CHAPTER IX

THE CIRCULATION

The blood circulates in a closed system of tubes, continuous from the heart back to the heart. The walls of these vessels separate the blood from the tissues. Nowhere, except in the spleen, does it come into contact with any cells other than the lining cells of the vessels in which it flows, and the exception made by the spleen is more apparent than real. The spleen (p. 79) is a kind of sponge invested with a firm capsule. Small arteries discharge their blood into its spaces; small veins collect it. But the organ is essentially a part of the vascular system. Its spaces take the place of the capillary vessels which connect arteries with veins in other situations.

The blood makes a double circuit. From the right heart it passes through the vessels of the lungs. Returning to the left heart, it is driven through the body. Although the heart consists of two separate pumps, it makes but a single organ. Its division into right auricle and ventricle and left auricle and ventricle is but slightly indicated on the surface. In most invertebrate animals the two pumps are distinct. In some the lung-heart and the body-heart are on opposite aspects of the body. But one must not, when thinking of the morphology of the vertebrate heart, picture it as formed by the juxtaposition of two, originally separate, pumps. Truly, in its very earliest stage of growth, it is represented by two tubes which lie, in the embryo, far apart. But these, before we can speak of the existence of a heart, fuse into a single tube, with four contractile bulbs in series. As the heart develops, the dilatation at its hinder or venous end and the dilatation at its anterior or arterial end disappear. A partition is formed which divides the two middle bulbs into right and left auricle and right and left ventricle respectively. Immediately after birth the lungs are, for the first time, distended with air. Up to that particular minute they have had no functional use. Nothing would be gained by compelling all the blood of the body to traverse the vessels of the embryo’s lungs. Until birth, therefore, the inter-auricular septum is perforate. The blood takes a short-cut, through the foramen ovale, from right auricle to left. But by birth-time a curtain has grown down on the left side of the foramen. When the lungs are expanded by the forcible enlargement of the chest-cavity which contains them, their bloodvessels are distended by the same extensile force. Blood is sucked into them from the right side of the heart. A difference in pressure on the two sides is established. A condition is set up which is favourable to what may almost be termed the adherence of the flap which hangs down on the left side of the foramen ovale. The growth of its margin very rapidly obliterates the hole. Occasionally the closure of the foramen is not complete. A child grows up with a perforate inter-auricular septum. If the aperture be very small it causes little inconvenience. Shortness of breath and blueness of lips indicate its existence if it be large enough to lead to deficient aeration of the blood.

The two sides of the heart being quite separate, it is clear that all blood ejected by the right ventricle into the lungs must return to the left auricle, to be driven by it round the body. Yet it does not follow that the heart must at each stroke drive exactly the same quantity of blood into the pulmonary artery and into the aorta. On the average, each of the two sides ejects the same amount—about 3 ounces. Nor does it follow that as much blood is lodged in the lungs as in the whole of the rest of the body. The amount varies, but on the average the lungs contain not more than one-fifteenth of the whole blood. The heart may be likened to two cogwheels; the blood-stream to a chain, folded into a figure eight, against which the cogwheels work. Synchronously each cogwheel lifts a link, the right one of the smaller, the left one of the larger loop. Any given link returns to its starting-place in half a minute. Such an illustration gives an idea of the arrangement of the circulation as a whole, although the motion of a fluid is widely different from the motion of a chain.

If, the jugular vein of the neck being cut, a colouring matter—such, for example, as ferrocyanide of sodium or methylene-blue—is injected into its central end towards the heart, it begins to appear in the blood which issues from its distal end in half a minute. In this short space of time it has passed through the right heart, through the lungs, through the left heart, and through the vessels, arteries, capillaries, and veins, of the head. Half a minute is therefore the “circulation-time.” Not that all the blood-corpuscles of the body make the circuit as rapidly as this. The time taken depends upon the particular route they follow in the greater or systemic circulation. Some traverse the vessels which supply the walls of the heart itself—a short journey; others go down to the foot and up again. But the average circulation time does not exceed a minute or a minute and a half. It is particularly in the veins of the liver and other abdominal viscera that blood tends to linger. Usually half the blood of the body, or even more, is lodged in these capacious reservoirs. It is thanks to their capacity for storing blood that a supply is provided adequate to meet any special demand. If a man runs a few hundred yards, two-thirds of the whole blood of the body is transferred to his limbs. It is quickly withdrawn from the abdominal vessels when it is wanted elsewhere; but, failing an efficient cause for removing it, its accumulation induces lethargy. Even tight-lacing has been defended by an eminent physiologist on the ground that it prevents accumulation of blood in the abdomen. But tight-lacing diminishes the capacity of the chest, hampers the action of the heart, checks the circulation, distorts the abdominal viscera, and generally deforms and jams the domestic machinery, even though the professor be right in his view that visceral compression may favour alertness of mind. More by token, it interferes with this admirable adjustment by which blood is distributed to the various parts of the body in proportion to their needs. The brain is the only organ which has any difficulty in securing all it wants, and its claim to so much blood might be disputed. Nature has not provided for long-continued passivity of the body associated with strained activity of mind. When the stimulus to mental activity is not unreasonable, most “nervous” people are apt to discover that their brains are better supplied with blood than is good for their health.

The effect upon the distribution of blood throughout the body of squeezing the viscera is experienced after taking a deep breath and contracting the muscles of the abdomen. The contents of the abdomen are compressed between the depressed diaphragm and its muscular wall.

Certain other forces co-operate with the beat of the heart in causing blood to circulate. Two such factors are especially deserving of attention. In the first place, the movement of blood in veins is largely dependent upon external pressure. The veins are valved at frequent intervals, the folds in their interior being of course directed towards the heart. Any external force which empties a section of a vein drives blood forward. A “good stretch” brings the lateral pressure of contracting muscles to bear upon the walls of the veins which lie between them, or beneath. More blood is delivered to the heart. The exercise of standing erect in the attitude of attention, and then slowly raising the arms until the thumbs meet above the head, and slowly lowering them again, has a remarkable effect in quickening the circulation—increasing the blood-supply of the brain. Changes of posture, by relieving pressure on subcutaneous veins, removes an impediment to the flow of blood.

The second of the factors to which we have referred as adjuvant to the heart’s action is the negative pressure of inspiration. In explaining the effect of this force upon the circulation, the relation of the lungs to the thorax must be taken into account. The box in which the lungs are enclosed is too big for them; nevertheless, being extensible and elastic, they always fill it. They follow its movements when in inspiration the muscles between the ribs enlarge it, and when in expiration it diminishes again. No air or fluid, save the moisture which lubricates the surface of the pleura, reducing friction, occupies the (potential) space between the lungs and the chest. But the moment the chest is punctured the lungs collapse. Air is sucked into the pleural cavity. The lungs fill the chest only so long as there is neither air nor fluid between it and them. Lung-tissue is extremely delicate. Each air-cell is a cup of thin membrane holding together a basket-work of capillary vessels. So long as the chest-wall is stationary the negative pressure in the pleural cavity has no effect upon these slender tubes. But when the chest expands, the capillaries are between two minus pressures, the pull of the chest-wall and the resistance offered to the entrance of air into the lungs by the passages through which it has to pass. The calibre of the lung-capillaries is increased, just as it would be increased were they hanging in an air-pump while the piston was drawn out. More blood passes to the left heart through the wider capillaries. Ejected into the aorta, it raises the pressure in the arterial system. A record of the pressure in an artery shows a rhythmic rise for each heart-beat. It shows also a rise with inspiration and a fall with expiration. These larger undulations correspond with the movements of the chest, although they are necessarily somewhat late on respiration, for the first effect of the dilatation of the capillaries is to cause them to hold more blood and to deliver less. The first effect of expiration, on the other hand, is to urge on the blood which the dilated vessels contain. In any case a single beat is needed to throw into the aorta the blood which has been received by the right auricle.

The expansion of the chest influences the flow of blood in yet another way. The heart and the great vessels which join and leave it are themselves enclosed within the chest, subject to the negative pressure produced within that cavity by the elasticity of the lungs. The lungs pull upon the pericardium, the membranous covering of the heart. When this pull is increased owing to the forcible expansion of the chest, blood is sucked into the great veins, just as air is sucked into the windpipe. The thick-walled aorta, containing blood at high pressure, does not feel the effect of slight variations in the pressure round it. The soft-walled veins are expanded during inspiration to a not inconsiderable degree. What relief a deep yawn gives by hastening a languid circulation! Leaning over an account-book late in the afternoon, every condition is unfavourable to the flow of blood. It accumulates in the legs and in the abdomen. The head is thrown back and the mouth opened wide, while the chest expands in a long deep inspiration. Down on the liver, stomach, and intestines presses the flattened diaphragm, squeezing their blood towards the heart. The negative pressure within the chest sucks this up, and draws down the blood contained in the great veins of the neck. The capillaries of the lungs are widened, allowing blood to pass more quickly from the right side to the left side of the heart. The heart responds to the call upon it, throwing all that it receives into the aorta. Only a great effort of the will had kept the pale brain at work; in the attic it suffers more than organs on the lower storeys from insufficient pressure. For a short time after the yawn it finds itself nourished with an adequate supply of blood.

The negative pressure in the thorax is considerable at all times. If a manometer—a =U=-shaped tube with mercury in its loop—be connected with a cannula passed through the wall of the chest, the difference of level of the mercury in the two limbs of the =U= is a measure of the force with which the lungs are endeavouring to shrink away from the chest-wall. Even at the end of expiration the mercury in the limb next the chest stands about 6 millimetres higher than the mercury in the outer limb. During a deep inspiration the pressure in the chest falls 30 millimetres below the atmospheric pressure. Hence a problem is presented of which no completely satisfactory solution has yet been given. How comes it that lymph is not sucked into the pleural cavity? In health there is no more pleural fluid than just suffices to keep the membrane moist. The endothelial cells which cover the surface of the pleura resist further exudation. Valves in the lymphatic vessels prevent backward flow. Yet in disease, when the pleura is inflamed, lymph pours out quickly, often to be reabsorbed with equal rapidity when the pleurisy subsides. This flow uphill, from a lower to a higher pressure, can be explained only as a phenomenon due to the “secretory” capacity of endothelium. As an answer to the hydrostatic problem this is hardly satisfactory.

The circulation of the blood is the result of the difference between the pressure in the vessels through which it leaves the heart, and that in the vessels through which it is returned. The pressure in the aorta amounts to about 200 millimetres of mercury. In the venæ cavæ it is nil, or, owing to the aspiration of the thorax, less than nil.

=The Heart.=—Inspection of the liver, the spleen, or the kidney helps but little to the comprehension of the mechanism of these organs. It is quite otherwise in the case of the heart. Its mechanics being comparatively simple, physiology is concerned with measurements, with the conditions under which it can and cannot work, and with the action upon it of the nervous system and of drugs. The heart of any mammal will suffice for anatomical study. A sheep’s heart is about the same size as that of a man, and exactly similar, save in minute particulars, which do not appreciably affect its mode of working.

The heart is a hollow muscle, composed of minute contractile cells. Each cell is a cylinder, about twice as long as it is broad, with an oval nucleus in its centre. There is no impropriety in speaking of the heart as a single muscle. Muscles which we can move at will, “voluntary muscles,” consist of fibres, each from 1 inch to 2 inches long, and of about the thickness of a piece of thread (Fig. 16). Every fibre is surrounded by a membranous sheath, its sarcolemma, which completely isolates it from the others. Each has its separate nerve-supply. A voluntary muscle-fibre is a cell-complex. The single embryonic cell which grew into the fibre underwent nuclear division until hundreds of nuclei were formed, but its cell-substance was not divided into territories appertaining to the several nuclei. In heart-muscle, on the other hand, nuclear division has been followed by cell division; but minute protoplasmic bridges are left between the cells. The whole of the heart-substance is thus in structural continuity. The cells are not invested with sarcolemma. As the result of this arrangement, an impulse started in one part of the heart spreads over the whole, with certain limitations as to the directions in which it is able to travel, whereas in voluntary muscle a separate impulse must be delivered to each fibre. The wave of contraction commences in the great veins, the venæ cavæ and pulmonary veins, near their junction with the heart, spreads from cell to cell throughout the auricles, and onwards down the ventricles to the apex of the heart. The substance of the heart has not, however, a homogeneous appearance. Its cells are collected into fascicles, which lie in various planes and cross the axis of the heart at various angles. In a boiled sheep’s heart it is easy to separate one fascicle from another, and to distinguish the sheets into which the fascicles are collected. The four valves of the heart lie in almost the same plane. They are supported by a fibrous plate divided into four rings (Fig. 11). Most of the fascicles are attached to this plate, though some which encircle the auricles are independent of it. With one or with both ends attached to the plate, fascicles loop over the auricles. They run down the ventricles with a twist from right to left. Those on the surface turn in at the apex of the heart, and run up the inner surface of the ventricles. Some of them go to form the free columns which are found on the inner surface of the ventricles, pointing towards the valves—musculi papillares. The fibrous plate which supports the valves cuts off almost all of the muscle which makes the walls of the auricles from that which constitutes the ventricular walls; but a thin sheet is continued from the inner surface of the auricles down the interventricular septum. To a considerable extent the walls of the two auricles and of the two ventricles are respectively continuous, insuring synchronous contraction.

The arrangement of the fascicles accounts for the changes in form which the heart undergoes when it contracts. Systole commences in the cardiac ends of the venæ cavæ and pulmonary veins. They empty the last of their blood into the auricles, and close to prevent regurgitation, their mouths not being valved. Then the auricles quickly shrink in all dimensions, and as soon as their contraction is at its height the ventricles contract, while the auricles relax. The ventricular wave runs from base to apex too rapidly to be followed with the eye, and ends, owing to the involution of the fascicles, in the musculi papillares. As soon as ventricular systole has commenced, the auricles relax. After emptying their contents into the aorta and pulmonary artery, the ventricles relax, their contraction giving way first at the apex, and being longest held at the base. Then follows a pause (diastole), during which both auricles and ventricles are flaccid. If the pericardium is open, the heart is seen to become round instead of oval in transverse outline during systole. It shortens. Its apex twists a little to the right, and projects forward. But if it is within its pericardium the shortening is not accompanied with any displacement of the apex. Instead of the apex mounting, the base descends. The front of the right ventricle, at some little distance from the apex, presses the chest-wall forwards in the fifth intercostal space, about an inch to the inner side of a line falling vertically through the nipple. This pressing forwards is felt as the “impulse of the heart.”

The contraction of the heart is not a see-saw of auricles and ventricles. During diastole blood is falling from the veins through the auricles into the ventricles. In a sense, the auricles are not necessary parts of the double pump. They collect blood while the ventricle is contracting, thus preventing it from heading up in the veins. They save time. Their contraction completes the filling of the ventricle, so that the instant the ventricular contraction begins blood enters the aorta and pulmonary artery.

=The Valves.=—If ever expressions of admiration were appropriate in a treatise on the animal body, such preface might be permitted to a description of the cardiac valves. Which means no more than this: Men make pumps. Therefore they are in a position to appreciate the mechanism of the heart. We cannot admire what we do not understand. If we made secreting organs or self-contracting springs, glands and muscles would evoke our commendation. We should recognize that Nature’s apparatus is even better adapted to its work than any that men can make. This is the admission which is forced from us when we study the heart.

The apertures connecting auricles and ventricles are extremely wide, allowing the contents of the former to be emptied into the latter almost instantaneously. If we attempted to make a pump fulfilling this condition, we should find that it failed in several respects. In the first place, the rush of fluid from the one chamber into the other would press the flaps of the valves back against the wall of the second chamber. They would cling to the wall, and would not float up quickly into place when the second chamber was squeezed. Let us call the two chambers A and V for brevity’s sake. When V contracted, some of the fluid would be thrown back into A, because, the resistance in that direction being lower than the resistance offered by the column of fluid above the pump (the resistance in the aorta is very high), the contents of V would rush past the margins of the A-V valve. This would happen even though its flaps were not pressed back against the wall. Further, at the height of contraction the membranous valve would bulge backwards into A, making a cup towards V which V could not empty. In the heart these difficulties have been overcome.

The tricuspid valve, which separates the right auricle from the right ventricle, has three flaps. The mitral valve, on the left side of the heart, has but two. The flaps are composed of tough membrane, but are comparatively thin. The following direction for deciding at an autopsy whether or not they were healthy at the time of death was given many years ago by a surgeon of repute: “You ought to be able to see the dirt under your thumbnail when you place it beneath one of the flaps.” Surgery has improved in cleanliness as well as in other ways; indeed, the possibility of advance has been due to the recognition of the need for transcendental cleanliness. But this is a digression. The margins of the flaps are crenulated. Threads—chordæ tendineæ—are attached to them like the stay-ropes of a tent. At their other end these tendons are attached to the musculi papillares already mentioned. The bunch of tendons from each papillary muscle spreads, to be inserted into the contiguous margins of two flaps. We have mentioned some of the difficulties which have been overcome in the construction of the pump. (1) The flaps do not flatten back against the wall of the ventricle during systole of the auricle. It must be remembered that during diastole of both chambers blood is flowing through the auricle into the ventricle. The latter being partly filled before systole of the auricle commences, the flaps are floated up. This is greatly favoured by the form of the inner wall of the ventricle. It is not flat, but raised in pillars—columnæ carneæ. The spaces between these pillars cause backwash currents, which lift the flaps and help to bring them into apposition as soon as systole of the ventricle commences. (2) No blood which has entered the ventricle is thrown back into the auricle. The valve “balloons” over the blood in the ventricle before the contraction of the auricle has ceased. The thin margins of its flaps come together with great rapidity. The tendinous cords holding their edges on the ventricular side, they meet, not edge to edge, but folded flap to folded flap. (3) The valve does not bulge into the auricle. On the contrary, at the height of systole it is pulled into the ventricle by the contracting musculi papillares. As the ring to which the valve is attached is diminished in size, by the contraction of the base of the heart, which continues, it will be remembered, until after the apex has begun to relax, the edges of the flaps are folded farther and still farther over by the pull of the musculi papillares, and the blood is squeezed out from between the wall of the ventricle and the indrawn valve.

The “semilunar valves,” which close the apertures into the aorta and pulmonary artery, have each three flaps. The aortic semilunar valve, which has the higher pressure to bear, shows its characteristic features in a rather more marked degree than the other. Each of its three flaps is a half-cup. At the centre of the margin of the half-cup is a small fibrous nodule. The edge of the cup on either side of this is very thin. Fine elastic fibres radiate from the nodule to all parts of the flap. The wall of the aorta shows three bays, or “sinuses,” one behind each flap. Hence, when the valve is forced by the rise of pressure in the ventricle, the flap is not flattened back against the wall of the aorta. There is always a certain amount of backwash in the pocket behind it. The instant the pressure in the ventricle begins to fall, the three flaps come together with a click, so smart as to be plainly audible over most of the front of the chest. The click is the “second sound” of the heart. The auriculo-ventricular valves also make a sound when they close; but this “first sound of the heart” has a different character. It is prolonged, soft, low-pitched. It is customary to represent the sounds by the syllables “lūbb dŭp—lūbb dŭp,” the pause during diastole being of about the same length as the sounds when the heart is beating with its normal rhythm. The duration of systole is little affected by variations in the rate of beat. It is diastole that is shortened or prolonged. The second sound is due entirely to the closure of the semilunar valves. It is heard most clearly when the stethoscope is placed over the region where the aorta comes nearest to the wall of the chest—at the second rib cartilage on the right side of the breast-bone. The first sound is loudest near the apex of the heart. It is generally agreed that it is not wholly due to the closure of the auriculo-ventricular valves, but possesses a second constituent. Some persons assert that they can with the ear distinguish the clearer valvular sound at the commencement from the general rumble which overtakes it. The main part of the sound, if it have two constituents, or the whole sound, if there be no distinguishable valvular constituent—observers differ—is just the noise of a distant cab (_bruit du cab_) or the waves on a far-off beach; it is the sound which the ear picks up from any irregular mixture of tones which it cannot analyse. It is the resonance-tone of the ear. That the membranous valves play the leading part in producing the first sound cannot be doubted, whether by their first closure or by their subsequent vibration. We should be inclined to attribute to them the whole performance, were it not that the first sound, or at any rate a sound, is heard during the beating of a bloodless heart. If an animal be killed and the heart removed from its thorax with the utmost despatch, it will beat for about a minute while lying in the palm of one’s hand. When a stethoscope is applied to the ventricle, a “first sound” is heard. This was described as a muscular sound, owing to a misconception. It is similar to the sound which is heard when a stethoscope rests upon a contracting biceps. Until recently the voluntary contraction of a muscle was believed to be vibratory—a tetanus. The sound corresponds to a rate of about thirty-six vibrations to the second. There being reasons for thinking that muscle contracting naturally does not vibrate as fast as this, the sound was interpreted as the first overtone of the muscle-note. Muscle was said to vibrate eighteen times a second. The similarity of the first sound of the heart and the ordinary muscle-sound led physiologists to infer that the contraction of the heart also was a tetanus. But this was a mistake. Neither voluntary muscular action nor the contraction of the heart is an interrupted contraction in this sense. In the case of the musculature of the heart especially, contraction is a steady shrinking, followed by a steady relaxation. The sound produced by the bloodless heart is due to the various displacements which occur when it contracts. Its interior is very irregular, with its columns, papillary muscles, tendinous cords, valves. The displacement of these various structures is responsible for the noise.

The sounds of the heart afford to the physician a means of ascertaining with the utmost nicety the condition of the valves. If the sounds are altered from the normal in the least degree, the valves are not healthy. Alteration of the structure of a valve is in ordinary parlance heart-disease. It is usually indicated by an addition to the normal sound. Such addition is termed a “murmur”; in French, _un bruit de souffle_. Either term is somewhat misleading to the tyro. We remember a fellow-student to whom our chief had in vain expounded the nature of a murmur. “Surely, Mr. S., you can hear the murmur in this case.” We others could hear it as we stood around the bed. After listening for a minute, S. replied: “I think I could hear it, sir, if the heart wasn’t making such a thundering noise.” The thundering noise was the murmur. It is the business of the physician to recognize that there is a departure from the normal, to analyse its character, to determine the time at which it is heard in relation to the cardiac cycle, and to locate the place on the chest where it is heard most loudly. He is then in a position to state which of the valves is affected and what is the nature of its lesion. Is it a lesion obstructing an orifice, or is it causing regurgitation of blood? Or is one of the valves, as is commonly the case in heart-disease, imperfect in both respects?

A murmur, in the strictest sense, is a sound added to a heart-sound. It is due in all cases to vibration of a fluid column (“fluid vein” is the term in physics). When fluid passing under pressure along a tube of a certain calibre enters a tube of smaller calibre, no vibration occurs. When it passes from a tube of smaller calibre into a larger tube or space, it is thrown into vibration. Under normal conditions no vibration occurs in the heart. The auriculo-ventricular orifices are so large that auricle and ventricle form a single cavity when the valve is open. The ventricles drive the blood into tubes of smaller dimensions than themselves. These are not the conditions which set up vibration in a fluid column. But if one of the orifices is constricted, owing to thickening or partial adhesion of its valve, the fluid column vibrates on entering the space beyond it. The sound is propagated forwards, beyond the constriction, not behind it, and transmitted to the wall of the ventricle, aorta, or pulmonary artery, as the case may be. When either of the auriculo-ventricular orifices is constricted, the vibration of the fluid column can be felt as well as heard. The finger placed against the chest-wall at the spot where the impulse of the heart occurs is sensible of a thrill. The vibration may occur whilst blood is flowing _through_ an auricle into a ventricle, before the auricle contracts. In time, it is presystolic. The murmur produced by regurgitation into an auricle is synchronous with systole. The murmur due to regurgitation into a ventricle past an incompetent semilunar valve is postsystolic.

We have said that the heart is so formed that no vibrating fluid vein is produced when it is functioning normally. Murmurs are due to alterations in the valves which are visible after death. This statement needs modification. Not infrequently functional murmurs are heard, which disappear again after a time—in a few weeks, or even days, perhaps. The explanation of murmurs of this class is very difficult. They are heard most frequently in anæmic persons, and appear in these cases to be due to the heart having shrunk, owing to the blood in circulation being deficient in quantity, until the cavities of the ventricles have a smaller diameter than that of the great arteries into which they expel their contents.

Such is the explanation of the physical cause of murmurs given by Chauveau and Marey, the physiologists who have paid most attention to this subject. But it must be remembered that the valves which, when diseased, are the sources of the murmurs are membranous structures. It may be that fluid veins would be produced by them if they were rigid ledges which jutted into the blood-stream; but, being membranous, they are capable of vibration. Certain physicists are of opinion that a murmur is caused, not by the vibration of a fluid vein, as such, but by the vibration of the membranous structure which impedes the passage of the fluid. The physics of the problem is of little consequence to the physician. The murmur is produced at the spot where a diseased valve is situated, and is propagated forwards. It enables him to ascertain with accuracy what is amiss with the heart.

=Bloodvessels.=—The greater circulation occurs through a closed system of vessels which unite the left ventricle with the right auricle. The aorta gives off lateral branches. Its branches branch. Subdivision continues until the vessels are just wide enough to allow blood-corpuscles to pass in single file, or but little wider. When a bough of a tree divides, the united cross-sections of its twigs, their soft bark being stripped off, may be a little larger than the cross-section of the bough; but the disparity is usually small. The united cross-sections of the smaller arteries is considerably greater than that of the trunks which give origin to them. By the time the capillaries are reached, their total bed—their united cross-section—is about 640 times as great as that of the aorta. This estimate is based upon the diminution in the rate at which blood flows through the vessels. The velocity with which a stream flows through a channel varies as the cross-section of the channel. In a capillary vessel the blood flows at the rate of from 0·5 millimetre to 1 millimetre per second. In the aorta the velocity is about 320 millimetres per second. In the re-formation of the venous system a converse process of reduction occurs, but not with anything like the same rapidity. The united calibre of the two venæ cavæ, in which the reduction is complete, is about twice that of the aorta. From this it follows that the veins hold much more blood than the arteries; and since veins are more easily distended, the amount that they can hold varies within wide limits. They constitute to some extent a reservoir for blood.

The capillary vessels are the tubes of the circulatory system in which blood comes into use. On the average they are about 0·5 millimetre long. Through them the blood flows slowly. Through their walls alone is there any exchange worth mentioning between the blood within the vascular system and the lymph by which it is surrounded. Interest therefore centres in these vessels. Their walls are formed of endothelial tiles. In the centre of each thin transparent tile is a boss, where its lens-shaped nucleus is situate. The outline of the tile is sinuous. Its margin dovetails with the margins of those adjacent to it. Oxygen and carbonic acid, nutrient substances and waste products, pass rapidly through the endothelial cells. Leucocytes have the power of pushing the cells aside, in order that they may make their way out of the blood into the lymph which fills the tissue-spaces. With the exception of the lens and cornea of the eye, cartilage, and the various epidermal structures, all tissues are traversed by capillary vessels. It is not difficult to calculate the number of such vessels in the body exclusive of the liver and the lungs. The diameter of the aorta is 28 millimetres, that of a capillary about 0·008 millimetre. The cross-section of all the capillaries added together is 640 times that of the aorta, as already stated.

Many schemata have been devised to illustrate the vascular system; but all are misleading, inasmuch as they fail to give any idea of the extent to which the subdivision of its vessels is carried. If the water-pipes supplying a town branched until the original conduit was represented by five to six thousand million little pipes, the friction which the pumping-station would have to overcome would be very great. But little force would remain in the water when it reached the smallest pipe. Still greater is the resistance to the flow of blood, which is slightly viscous, and contains solid corpuscles, which increase friction. Two thousand miles of capillary tubing in the body of a man, without reckoning the vessels of his liver and lungs!

Water is supplied to houses in rigid tubes. Arteries are elastic, and their elasticity is self-regulating. The cause of this will be apparent if a section of an artery is examined. It contains much elastic tissue. It also contains plain muscle-fibres. The smaller the artery, the greater is the amount of muscle relatively to the other constituents of its wall. The wall of a vein contains very little muscle, and not much elastic tissue. The muscle of all arterial walls is in a chronic state of tone. To some extent the degree of tone is varied automatically. Pressure within an artery acts as a stimulus to the muscle-fibres of its wall. Any increase leads the fibres to contract more strongly. Any diminution induces them to relax. The arteries resist distension; they do not narrow to any great extent when pressure falls. But more important than this automatic mechanism for maintaining a uniform pressure in the capillaries in general are the changes of pressure in particular localities, brought about by the mediation of vaso-constrictor and vaso-dilator nerves. In almost all organs and parts of the body the automatic tone of arteries is enhanced by impulses which flow continuously down vaso-constrictor nerves. These impulses start from, or, to speak more accurately, pass through, the vaso-motor centre in the medulla oblongata. From every part of the body impulses ascend to this centre, urging it to keep up the blood-pressure by universal constriction. Yet no separate organ would be interested in sending such a message if it were not open to it to ask at the same time that the constriction of its own vessels might be relaxed. Hence it may be said that every individual in the community is crying out for universal economy, with more generous treatment of himself. The response made by the State to the latter part of his demand is in proportion to the vehemence with which it is presented.

If the spinal cord of an animal be cut across near the medulla oblongata, respiration being maintained by pumping air into and out of the lungs, the heart continues to beat with undiminished force, but the pressure in the large arteries falls to one-third of its normal height. Constricting impulses no longer pass down the spinal cord from the vaso-motor centre. This experiment also illustrates the truth of the statement that models of the vascular system—arrangements of pumps and indiarubber tubes—are more likely to mislead than to inform. In an artificial schema the relaxation of the constriction of the small tubes on the proximal side of the capillary vessels would reduce friction. Fluid would reach the capillaries in larger quantity, and pass through them more quickly. The pressure in the tubes which represented veins would consequently approach more nearly to that on the arterial side. But when the spinal cord is divided the pressure falls in the veins, as well as in the arteries. This is due to another factor, and one of very great importance in the regulation of the circulation. The blood from the digestive organs is collected by the “portal system” of veins. These do not join the inferior vena cava; they go to the liver, where they again break up into capillaries. It is not until after this second distribution through minute vessels that the blood is re-collected by the hepatic veins and forwarded to the heart. As in the case of the arteries, the portal system of vessels is controlled by the nervous system. When the spinal cord is divided they also dilate. The whole vascular system becoming more capacious, blood-pressure falls in veins as well as in arteries.

When the digestive organs are active, other parts of the body are kept short of blood. It chanced to the writer, in his student days, to spend the early summer in Paris, with a big healthy Yorkshireman as companion. We dined together each night at one of the restaurants of the Palais Royal _à prix fixe_. After dinner, with British regularity, my friend called for the _Times_. Then followed a short period of placid reading, interrupted by the remark: “How cold it is!” Half an hour later, giving himself a shake: “Suppose we go and dine somewhere else?” His well-ordered digestive organs had made short work of the two-franc dinner. They had been ably supported by the vaso-motor system of nerves which provided them with the bulk of the blood, while limbs and skin ran short.

Vaso-constrictor nerves leave the spinal cord by the roots (called “rami communicantes”) of sympathetic ganglia. Beyond the ganglia they apply themselves to the large arteries whose course they follow. The constrictor nerves for the face and neck leave the spinal cord within the chest by the roots of the first four thoracic nerves. They do not at once apply themselves to the great artery of the head. Until the upper part of the neck is reached, they traverse the ganglionated sympathetic cord, which lies behind the carotid artery and internal jugular vein. If in a rabbit this cord be cut, the vessels of its ear dilate, as evidenced by the rosy blush which is observed when a light is held behind it. If the upper part of the sympathetic cord be stimulated, the ear grows pale. The redness of the ear remains for many days after section of the nerve; but gradually the engorgement diminishes, and the vessels acquire the power of automatically regulating the flow.

The classical experiment with the rabbit’s ear suffices to show the relation of bloodvessels and nerves which holds good for all areas of the skin. The condition of the skin is the chief factor in regulating the temperature of the body. In a cold atmosphere its vessels are severely constricted to limit loss of heat. When one passes into a warm room the constriction is relaxed. The skin is flushed; heat is thrown off by radiation. The sweat-glands secrete water, which is evaporated by the heat of the skin. Constriction and remission of constriction are the processes which diminish or increase loss of heat.

This mechanism is different in the case of glands and some other structures which, when active, require an abundant supply of blood. Such organs are provided with vaso-dilator in addition to vaso-constrictor nerves. The most conspicuous example of this is to be seen in the case of the submaxillary gland. The nerve to this gland runs for some distance as an isolated thread—the chorda tympani. Stimulation of the chorda tympani has the double effect of dilating the arteries of the gland and of causing it to secrete. But the administration of atropin prevents secretion. Vaso-dilation is then the only visible effect. Stimulation may increase sixfold the outflow of blood from the veins of the gland. It rushes through with such rapidity that it retains its bright arterial hue. The gland also receives a twig from the sympathetic cord in the neck, which, as already stated, controls the vessels of the face. By stimulating the one nerve or the other the physiologist can at will increase or diminish the amount of blood flowing through the submaxillary gland. Stimulating any sensory nerve causes in a reflex manner an increased outflow of constrictor impulses from the centre in the medulla oblongata to all parts of the body, with the exception of the part to which the sensory nerve appertains. Its own constituency receives an increased supply of blood. It is not difficult to appreciate the importance of this double action. A part is injured. The restrictions placed upon its supply of blood are suspended. Lest its increased consumption should lead to a general fall in pressure, all other parts have their supply curtailed. The effect is even more pronounced than this. The whole blood-pressure is raised above its ordinary level. The flow of blood to the injured part is therefore greater than it would be were relaxation of its arteries the only change.

The most important of all constrictor nerves are the splanchnics which control the supply to the stomach and intestines. When these nerves are cut, the digestive organs become engorged to such an extent that a pronounced fall of the general blood-pressure is the result. Their stimulation renders the digestive organs anæmic. We have already shown that the relaxation of vaso-constriction occurs in a reflex manner. The reflex relaxation of the splanchnic area is a matter of great importance, because it can be brought about by stimulation of one of the sensory nerves of the heart. The higher the blood-pressure, the harder the heart would work if left to itself. It is an impetuous organ, always trying to quicken its pace and to increase the force of its beat. Excessive zeal would get it into trouble if severe precautions were not taken to hold it in check. True, it is encouraged by certain “accelerator nerves”—sympathetic filaments which leave the spinal cord by the anterior roots of the second and third thoracic nerves; but the influence which the accelerators exert under normal conditions is not, it would seem, very pronounced. The nerves which restrain the heart are much more in evidence than those which urge it on. The arrangements for diminishing the work of the heart are of two kinds. In the first place, branches derived from the vagus act as a continuous check. From a certain spot in the medulla oblongata, the cardio-inhibitory centre, impulses are always descending to slow the heart. They are of reflex origin, but a high blood-pressure in the centre increases the facility with which they are transmitted. Some of these stimuli originate in the heart itself, ascending and descending the vagus nerve. The remainder come from various sources. A severe injury to any part of the body slows the heart. Injury to the intestines, such as occurs in peritonitis, is particularly effective in increasing vagus inhibition. Slowing of the heart lowers blood-pressure. When both vagi are cut, the heart begins to gallop whatever may be the pressure against which it has to work.

A sensory nerve of the heart, termed the “depressor,” is the chief agent in lowering blood-pressure. Its course is not the same in all animals, but it runs more or less in conjunction with the vagus. Usually it joins its superior laryngeal branch. Impulses which ascend this nerve inhibit the constriction of the splanchnic vessels. They open a floodgate which brings down the general pressure. The severe pain and extreme distress of angina pectoris are the cry of the heart when blood-pressure is too high—when it feels unable to work against it. This was recognized by physiologists long before a remedy was known. A systematic search was instituted for a drug which could be used with safety to lower blood-pressure. The discovery that the inhalation of amyl nitrite answers this purpose and fulfils this condition was the result.

When we consider the hydrostatics of the circulation, it becomes evident that changes in the force with which the heart beats, and changes in the calibre of the bloodvessels, work together in determining blood-pressure. Both vessels and heart contract automatically—the former continuously, the latter rhythmically. The heart of a frog, if it is enclosed in a moist chamber, beats for a long time after its removal from the animal. Even when cut in pieces, in certain ways, the separate pieces beat. A strip from the ventricle of a tortoise’s heart, kept gently stretched by the weight of a light lever attached to one of its ends, continued to contract rhythmically for forty-eight hours. When the heart has come to a pause, it cannot be started again by stimulating any nerve. It has in the most marked degree its own views as to the rapidity and force with which it ought to beat. But within certain limits it is under nervous control. The accelerators hasten it, to its own detriment. They belong to the division of katabolic nerves—a name given them to indicate that they waste the tissues, impoverishing their condition. The vagus nerve slows the heart. It protects it from itself. Its action is anabolic. The condition of the heart is improved under its influence. If it has been kept in check for a time by stimulation of the vagus, the heart beats more strongly when this nerve ceases to act than it did before it was induced to rest.

The arteries also are under the influence of two antagonistic sets of nerves. Those which increase their tonic contraction are almost universal in their distribution. It may be that those which actively check it are equally widespread, but the evidence is not altogether free from ambiguity. On certain organs—such as the salivary glands, already instanced—which require great variations in the amount of blood supplied to them, the influence of dilator nerves is very marked. The simplest hypothesis as to the mode of action of vaso-constrictor and vaso-dilator nerves leaves the initiative with the muscle-ibres of the vessel-wall. The distending internal pressure of blood is the stimulus which induces the muscle to contract. In some invertebrate animals—the snail, for example—if blood be prevented from entering the heart, so that there is no distending pressure, the heart stops. In higher animals the heart has acquired a habit of contracting, which keeps it going in the absence of its proper stimulus. The two classes of nerves exercise opposing influences on the muscle. Vaso-constrictor nerves increase the excitability of its fibres; vaso-dilator nerves diminish it. Only thus can we explain their action on a common basis. A good deal might be said as to the reasonableness of such an explanation. Our views as to the relation of nerve-influence and muscle-contraction are apt to go astray, owing to the fact that generations of physiologists have observed the phenomenon of a spasm of a muscle following on a sudden stimulus to a nerve. The two events are evidently related. The stimulus appears to set up a new condition in the nerve—to initiate a process which was not occurring before the electric current was passed through it. The muscular spasm equally appears to be an isolated event. As usual, we are misled by the analogy of human inventions. We compare the nerve-impulse to the fall of a hammer, the muscle-spasm to the explosion of gunpowder. We forget that nerve and muscle are in permanent connection; that the impulse is a sudden exaggeration of an influence which the nerve is continuously exerting, the contraction an exaggeration of metabolic changes which are constantly occurring in muscle. (See in this connection the explanation of muscle-tone, p. 273.) In the case of plain muscle, nerve stimuli do not cause contraction; they merely increase the excitability of the muscle. It may be more difficult for us to figure to ourselves the way in which dilator nerves diminish excitability; but the existence of such an anabolic influence is beyond the reach of doubt. Heart and bloodvessels are part of the same system. The heart has its accelerator and inhibitory nerves, the bloodvessels their constrictor and dilator nerves. For both vessel-wall and heart the stimulus to contraction is the distending pressure of blood—although it is not altogether necessary that this stimulus should be acting at the time. Sympathetic and vagus nerves can to a certain extent control the beating of a bloodless heart. The heart-tissue has acquired the habit of beating, and the habit of listening to advice conveyed to it through these nerves.

The self-adjustment of the blood-tubes to the pressure to which they are exposed is exhibited in the adaptation of their degree of contraction to the position of the body—to the weight, that is to say, of the column of fluid which they have to support. Everyone has played the game of “right hand or left.” When the hand is held above the head the blood leaves it, and the hand becomes cold; but if there be need for adjustment, and time is given for the mechanism to come into play, it works to perfection. When we are standing erect, there is neither too much blood in the feet nor too little in the head. But after a fortnight in bed a convalescent finds, the first time that he stands upright, that his legs are quickly engorged—his slippers after a few minutes feel too tight for him—whereas the brain becomes so anæmic that he turns giddy, or even faints.

Numberless illustrations of vaso-motor action are met with in daily experience. It is a curious fact that the nerves which control the calibre of the bloodvessels tend to overact their part. When an organ demands more blood, it is supplied at the expense of the rest of the body, and especially of the parts most nearly adjacent. This is partly a mechanical effect. If all the houses in a terrace are supplied with water from a common main, the bursting of a water-pipe in one of them will reduce the supply of its neighbours more than it will reduce the supply of houses in distant parts of the town. But vaso-motor nerves, in their compensating adjustment, go farther than this. A thimbleful of blood removed by a leech produces an effect upon an underlying engorged organ altogether out of proportion to the hydrostatic requirements of the case. “Cupping” the loins diminishes the congestion of the kidneys. This is the explanation of the curative efficacy of various agents which, with improvements in surgery and the introduction of more reliable drugs, have almost disappeared from the surgeon’s armamentarium—scarification, blisters, setons, and the like. Such methods have been relegated to veterinary practice.

There is a marked tendency to see-saw between the skin and the mucous membrane of the alimentary canal. During active digestion, when the “splanchnic area” is full of blood, the skin is cold. Hot fomentations, by dilating the vessels of the skin, diminish congestion of the alimentary tract. An inflamed throat is relieved by a compress round the neck. Conversely, it must be admitted that, in certain persons, slight constriction of the vessels of the skin induces inflammation of the mucous membrane. This is one reason for the almost universal dread of draughts. A draught cools a limited area of the skin. Some of us cultivate a love of draughts. They are the sensible evidence of the entrance of fresh air. Yet we admit reluctantly that certain fragile mortals are not altogether fanciful in supposing that a draught may give them a catarrh or a toothache. If asked why they object to draughts, many persons answer that they “are afraid of catching a chill”—carrying us back to the time before the clinical thermometer was invented; to days when the shivering fit, or “rigor,” which first calls attention to the fact that the temperature is already two or three degrees above the normal, was supposed to be the commencement of the illness. The patient imagined that the “chill” caused him to shiver, and that if he had not “caught” it he would not have been ill. The substitution of the term “cold” for “rheum,” naming the malady after one of its prominent symptoms, has done much to perpetuate this superstition. “Chill” is a word we scarcely dare to mention. When doctors could no longer attribute to witchcraft the occurrence of disorders for which they had no other explanation, they invented the luminous theory that inflammatory diseases—especially those of the stomach, liver, and lungs—were produced by “a chill.” At one time all diseases which were not evidently infectious were caused by chill. The discovery of germs and the recognition of their maleficent activity has stripped this cloak of ignorance off almost every case of abnormal tissue-metabolism. It is recognized now that the germ _is_ the disease, not the effects which the germ produces. Pneumonia is impossible in the absence of the pneumococcus, however severe the chill to which the patient was exposed when out in the cold and wet. Consumption is the effect produced by the tubercle bacillus. If there are no bacilli, there can be no consumption. Yet these two diseases illustrate the possibility of the use of the term “chill” without impropriety. The coccus of pneumonia may frequently be found in the mouth of a healthy person. If everyone with whom the tubercle bacillus has at some time come in contact were inevitably its victim, no human being would be free from phthisis, if any still survived. There are conditions of health, or rather of unhealth, in which the economy is less resistant than usual to the germs. Apparently the vaso-motor disturbances of internal organs caused by the cooling of the surface of the body, if it occur when health is otherwise depressed, contributes to the production of such a state.

The vaso-motor system is influenced by emotions. It is a little difficult to express accurately the relation between emotion and vaso-motor change. Some psychologists regard the vaso-motor change as the emotion. “All emotions,” says a prominent exponent of this view, “are wholly due to excitation of a particular kind of the vaso-motor centre.” The person about to be subject to an emotion of shame, anger, fear, disgust, recognizes a fact or circumstance, or conjunction of circumstances, which justifies the emotion. (We are assuming that emotions may be justified; that the intellectual appreciation of a situation and reasoned decision regarding the action which it demands is not sufficient.) This recognition as an intellectual act of the higher brain is accompanied by certain forms of enhanced activity or inhibition of activity of the vaso-motor centre in the medulla oblongata which cause changes in the degree of contraction of the bloodvessels of certain organs. The vascular changes produce an alteration in the state of the organ which is reflected in nerve-currents sent back to the brain, providing the background of feeling which constitutes emotional tone. We are not prepared to endorse this extreme view of the nature of an emotion. A maiden’s blush is not an emotion of embarrassment or shame. It is its harmony. Her mind plays the air. The sensations which originate in the flushed skin of the face sustain it with their accompaniment. The emotional tone keeps attention fixed on the fact or circumstance which led her to conclude, by the exercise of her reason, that she was placed in an awkward situation. This fixing of attention is frequently so pronounced as to inhibit all other intellectual action. The maiden is less quick than she would have been, had the emotion not glued her thoughts together, in recognizing the readiest means of extricating herself from embarrassment. All nerves found within the chest and abdomen were in very early times termed “sympathetic.” The cord in the neck was the “little sympathetic.” The name explains itself; but it will be understood that it implied much more in the days when the liver, spleen and heart were supposed to pour out emotions than it does now. The vagus nerve was termed the “middle sympathetic.” Shame inhibits the activity of the vaso-constrictor nerves of the face; dilation of the vessels which they supply is accompanied with constriction of other cutaneous nerves. Kipling must, we think, have embellished Nature when he represents the very unimpressionable hero of Lungtungpen as admitting “I niver blushed before or since; but I blushed all over my carkiss thin.” Usually the carmine of the face contrasts with the pallor and coldness of the hands. Still, we are not prepared to assert that it is impossible, under circumstances as trying as those in which Private Mulvaney and his companions were placed, for all the cutaneous constrictor nerves to let go their grip at the same time. Terror heightens the control of the vaso-motor centre over the vessels of the skin; it increases vagus inhibition of the heart. Even disgust evoked by a revolting sight or a foul smell may call the vagus so forcibly into action as to bring the heart to a standstill.

=The Pulse.=—The arterial system is always distended. The pressure in the largest arteries amounts to about 140 millimetres of mercury. The source of pressure is the beat of the heart pushing the blood forward against the resistance offered to its flow by the smallest vessels. At every stroke another 3 ounces is added to the already overfull vessels. In the aorta, therefore, the blood moves forward with jerks, but by the time it reaches the capillaries the intermittent accessions of force have been taken up by the elastic walls of the vessels and returned to the stream in the form of constant pressure. In the very smallest arteries the blood flows in a steady stream. If the corpuscles in a capillary vessel are watched under the microscope, they show no variations in rapidity synchronous with the beat of the heart. The “pulse” in the larger arteries is the push given to the column of blood by the sudden contraction of the left ventricle. Its propagation along the arteries will be understood if it is remembered that the blood is contained within elastic tubes. The first effect of the ejection into the aorta of an additional quantity of blood is the distension of its wall. The wave of distension travels down all the arteries of the body with gradually decreasing force.

Much may be learned from the pulse with regard to the condition of the vascular system, although it is impossible to balance the effects of the several factors which go to the production of its various modifications. The character of the pulse depends upon the vigour with which the heart is beating, the efficiency or otherwise of the cardiac valves, the quantity of blood in circulation, the suppleness of the arterial walls, the degree to which they are contracted, the resistance offered by the smaller vessels. Departures from the normal may take the direction of unduly high tension or of unduly low tension. In place of the sudden rise and more or less gradual fall, with the slightest possible roughness due to secondary waves, which constitutes a healthy pulse, the rise may be shorter, its subsidence prolonged. This is a high-tension or hard pulse. The pressure in the arteries is unduly high, or the walls of the vessels are not as elastic as they should be. Considerable pressure is needed to obliterate such a pulse—_i.e._, to prevent it from passing on beneath the finger. As the converse of this condition, the difference between the beginning of the pulse and its end may be very marked, the vessel suddenly dilating and as suddenly collapsing. But little pressure is needed to stop such a low-tension pulse from passing beneath the finger. Usually it has a distinct secondary or dicrotic wave. Some tactile education is needed by the finger that aspires to read the pulse. It was hoped that the personal equation would be of less importance if mechanical records were substituted for statements as to the impression produced upon the observer. Various forms of sphygmograph (σφυγμός, pulse) have been invented for this purpose. The form commonly used (Fig. 14) consists of a metal spring which is adjusted so that a button beneath its free end presses on the radial artery at the wrist. The force with which it presses is regulated by a screw. At each pulsation its free end is lifted by the distension and rounding of the artery. Its movement is transmitted by means of a continuous screw, attached to it vertically, to a cogged wheel, which in its turn raises a lever. The end of the lever scratches blackened paper fastened on a plate moved by clockwork. Records made in this way are useful for future reference. They are not, however, so valuable as it was anticipated that they would be. The form of the tracing depends to so large an extent upon the amount of pressure exerted by the spring, and the amount of pressure must be adapted to the vascular tone in every case. Some of the most interesting tracings are obtained from old people affected with atheroma of the arteries. This is a condition in which, owing to old-standing inflammation of the subepithelial coat of the vessels, the arteries have lost their suppleness. They are hard and inelastic. Instead of showing the normal steep face of the pulse-wave rising abruptly to its highest point, the tracing rises vertically for a short distance, and then slopes upwards. The wave is flat-topped or hog-backed.

All pulses are dicrotic, although the dicrotism may not be sufficiently pronounced to be felt with the finger. The notch which divides the primary from the secondary wave is produced by the closure—that is to say, by the falling down of the aortic valve. The wave from the commencement of its ascent to the dicrotic notch corresponds to the period during which blood is passing from the heart into the aorta. This part of the tracing represents systole of the ventricle after the semilunar valve has been forced. It is the push given to the bottom of the column by the additional 3 ounces of blood thrust into the aorta. The effort of the ventricle then comes to an end. The pressure beneath the semilunar valve is less than that above it. The valve closes. If the blood were contained in an open tube, the wave would now end, save for secondary oscillations, due to inertia of the fluid. But the arterial system is practically closed owing to the fineness of the tubes into which it ultimately divides. Its walls are elastic. They distend, taking up the pressure and returning it again in the second half of the wave. In fever, after the consumption of alcohol, and in other conditions in which the finest bloodvessels are dilated, the division between the two parts of the wave is very marked. Dicrotism is plainly felt. We have used the expression “finest vessels” rather than “capillaries,” because the ascription to the capillary vessels of all peripheral resistance has led to misunderstanding. Resistance is offered throughout the whole vascular system, with the exception of the largest veins. It is greatest in the small arteries, capillaries, and small veins. It is so adjusted as to fall to zero just before the blood reaches the heart.