SEMILUNAR VALVES.

235

whilst that on the left side has only two, so as to bear some resemblance to a bishop's mitre, whence it is called the mitral valve.

273. The aorta and pulmonary artery are in like manner furnished with valves, which prevent the blood that has been forced into them by the contraction of the ventricles, from returning into those cavities when they begin to dilate again. These valves, however, are formed upon a different plan, and more resemble those of the veins, which will be presently described. They consist of three little pocket-shaped folds of the lining membrane of these arteries (similar to those at b b, fig. 128), which are pressed flat against the walls of those tubes when the blood is forced into them; but as soon as they are filled, and the ventricles begin to dilate, so that the blood has a tendency to return, it presses upon the upper side of these pockets, and fills them out against one another, in such a manner as completely to close the entrance into the ventricle. The three little pocket-shaped folds, however, would not close the centre of the aperture, were it not that each of them has a little projection from its most prominent part, which meets with those of the others, and effects the requisite end. The situation of these valves (which are termed semilunar from their half-moon shape) is seen at g, fig. 127, f being the pulmonary artery.

274. The amount of blood sent-out from either ventricle at each contraction, in a middle-sized man, seldom exceeds 3 ounces; but the whole quantity of blood contained in the body is not less than 18 lbs.: hence, it would require 96 contractions of the heart to propel the whole of this blood through the body, and these (at the ordinary rapidity) would occupy about 1 minute. It has been calculated, from recent experiments, that the usual force of the heart in man would sustain a column of blood about 7 feet 2 inches high, the weight of which would be about 4 lbs. 3 oz. on every square inch. The backward pressure of this column upon the walls of the heart, or in other words, the force which they have to overcome in propelling the blood into the aorta, is estimated at about 13 lbs.

275. From the mode in which the blood is forced into the arterial system by a series of interrupted impulses, it might be supposed that its course would be a succession of distinct

236

EQUALIZING ACTION OF ARTERIES:-PULSE.

jets; but this is prevented, so that the current is reduced to an equable stream by the time it reaches the capillaries, through the elasticity of the walls of the arteries. In order to comprehend how this acts, we may suppose a forcing-pump (§ 270) to propel its fluid, not into a hard unyielding tube of iron or lead, but into an elastic tube of india-rubber. The effect of each stroke of the pump will be partly expended in distending the tube, so as to make it contain an additional quantity of water; and the suddenness of the jet at its opposite extremity will be diminished. In the interval of the stroke, the elasticity of the wall of the tube will cause it to contract again, and to force-out the added portion of its contents; this it will not have completed by the time that the action of the pump is renewed; and in this manner, instead of an interrupted jet at the mouth of the tube, we shall have a continuous flow, which, if the tube be long enough, will become quite equable.* It is precisely in this manner that the elasticity of the arteries influences the flow of blood through them, by converting the interrupted impulses which the heart communicates to it, into a continued force of movement. In the large arteries, these impulses are very evident; in the smaller branches they are less so, but they still manifest themselves by the jerking in the stream of blood proceeding from a wound in one of these vessels; whilst in the capillaries, the influence of the heart's interrupted impulses cannot usually be seen at all, the streams that pass through them being perfectly equable.

276. The phenomenon which we call the pulse, is nothing else than the change in the condition of the artery occasioned by the increased pressure of the fluid upon its walls, at the moment when the heart's contraction forces an additional quantity of blood into the arterial system. By the frequency and force of this change, we can judge of the power with which the blood is being propelled. But the pulse can only be well distinguished, when we can compress the artery against some resisting body, so that there is a partial obstruction to the flow of blood through it, which causes the distension to be more powerful; the most convenient artery for this

* The same effect is obtained in an ordinary fire- or garden-engine, by the interposition of an air-vessel, in which the elasticity of compressed air is substituted for that of the wall of the pipe.

PULSE-WOUNDS OF ARTERIES.

237

purpose is the radial artery (fig. 124) at the wrist; but the carotid artery in the neck, and the temporal artery in the temple, may be felt, when it is desired to know the force of the circulation in the head; as may the arteries supplying other parts, when we wish to gain information respecting the organs they supply. An increased action in the organ, whether this be due to inflammation, or to a state of unusual activity of its function, causes an increase of size in the artery which supplies it; and thus the pulsation may be unusually strong in a particular trunk, when the heart's action and the general circulation are not in a state of excitement. For instance, a whitlow on the thumb will occasion its artery to beat almost as powerfully as the radial artery usually does; and excessive activity of the mind, prolonged for some hours, greatly increases the force of the pulsations in the carotid arteries, from which the brain is chiefly supplied.

277. When an artery is wounded, there is often great difficulty in controlling the flow of blood; for pressure can seldom be effectually applied in the situation of the wound; and the surgeon is generally obliged to tie the vessel above the orifice. As a temporary expedient, the loss of blood may be prevented by making firm pressure upon the artery above the wounded part, that is, nearer the heart; and many valuable lives have been saved by the exercise of presence of mind, guided by a little knowledge. The best means of keeping-up the requisite pressure, until the proper instrument (the tourniquet) can be applied, is to lay over the artery (the place of which may be found by its pulsation) a hard pad, made by tightly rolling or folding a piece of cloth; this pad and the limb are then to be encircled by a bandage, by which the pressure is maintained; and this bandage may be tightened to any required degree, by twisting it with a ruler or a piece of stick. Thus a constant pressure may be exercised upon the artery, which will be generally sufficient to control the bleeding from it. But there are, unfortunately, many cases in which pressure of this kind cannot be applied; as for instance when the femoral artery is wounded high up in the thigh, or the carotid artery in the neck. And nothing else can then be done, but to compress the artery with the thumb, or with some round hard substance (such as the handle of an awl), until proper assistance can be obtained.

238

FLOW OF BLOOD THROUGH THE VEINS.

278. The impulse of the heart, and the elasticity of the arteries, which together propel the blood through the capillary system, continue to act upon it after it is received into the veins; and are in fact the chief causes of its movement in them. If we interrupt the current of blood through an artery by making pressure upon it, and open the corresponding vein, the fluid will continue to flow from the latter, so long as the artery contains blood enough to be forced into the vein by its own contraction; but as soon as it is emptied, the flow from the orifice in the vein will cease, even though the vein itself remains nearly full. If the pressure be then taken off the artery, there is an immediate renewal of the stream from the vein, which may be again checked by pressure on the artery. In the ordinary operation of bleeding, we cause the superficial veins of the arm to be distended, by tying a bandage round them above the point at which we would make the incision; and when an aperture is made, the blood spouts forth freely, being prevented by the bandage from returning to the heart." But if the bandage be too tight, so that the artery also is compressed, the blood will not flow freely from the vein; and the loosening of the bandage will then produce the desired effect. When a sufficient quantity of blood has been withdrawn, the bandage is removed; and the return-flow through the veins being now unobstructed, the stream from the orifice immediately diminishes so as to be very easily checked by pressure upon it, or may even cease altogether.

b

b

a

279. The veins contain a great number of valves, which are formed, like the semilunar valves of the aorta (§ 273), by a doubling of their lining membrane. Their situation may be known by the little dilatations they which the veins exhibit at the points where they occur; and which are very obvious in the arm of a person not too fat, when it is encircled by a bandage that causes distension of the superficial veins. The structure of these valves is seen at bb,

FLOW OF BLOOD THROUGH THE VEINS.

239 fig. 128; they consist of pocket-like folds of the lining membrane, which allow the blood free passage as it flows towards the heart, but check its reflux into the arteries. Hence it follows, that every time pressure is made upon the veins, it will force towards the heart a portion of the blood they contain, since this cannot be driven in a contrary direction. Now, from the manner in which the veins are distributed, some of them must be compressed by almost every muscular movement; these will become refilled as soon as the muscles relax; and they will be again pressed-on, when the movement is repeated. Hence a succession of muscular movements will act the part of a diffused heart, over the whole of the venous system, and will very much aid the flow of blood through its tubes. It is partly in this manner, that exercise increases the rapidity of the circulation. If the blood is brought to the heart by the great veins more rapidly than usual, the heart must go through its operations more rapidly, in order to dispose of the fluid; and if these actions be impeded, great danger of their entire cessation may exist. Hence the importance of bodily tranquillity to those affected with diseases of the heart or lungs (§ 271).

280. Besides the aid thus afforded to the venous circulation, it is probable that there is another cause of the motion of the blood in them, which is independent of the action of the heart and of the arteries. Many facts lead to the belief that a new force is produced, while the blood is flowing through the capillary vessels, a force which may, in some instances, maintain the circulation by itself alone. Thus in many of the lower animals, it seems as if the power of the heart were so unequal to the maintenance of the circulation, that this must partly depend upon some other influence; and even in the highest, there is evidence that the movement of blood in the capillaries may continue for a time, after the action of the heart and of the arteries has ceased to affect it.* This movement seems intimately connected with the changes to which the blood is subservient in the capillaries; for, if these be checked, not even the heart's action can propel the blood through them, although no mechanical

*For a full consideration of this question, see the Author's Principles of Comparative Physiology (4th edition), §§ 247-251; and Principles of Human Physiology (5th edition), §§ 267-275.