Circulation. Under this title we shall examine only the circulation of blood in the animal economy, omitting all that relates to the circulation of lymph and chyle, and to the circulation of the nutritive fluids in plants. Before stating by what decisive experiments and reasonings the eminent William Harvey (1619, 1G28) proved the circulation of the blood, we must remind the reader of some points in the anatomy of man and of the higher animals which are essential to the understanding of our subject. The heart, which is at least one of the principal organs that circulate the blood, is a complicated muscular apparatus, composed of walls separating four cavities, two on the left and two on the right of the organ. Although there is only a muscular Avail between these two sets of cavities, and although they belong to one single organ or apparatus, the two sides of the heart, in which are these two sets of cavities, are sometimes called, for the sake of brevity, the right heart and the left heart. Of the four cavities of the heart and of their walls, two, the upper ones, are called the auricles, while the two lower ones are called the ventricles; so that there is a left auricle and a left ventricle, constituting the left heart, and a right auricle and a right ventricle, constituting the right heart.

The circulation of the blood takes place in the following way: From all the parts of the body black blood, called also venous blood because it comes by the veins, reaches the right auricle, which propels it into the right ventricle, from which it is sent to the organs of respiration, i. e., the lungs. The blood vessel that brings the blood from the right ventricle into the lungs is the pulmonary artery. The lungs send back this blood to the heart through blood vessels which, on account of their structural resemblance to veins, although they do not carry black or venous blood, are called pulmonary veins. In the lungs the blood by the influence of respiration becomes red, and in consequence the heart receives red blood from the pulmonary veins. The left auricle is the place of reception for this blood, which is thence carried into the left ventricle. By a large blood vessel called the aorta the blood expelled from the left ventricle passes into the arterial system, and is thence distributed to all the parts of the body. So that, as may be now easily understood, there is apparently a double circulation, one through the whole body, and one through the lungs and the heart.

The blood from all parts of the body comes to the heart through the veins; it passes from the trunks of these vessels, the verm cavae, into the right auricle, thence into the right ventricle, which sends it to the lungs through the pulmonary artery; thence it returns to the heart by the pulmonary veins, and enters the left auricle, which impels it into the left ventricle, from which it goes to the whole body through the aorta and its ramifications, the various arteries; finally, from the arteries it passes into the veins through the very minute blood vessels called capillaries. In writing the names of the parts through which the blood circulates, we may begin with either, as these parts form a circuit; thus:

10. General capillaries.

1. Veins.

Right heart.

2. Right auricle. 3. Right ventricle

4. Pulmonary artery

9. Aorta and arteries.

8. Left ventricle. 7. Left auricle.

Left heart.

6. Pulmonary veins.

5. Capillaries of the lungs.

If, instead of beginning with number 1, we begin with number 7, the blood then passes from 7 to 8, to 9, to 10, to 1, to 2, to 3, to 4, to 5, to 6, and thence reaches again number 7. The principal facts discovered or clearly proved for the first time by Harvey are: 1, that the movements of the heart are similar to those of the. muscles of the limbs, etc, as regards the parts which produce them; 2, that the arteries become full at the time the ventricles of the heart expel the blood they contain; 3, that the pulmonary artery receives blood at the same time the aorta and other arteries receive it, and therefore that the two ventricles contract and expel the blood at the same time; 4, that the two auricles contract simultaneously, and that their contraction precedes that of the two ventricles; 5, that after a ligature has been applied on an artery this vessel becomes quite distended with blood between the ligature and the heart, and empty in the other parts, showing that the blood comes from the heart into the arteries; 6, that after the application of a ligature on a vein the blood disappears above the ligature, i. e., in the direction toward the heart, while it accumulates in the vein below the ligature, i. e., in the part which seems to be separated from the heart; 7, that the valves in the veins prevent the blood from going in a wrong direction.

From these facts and the natural conclusions that may be drawn from them, and also from many other facts and reasonings. Harvey deduced the theory of the circulation of the blood, which we have stated, and which is now universally admitted. Harvey, in giving this complete demonstration of the circulation of the blood, achieved the most important discovery yet made in physiology. - We pass now to the examination of the most interesting questions concerning the circulation of the blood, omitting all those relating to the frequency, strength, and causes of the movements of the heart, for which see Heart and Pulse. There are four principal members or parts of the circulatory apparatus, each of which has a special office to perform in the function as a whole. These are: 1, the heart; 2, the arteries; 3, the capillaries; and 4, the veins. We shall describe the action of these different organs or groups of organs in succession. - The heart is the main cause of the movement of the blood in the circulation. It is, as Harvey first demonstrated, a muscular organ, which contracts upon the blood contained in its cavities and drives it out into the arterial trunks, exactly like an animated force pump.

The left ventricle, which is by far the thickest and strongest of all the chambers of the heart, nearly empties itself at each contraction, and the blood, being prevented from regurgitating into the auricle by the shutting back of the ventricular valves, is forced onward by a vigorous impulse into the aorta. The elastic resistance of the arterial system, being inferior in force to the muscular contraction of the heart, yields before it, and the blood which previously filled the left ventricle is thus suddenly transferred to the cavity of the aorta. The contraction of the heart is then at once followed by a condition of relaxation, and the ventricle begins to be again passively filled by the blood flowing into it from behind, from the veins and through the auricle. Thus the circulation through the heart is intermittent. The blood is at one instant driven out of the ventricle in a powerful stream, and is then temporarily arrested until another muscular contraction takes place, to repeat the same phenomenon. - Movement of the blood in the arteries. The most marked physical property of the arteries, as a whole, is their elasticity. In this respect they are like a series of India-rubber tubes, owing to the abundant development of elastic fibres in their middle coat.

Accordingly they are distensible, but offer nevertheless a certain amount of resistance to any distending force. This resistance, as we have remarked above, being less than the impulse with which the blood is injected into them from the heart, the arteries expand at the instant of the heart's contraction, and are thus made to contain a larger quantity of blood than before. This expansion of the arteries under the heart's action, which is nearly simultaneous all over the body, can easily be perceived by the finger placed above an artery of moderate size, and is known as the arterial pulse. As each stroke of the pulse is caused by and is synchronous with a cardiac pulsation, its frequency becomes a valuable and convenient means by which the physician estimates the rapidity of the heart's action. As soon, however, as the cardiac contraction which caused the dilatation of the arteries comes to an end. and is followed by relaxation, then the elastic reaction of the arteries themselves, compressing the blood, drives it onward toward their terminal ramifications and into the capillary system.

Under these circumstances the blood would be partly forced backward into the relaxed ventricle, were it not for three thin but strong membranous valves which guard the orifice of the aorta at its junction with the heart, and which shut backward at the instant of the heart's relaxation, and thus prevent any backward regurgitation. Thus the blood, forced into the arteries by the muscular power of the heart, is driven onward to the capillaries by the elastic force of the arteries themselves. Nevertheless the heart is the essential cause even of the arterial circulation; for the elasticity of the arteries, which is a passive physical property, like that of India rubber, is called into exercise only by the distention of these vessels under the cardiac impulse; and the blood would soon cease flowing were it not, at each successive contraction of the ventricles, thrown into the arterial system in superabundant quantity. The arterial circulation, carried on under the combined influence of the two forces just described, presents certain peculiarities which are worthy of notice. First, its rapidity varies at different periods during a single pulsation.

At the instant of the heart's contraction the blood moves with a maximum velocity; in the intervals of the heart's action, when it is compressed by the arterial elasticity alone, its velocity is diminished. Consequently, if an artery of medium size be wounded, the blood escapes from it in jets. At each cardiac pulsation the stream of blood rises and flows more rapidly; at each relaxation it falls to a lower level and runs more slowly. The stream is never entirely interrupted. On the contrary, it flows always with a considerable degree of force; but its increased velocity at each period of the heart's action is abundantly visible. It is this pulsatile character of the haemorrhage from arteries, as well as the florid color of the arterial blood, which enables us at once to distinguish it from haemorrhage which takes place from veins or capillaries. Chau-veau has succeeded in measuring these variations of rapidity in the arterial circulation, by an ingenious instrument adapted to the carotid artery of the horse.

He finds that at the instant of the heart's contraction the blood is suddenly put in motion with a comparatively high degree of rapidity, amounting on the average to 20 inches per second; the movement of the blood is then diminished or brought to a standstill immediately before the closure of the aortic valves; on the closure of the valves the blood again moves forward with a velocity of about 8 1/2 inches per second; and it then diminishes gradually, falling to a little less than 0 inches per second by the end of the period of relaxation. Secondly, the difference between the two conditions of distention and collapse of the artery diminishes from the heart outward; for at a distance from the heart the force of the cardiac pulsation has been partly expended in distending the larger vessels in its neighborhood, and consequently the distention of the remote artery is less; and at the time of cardiac relaxation the smaller artery is still kept partly distended by the elastic reaction of the larger ones behind.

Thus, as in the case of water driven by the strokes of a force pump through an elastic air chamber, the intermittent action of the heart upon the blood becomes more and more equalized, from the centre of the circulation outward; and in the smallest arterial ramifications the pulsating character of the flow is hardly visible to the naked eye, though still perceptible under the microscope. The pressure exerted upon the blood in the arterial system may be measured by attaching the carotid artery of the living animal to a reservoir of mercury provided with an upright open tube or pressure gauge. When the cavity of the artery is allowed to communicate freely with the interior of the reservoir, the mercury, under the pressure of the blood, rises in the upright tube, and the height of the mercurial column thus becomes an indication of the pressure to which the blood itself is subjected within the artery. The arterial pressure, measured by this test, is found to equal on the average a column of mercury 150 millimetres (6 inches) in height. Various oscillations of the level of the mercury show the variations in pressure upon the blood during the different periods of a pulsation; but in the healthy condition it seldom or never falls below 130 millimetres. - Movement of the hlood in the cajyillaries.

In the capillary vessels the movement of the blood at once loses all trace of the pulsating character which it presented in the arteries, and becomes perfectly steady and uniform. This is because, on leaving the arteries and passing into the minute and excessively abundant capillary vessels, the same quantity of blood at once becomes subjected to the influence of a much greater extent of elastic surface; for the walls of the capillaries are themselves elastic, and the substance of the organs in which they ramify has also to a greater or less degree the same property. The effect of this increased elasticity of the surrounding parts upon a given quantity of blood is to equalize its rate of movement, and convert it into that of a completely uniform current. Nevertheless, the physical cause of the passage of the blood through the capillary vessels is simply the pressure from behind to which it is subjected in the arterial system. This is fully evident from the following considerations: If the nozzle of an injection pipe be placed in the femoral artery of a recently killed animal, and liquid blood be forced by it through the terminal ramifications of the artery and the capillary vessels of the limb, so as to return in a full stream by the femoral vein, it is found that the force requisite to accomplish this result, as measured by a pressure gauge connected with the syringe, is from 120 to 130 millimetres of mercury.

Prof. Sharpey of London found that an injection might be made to pass in this way from the mesenteric artery through both the two successive capillary systems of the intestine and the liver, and return in a full stream by the vena cava, under the pressure of a column of mercury 130 millimetres in height. But we have already shown that the blood is habitually subjected in the arteries of the living animal to an average pressure of 150 millimetres. This force accordingly is amply sufficient to account for the movement of the blood through the capillaries generally, from the arteries to the veins. The rapidity of movement of the blood in the capillaries is much less than in the arteries. This is due to the fact that the extent of vascular surface with which the blood comes in contact on entering the capillary system is vastly increased owing to the small size and great number of the vascular canals through which it now flows; and consequently the mechanical resistance to its passage is increased in corresponding ratio. It is estimated, from data derived from microscopic inspection of the capillary circulation in transparent tissues, that the rate of movement of the blood in these vessels is about 1/30 of an inch per second.

It must be remembered, however, that while the capillary vessels are excessively numerous, and their united calibre therefore very large, their length, on the other hand, is very small. The effect of this anatomical arrangement accordingly is to disseminate a comparatively small quantity of blood, while it is contained in the capillary vessels, over a very large space; so that the physiological and chemical reactions taking place between the blood and the substance of the tissues are accomplished almost instantaneously. Furthermore, although the rate of movement of the blood in these vessels is very slow, yet as the distance to be passed over between the arteries and the veins is very small, the blood requires but a few seconds to traverse the capillary system, and to commence its returning passage by the veins. - Movement of the blood in the veins. In the veins the blood is gradually collected from the peripheral parts of the circulatory system and returned to the right side of the heart. Beginning by small rootlets directly connected with the capillaries, the veins constantly unite with each other into larger branches and trunks, converging in this way toward the centre of the circulation, until they terminate in the right auricle of the heart by two great canals, one coming from the upper and one from the lower parts of the body, and named accordingly the vena cava superior and the vena cava inferior.

The walls of the veins are thinner than those of the arteries, and are less distensible and less elastic; but they have great strength owing to the large proportion of white fibrous tissue in their structure, and are able to resist without laceration an equal or even greater pressure than the arteries of corresponding size. They are provided at various intervals with membranous semi-lunar valves, which open toward the heart and shut backward in the opposite direction. The main cause of the movement of the blood in the veins is the pressure from behind exerted by the blood as it is carried through the capillary circulation. It thus fills the rootlets and smaller branches of the veins with a constantly increasing supply, and urges the blood which they already contained onward to the heart. If this were the only force at work, the venous system would after a time become so filled with blood that its resistance would counterbalance the pressure of blood from the arteries, and the circulation would come to an end by simple engorgement of the veins. But in point of fact the right ventricle of the heart at each pulsation discharges into the pulmonary artery, and, as it were, lifts away from the venous system a portion of the blood which had accumulated in it.

Thus the veins are protected from engorgement, and their backward resistance is kept always inferior to the pressure by which the blood enters them from the capillaries. The venous circulation is also much facilitated by the alternate contraction and relaxation of the voluntary muscles. When any one of these muscles contracts, it increases in thickness exactly in proportion as it diminishes in length. The effect of this lateral swelling of the muscles in activity is to compress the veins situated between them, and the blood is thus forced out of the compressed portion. The valves of the veins, already mentioned, prevent the blood from regurgitating toward the extremities, and it is consequently driven onward toward the heart, in which direction alone the passage for it is clear. "When the muscles relax, the corresponding portion of the vein is again rapidly filled with blood coming from the extremities, and the circulation goes on with increased facility from the circumference toward the centre. - Variations of the circulation in different parts. We know that the capillary circulation in various parts of the body is subject to marked variations, dependent on external or internal causes.

Thus a mustard poultice, or a sponge moistened with water of ammonia, applied to the skin, will cause a local and circumscribed redness, due to an increased capillary circulation in the part. The face will become flushed or pallid under the influence of mental emotion, and cold or hot applications will cause a similar change of color in those parts of the skin with which they are brought in contact. Even the internal organs exhibit fluctuations of the circulation of the same kind, to a marked degree. When a glandular organ is about to enter into active secretion, it is the seat of a kind of physiological congestion. Its capillary blood vessels enlarge in diameter, and admit a greater quantity of blood than before. The rapidity of the capillary circulation is at the same time increased, so that a larger quantity of blood than usual passes in a given time through the vessels of the part. "When the period of functional activity comes to an end, the capillary vessels return to their original diameter, admit a smaller quantity of blood, and the color of the organ resumes its ordinary pallid state. These variations in the capillary circulation are due to the action of the smaller arteries.

The arteries generally, as we have already shown, are characterized mainly by their passive physical property of elasticity. But the medium-sized and especially the smaller arteries are provided also with minute muscular fibres, running round them in a circular or transverse direction, and forming part of their middle coats. By the greater or less degree of contraction of these muscular fibres, the calibre of the arterial twig is enlarged or diminished, and consequently the quantity of blood admitted by it to the capillary vessels varies in a corresponding degree. Thus, although the circulation everywhere is kept up by the central force of the heart's action, yet its influence on particular parts is modified in degree by the contracted or relaxed condition of the smaller arteries. - General rapidity of the circulation. The time required for the blood to traverse the entire round of the double circulation, that is, through the lungs and the general capillary system, returning again to the right side of the heart, has been found by experiment to be very much shorter than was formerly supposed.

The observations of Hering, Poisseuille, Blake, and Matteucci on this subject were made, for the most part, by injecting into the jugular vein of one side upon the living animal some substance, like ferrocyanide of potassium, which could readily be recognized by its chemical reactions. Blood was at the same time drawn from the jugular vein of the opposite side; and the interval which elapsed before the appearance of the foreign salt in the blood drawn from this second opening indicated the time required for the blood to pass from the point of injection through the vena cava to the heart, from the right side of the heart through the lungs to the left cavities, from the left ventricle through the carotid arteries and the capillary vessels of the head, and thence downward to the jugular vein on the opposite side. The average results obtained from these experiments were as follows:

TIME REQUISITE FOR THE COMPLETE CIRCULATION OF THE BLOOD.

In man the complete double circulation probably requires from 15 to 25 seconds.