ticity, but this is even less called upon than that of the capillaries; and, presently, in the larger veins, the moving blood is found to press no harder from within than the atmosphere from without.

Subsidiary Forces which Assist the Flow in the Veins.-There are certain forces which, occasionally or regularly, assist the heart to return the venous blood into itself. Too much stress is often laid upon these; for it is easy to see by experiment that the heart can maintain the circulation wholly without help. The origins of these subsidiary forces are, first, the contraction of the skeletal muscles in general; second, the continuous traction of the lungs; third, the contraction of the muscles of inspiration.

The Skeletal Muscles and the Venous Valves.-A vein may lie in such relation to a muscle that when the latter contracts the vein is pressed upon, its feeble blood-pressure is overborne, the vein is narrowed, and blood is squeezed out of it. The veins in many parts are rich in valves, competent to prevent regurgitation of the blood while permitting its flow in the physiological direction. The pressure of a contracting muscle, therefore, can only squeeze blood out of a vein toward the heart, never in the reverse direction. Muscular contraction, then, may, and often does, assist in the return of the venous blood with a force not even indirectly derived from the heart. But such assistance, although it may be vigorous and at times important, is transient and irregular. Indeed, were a given muscle to remain long in contraction, the continued squeezing of the vein would be an obstruction to the flow through it.

The Continuous Pull of the Elastic Lungs.-The influence of thoracic aspiration upon the movement of the blood in the veins deserves a fuller discussion. The root of the neck is the region where this influence shows itself most clearly, but it may also be verified in the ascending vena cava of an animal in which the abdomen has been opened. The physiology of respiration shows that not only in inspiration, but also in expiration, the elastic fibres of the lungs are upon the stretch, and are pulling upon the ribs and intercostal spaces, upon the diaphragm, and upon the heart and the great vessels. This dilating force at all times exerted upon the heart by the lungs is of assistance, as we shall see, in the diastolic expansion of its ventricles. In the same way the elastic pull of the lungs acts upon the venæ cave within the chest, and generates within them, as well as within the right auricle, a force of suction. The effects upon the venous flow of this continuous aspiration are best known in the system of the descending vena cava. This suction from within the chest extends to the great veins just without it in the neck. In these, close to the chest, as we have seen, manometric observation reveals a continuous slightly negative pressure. A little farther from the chest, however, but still within the lower portions of the neck, the intravenous pressure is slightly positive. The elastic pull of the lung, therefore, continuously assists in unloading the terminal part of the venous system, and thus differs markedly from the irregular contractions of the skeletal muscles.

The Contraction of the Muscles of Inspiration.-But some skeletal

oscillate, come to a standstill, and then reverse the direction of their movement, and return to the capillary whence they had started. Naturally, no such reversal will ever be seen in a capillary which springs directly from an artery or which directly joins a vein. It will be remembered, however, that any apparent speed of a corpuscle is much magnified by the microscope, and that therefore the variations referred to are comparatively unimportant. We may, in fact, without material error, treat the speed of the blood in the capillaries which intervene between the arteries and veins of a region as approximately uniform for an ordinary period of observation, as the minute variations will tend to compensate for one another. This speed is sluggish, as already noted. In the capillaries of the web of the frog's foot it has been found to be about 0.5 millimeter per second. The causes of this sluggishness will be set forth later. That the very short distance between artery and vein is traversed slowly, deserves to be insisted on, as thus time is afforded for the uses of the blood to be fulfilled.

Capillary Blood-pressure. The pressure of the blood against the capillary wall is low, though higher than that of the lymph without. This pressure is subject to changes, and is readily yielded to by the elastic and delicate wall. From these changes of pressure changes of calibre result. The microscope tells us less about the capillary blood-pressure than about the other phenomena of the flow; but the microscope may sometimes show one striking fact. In a capillary district under observation, a capillary not noted before may suddenly start into view as if newly formed under the eye. This is because its calibre has been too small for red corpuscles and leucocytes to enter, until some slight increase of pressure has dilated the transparent tube, hitherto filled with transparent plasma only. This dilatation has admitted corpuscles, and has caused the vessel to appear.

That the capillary pressure is low is shown, moreover, by the fact that when one's finger is pricked or slightly cut, the blood simply drips away; that it does not spring in a jet, as when an artery of any size has been divided. That the capillary pressure is low may also be shown, and more accurately, by the careful scientific application of a familiar fact: If one press with a blunt lead-pencil upon the skin between the base of a finger-nail and the neighboring joint, the ruddy surface becomes pale, because the blood is expelled from the capillaries and they are flattened. If delicate weights be used, instead of the pencil, the force can be measured which just suffices to whiten the surface somewhat, that is, to counterbalance the pressure of the distending blood, which pressure thus can be measured approximately. It has been found to be very much lower than the pressure in the large arteries, considerably higher than that in the large veins, and thus intermediate between the two; whereas the blood-speed in the capillaries is less than the speed in either the arteries or the veins. The pressure in the capillaries, measured by the method just described, has been found to be equal to that required to sustain against gravity a column of mercury from 24 to 54 milli

meters high; or, in the parlance of the laboratory, has been found equal to from 24 to 54 millimeters of mercury.1

Summary of the Capillary Flow.-Whether in the lungs or in the rest of the body, the general characters of the capillary flow, as learned from direct inspection and from experiment, may be summed up as follows: The blood moves through the capillaries toward the veins with much friction, continuously, slowly, without pulse, and under low pressure. To account for these facts is to deal systematically with the mechanics of the circulation; and to that task we must now address ourselves.

C. THE PRESSURE OF THE BLOOD IN THE ARTERIES, CAPILLARIES, AND VEINS.

Why does the blood move continuously out of the arteries through the capillaries into the veins? Because there is continuously a high pressure of blood in the arteries and a low pressure in the veins, and from the seat of high to that of low pressure the blood must continuously flow through the capillaries, where pressure is intermediate, as already stated.

Method of Studying Arterial and Venous Pressure, and General Results. Before stating quantitatively the differences of pressure, we must see how they are ascertained for the arteries and veins. The method of obtaining the capillary pressure has been referred to already. If, in the neck of a mammal, the left common carotid artery be clamped in two places, it can, without loss of blood, be divided between the clamps, and a long straight glass tube, open at both ends, and of small calibre, can be tied into that stump of the artery which is still connected with the aorta, and which is called the "proximal" stump. If now the glass tube be held upright, and the clamp be taken off which has hitherto closed the artery between the tube and the aorta, the blood will mount in the tube, which is open at the top, to a considerable height, and will remain there. The external jugular vein of the other side should have been treated in the same way, but its tube should have been inserted into the "distal" stump-that is, the stump connected with the veins of the head, and not with the subclavian veins. If the clamp between the tube and the head have been removed at nearly the same time with that upon the artery, the blood may have mounted in the upright venous tube also, but only to a small distance. To cite an actual case in illustration, in a small etherized dog the arterial blood-column has been seen to stand at a height of about 155 centimeters above the level of the aorta, the height of the venous column about 18 centimeters above the same level. The heights of the arterial and venous columns of blood measure the pressures obtaining within the aorta and the veins of the head respectively, while at the same time the circulation continues to be free through both the aorta and the venous network. Therefore, in the dog above referred to, the aortic pressure was between eight and nine

1 N. v. Kries: "Ueber den Druck in den Blutcapillaren der menschlichen Haut,” Berichte über die Verhandlungen der k. sächsischen Gesellschaft der Wissenschaften zu Leipzig, math.-physische Classe, 1875, S. 149.

times as great as that in the smaller veins of the head. As, during such an experiment, the blood is free to pass from the aorta through one carotid and both vertebral arteries to the head, and to return through all the veins of that part, except one external jugular, to the vena cava, it is demonstrated that there must be a continuous flow from the aorta, through the capillaries of the head, into the veins, because the pressure in the aorta is many times as great as the pressure in the veins. Obviously, such an experiment, although very instructive, gives only roughly qualitative results.

Two things will be noted, moreover, in such an experiment. One is that the venous column is steady; the other is that the arterial column is perpetually fluctuating in a rhythmic manner. The top of the arterial column shows a regular rise and fall of perhaps a few centimeters, the rhythm of which is the same as that of the breathing of the animal; and, while the surface is thus rising and falling, it is also the seat of frequent flickering fluctuations of smaller extent, the rhythm of which is regular, and agrees with that of the heart's beat. At no time, however, do the respiratory fluctuations of the arterial column amount to more than a fraction of its mean height; compared to which last, again, the cardiac fluctuations are still smaller. It is clear, then, that the aortic pressure changes with the movements of the chest, and with the systoles and diastoles of the left ventricle. But stress is laid at present upon the fact that the aortic pressure at its lowest is several times as high as the pressure in the smaller veins of the head. Therefore, the occurrence of incessant fluctuations in the aortic pressure cannot prevent the continuous movement of the blood out of the arteries, through the capillaries, into the veins.

The upright tubes employed in the foregoing experiment are called "manometers." They were first applied to the measurement of the arterial and venous blood-pressures by a clergyman of the Church of England, Stephen Hales, rector of Farringdon in Hampshire, who experimented with them upon the horse first, and afterward upon other mammals. He published his method and results in 1733. The height of the manometric column is a true measure of the pressure which sustains it; for the force derived from gravity with which the blood in the tube presses downward at its lower opening is exactly equal to the force with which the blood in the artery or vein is pressed upward at the same opening. The downward force exerted by the column of blood varies directly with the height of the column, but, by the laws of fluid pressure, does not vary with the calibre of the manometer, which calibre may therefore be settled on other grounds. It follows also that the arterial and venous manometers need not be of the same calibre. Were, however, another fluid than the blood itself used in the manometer to measure a given intravascular pressure, as is easily possible, the height of the column would differ from that of the column of blood. For a given pressure the height 1 From μavós, rare. The name was given from such tubes being used to measure the tension of gases.

Stephen Hales: Statical Essays: containing Hemastaticks, etc., London, 1733, vol. ii. p. 1.

of the column is inverse to the density of the manometric fluid. For example, a given pressure will sustain a far taller column of blood than of mercury.

The

The Mercurial Manometer.The method of Hales, in its original simplicity, is valuable from that very simplicity for demonstration, but not for research. clotting of the blood soon ends the experiment, and, while it continues, the tallness of the tube required for the artery, and the height of the column of blood, are very inconvenient. It is essential to understand next the principles of the more exact instruments employed in the modern laboratory.

In 1828 the French physician and physiologist J. L. M. Poiseuille devised means both of keeping the blood from clotting in the tubes, and of using as a measuring fluid the heavy mercury instead of the much lighter blood. He thereby secured a long observation, a low column, and a manageable manometer.1 The "mercurial manometer" of to-day is that of Poiseuille, though modified (see Fig. 15). In an improved form it consists of a glass tube open at ends, and bent upon itself to the shape of the letter U. This is held upright by an iron frame. If mercury be poured into one branch of the U, it will fill both branches to an equal height. If fluid be driven down upon the mercury in one branch or "limb" of the tube, it will drive some of the mercury out

RT

P

D

R

L

F

M

FIG. 15.-Diagram of the recording mercurial manometer and the kymograph; the mercury is indicated in deep black: M, the manometer, connected by the leaden

pipe, L, with a glass cannula tied into the proximal stump of the left common carotid artery of a dog; A,

both the aorta; C. the stop-cock, by opening which the man

ometer may be made to communicate through RT, the rubber tube, with a pressure-bottle of solution of sodium carbonate; F, the float of ivory and hard rubber; R, the light steel rod, kept perpendicular by B, the steel bear

ing; P, the glass capillary pen charged with quickly drying ink; T, a thread which is caused, by the weight of a light ring of metal suspended from it, to press the pen obliquely and gently against the paper with which is covered D, the brass "drum " of the kymograph, which

drum revolves in the direction of the arrow. The supports of the manometer and the body and clock-work of the kymograph are omitted for the sake of simplicity.

The aorta and its branches are drawn disproportionately large for the sake of clearness.

of that limb into the other, and the two surfaces of the mercury may come to rest at very unequal levels. The difference of level, expressed in millimeters, 1 J. L. M. Poiseuille: Recherches sur la force du cœur aortique, Paris, 1828.