposed on them. In abnormal circumstances the process of hypertrophy is brought about by the power which the structures have of responding to the demands made upon them; and so long as the process is adequate, all disturbances may be averted. As an example of such readjustment may be cited the fact that in chronic renal cirrhosis, with increased thickness of the middle tunic of the arteries, there is hypertrophy of the left ventricle.

Dilatation of the heart is due to the inability of the heart muscle to expel the contents of its cavities. It may occur from temporary overstress or in the failing compensation of valvular disease, or may accompany pathological changes in the muscle such as myocarditis or one of the degenerations.

From the presence of toxic substances in the blood (whether introduced from without or arising within the body) the cells of the cardiac muscle fibres are apt to undergo what is termed cloudy swelling - the simplest form of degenerative process. The cells become larger and duller, with a granular appearance, and the nuclei are less distinct. As a result of interference with nutrition, whether by simple diminution or perverted processes, fatty degeneration ensues. It may be associated, but is not necessarily connected, with adipose accumulation and encroachment commonly termed infiltration. In true fatty degeneration the muscle cells have part of their protoplasm converted into adipose tissue. The fibres become granular, and the cells lose their definition, while the nuclei are obscure.

The myocardium undergoes both acute and chronic reaction changes. In the former there is enlargement of the nuclei, with proliferation but without karyokinesis. The muscle cells become swollen and lose their striation, while they are softer in texture and altered in outline. The intermuscular tissues are swollen, and may be invaded by leucocytes; this may end in abscess formation or in the production of newly formed fibrous tissue. Chronic processes affecting the myocardium give rise to a large amount of fibrosis, and the newly formed fibrous tissue separates and compresses the areas of muscle fibres, giving rise to what is commonly known as chronic interstitial myocarditis.

Restitution or recovery may occur to a varying extent in almost all of the disease-processes which have been considered, but it has to be kept in view that in certain of the degenerative affections there is little if any possibility of getting rid of the results of the process, which in the reactive changes terminating in the formation of much fibrous tissue, or its conversion into adipose or calcareous material, the same holds true. Many of the changes, which are no doubt in their essence conservative, lead to far-reaching consequences, by their interference with nutritive possibilities.

Diseased conditions of the myocardium are frequently associated with atheromatous degenerations of the coronary arteries, and angina pectoris is said to depend upon such state of malnutrition.

The causes which operate by means of the myocardium are almost invariably of a secondary character. The various degenerations already detailed, and the different forms of myocarditis, as well as simple debility of the muscle, are all examples of changes due to general or local disturbance. All processes which directly or indirectly interfere with the energy of the walls of the heart produce twofold effects, by diminishing the aspiratory or suctionpump action during diastole, and by lessening its expulsive or force-pump action during systole. The immediate result upon the heart itself of such disturbances is dilatation of that cavity immediately affected. This may occur under perfectly healthy conditions. In these, however, the dilatation is evanescent, while in the circumstances now under consideration it is permanent, and, although compensated, it leads to persistent dilatation. Upon the blood vessels the result, whether on account of diminished aspiratory or propulsive energy, is that the amount of blood in the arterial system is decreased, while it is increased in the venous. It is not a necessary consequence that because there is less blood in the arteries the arterial pressure will be diminished, or the venous pressure increased because the veins contain more than their normal amount of blood, seeing that the blood pressure depends upon many different factors. It is a fact, nevertheless, that in consequence of the alteration in the relative amount of blood in the arteries and veins there is a considerable disturbance of blood pressure. Gravitation may overcome the contractile and elastic factors, and several consequences arise from the resulting venous engorgement. From transudation, oedema of the dependent parts of the body and the serous membranes occurs. From the sluggish nature of the current, the blood absorbs too much carbonic acid and loses too much oxygen, hence cyanosis is the result. On account, also, of the slowness of the circulation, there is a longer period for radiation of heat, and the superficial parts of the body accordingly become cold.

The engorgement of internal organs leads to distinct changes in them. The solid viscera, such as the liver, the spleen, the kidney and the lung, become enlarged and hyperaemic, and if the disturbance be continued, cyanotic atrophy ensues. Change in structure, with loss of function, takes place from blocking of the vessels by blood-clot, whether due to coagulation on the spot, or by the conveyance thither of clots formed elsewhere; a cirrhotic termination also is not infrequent, although there is still some doubt whether in this latter condition other concomitant causes have not at the same time been operative. The brain, although suffering less from hyperaemia, is subject to disturbance of the circulation through it, while it is a common seat of embolic and thrombotic processes. The heart itself, lastly, suffers in consequence of the disturbed circulation through it, and by undergoing venous stasis, with weakening of its walls and increase of its fibrous tissue, it completes the final link in a vicious circle. Effusion into the serous sacs, such as the pleura, the pericardium and the peritoneum, leads to great disturbance of the viscera with which they are connected. The mucous membranes, both respiratory and digestive, become the seat of catarrhal changes in consequence of the backward pressure and impure blood.

(4) Changes in the Endocardium. - In endocarditis, or inflammation of the lining membrane of the heart, that portion of the membrane which covers the valves is invariably affected first. Two varieties of endocarditis are described, simple and infective or ulcerative, but it is difficult to separate them pathologically. Both result from poisoning of the membrane by micro-organisms and their toxins; the main difference seems to lie in the variety of micro-organism present. Simple endocarditis may be associated with a variety of diseases, acute rheumatism and scarlet fever being the most frequent. In many fatal cases of chorea associated with endocarditis the micrococcus rheumaticus has been found in the endocardium, while the streptococci present in tonsilitis have produced endocarditis in animals. The membrane covering the valves loses its smoothness, granulations or elevations forming on the free edges; then the endothelium proliferates and is destroyed and fibrin becomes deposited, producing what is termed a " vegetation." In the lower layers of this vegetation microorganisms can be demonstrated. Finally, portions of the vegetations may be broken off and carried as emboli in the blood stream, or two valves may become glued together, narrowing the opening and producing stenosis, or the deformed valves may be unable to close properly and regurgitation takes place. Thus the lesions of valvular disease are produced. In infective or ulcerative endocarditis, occurring in conjunction with such diseases as pyaemia, septicaemia, smallpox and pneumonia, pyogenic micrococci are carried into the blood stream, and purulent deposits take place around the valves. In this case, however, the emboli are septic, and when carried to distant tissues produce there ulceration and pus-formation. Numerous abscesses may occur in the wall of the heart muscle itself.

All the valves of the heart are not equally liable to disease; those most frequently affected are the aortic and mitral valves. We have seen how the lesions of the valves are brought about. A valvular lesion may act in two ways: it may impede the onward flow of the blood by narrowing the orifice, or the anal-closure of the valves may allow a reflux of blood. Either of these processes may occur at any of the valvular orifices of the heart. Obstruction is usually complicated by some regurgitation as well, though the converse does not hold good. An increase of the quantity of blood in the auricles, particularly the left, has a less marked effect on the heart itself than an increase in the contents of the ventricles, owing to the left auricle being in continuity with the pulmonary system; whereas if the amount of blood in the left ventricle be doubled the ventricle musts dilate in order to accommodate it. The reserve power of the heart is called upon to meet the dilatation, the muscular tissues becoming hypertrophied, and a more powerful systole is produced. As the left is the chief ventricle to undergo this change, the apex of the heart becomes displaced downwards. Similar changes take place in the right ventricle in pulmonary stenosis or tricuspid incompetency. Changes in the right ventricle other than primary valvular disease of the right side of the heart are frequently preceded by mitral incompetence, and are due to extra pressure being thrown upon the pulmonary semilunar valves by the pressure in the overfull pulmonary system. In mitral regurgitation the accumulation of blood in the right auricular cavity leads to its dilatation and an engorgement of the pulmonary vessels, pulmonary oedema and induration of the lung, which in turn affects the right heart. Should compensatory hypertrophy of the right ventricle fail to be established, we get the general venous congestion, dropsy and sequence before alluded to.

Cardiac rhythm may be modified in several ways; there may be variation in either the length or the strength of the beat, or the beats may not be asynchronous. In palpitation or tachycardia its frequency is increased. This increase depends upon the inhibition of the action of the cardio-inhibitory centre, impulses passing to it from the stomach (as in dyspepsia) or from other organs. Tachycardia is also produced by toxic action, as in diphtheria and Graves's disease. In bradycardia the frequency is diminished. It may be due to toxins or to degenerative changes. Intermittence may simulate bradycardia, though the actual rate of the beat is not lessened; but the weak beats fail to reach the periphery. Various irregularities may take place, dependent upon perverted nerve action. It is considered that the intrinsic nerve elements play a large part in these, and in some forms of disease the irregularity is of myocardial origin.

The blood vessels possess the properties of contractility and elasticity in different degrees. Their contractility is characterized by great tonicity, considerable rhythmic action and little or no rapidity of contraction. Their p y elasticity stores up energy in a potential condition, and this may be liberated in kinetic form as required. The vessels are supported in various degrees by the different tissues in which they are found. In the more solid viscera they are strongly supported, as in the liver and kidney, while in those which are less dense, as in the case of the brain and the lungs, they are not so well sustained.

In many conditions the contractility and elasticity of the blood vessels become diminished according as they may be involved in various pathological processes - purulent, tuberculous or syphilitic. Chronic toxic conditions lead to numerous degenerations, such as fatty degeneration or hyaline degeneration of muscle fibre, apparently as the effect of coagulative processes. The tissues assume a somewhat glassy appearance, with a distinct tendency towards segmentation. Calcareous infiltration is brought about by the deposition of lime salts in tissues which have previously undergone fatty or fibroid changes; it particularly affects the arteries in senile affections. In consequence of many toxic agencies as part of a senile change, and as the effect of long-continued stress, the blood vessels undergo a loss of their normal properties. This is compensated by the growth of an excessive amount of fibrous tissue, leading to various forms of arterial sclerosis, of which the best known are endarteritis obliterans, which affects the smaller arteries and is due to a toxic irritant and may occur at any age, and endarteritis deformans (atheroma), which affects the larger arteries during middle age, and is usually due to mechanical irritation. As the result of these fibrous changes there is interference with the blood current, since the vessels become unyielding yet frangible, instead of distensile and elastic, tubes. The sclerotic changes lead, moreover, to dilatation of blood vessels, as well as to the formation of definite aneurysms. They also pave the way for coagulation of blood within them, i.e. thrombosis, while in certain situations, more particularly in the brain and in the kidney, rupture is apt to take place. Upon the heart also these changes bring about far-reaching effects. Dilatation, accompanied by hypertrophy, is a certain result of generalized arterial degeneration, while changes in the coronary arteries lead to some of the definite results in the walls of the heart which have already been considered. Veins are subject also to mechanical and toxic effects. The pressure of abdominal tumours, the effects of the weight of a column of blood on a long vein, constipation or obstruction to the venous return may cause dilatations or varicosity. The dilatation thins the walls of the veins and the valves become incompetent; the dilated vessel then becomes twisted and the surrounding tissues thickened by the growth of fibrous tissue. The thinned walls may rupture, and, owing to the loss of the valves, extensive haemorrhages may take place. Thrombosis may follow the slowing of the blood current, and phleboliths are produced by the deposit of lime salts in it. Phlebitis is an acute inflammation of a vein. Apart from injury it usually follows invasion by a septic thrombus, as in the well-known phlegmasia alba dolens, when an infective clot from the uterine sinuses reaches the iliac veins. The pathology of the blood itself is treated under Blood.