upper end (b) of the instrument, while its peripheral extremity was fastened in the same manner to the lower end (c). The blood accordingly still kept on its usual course; only passing for a short distance through the artificial tube (a), between the divided extremi

VOLKMANN'S APPARATUS for measuring the rapidity of the arterial circulation.

ties of the artery. The instrument, however, was provided, as shown in the accompanying figures, with two transverse cylindrical plugs, also perforated; and arranged in such a manner, that when, at a given signal, the two plugs were suddenly turned in opposite directions, the stream of blood would be turned out of its course (Fig. 95), and made to traverse a long bent tube of glass (d, d, d), before again finding its way back to the lower portion of the artery. In this way the distance passed over by the blood in a given time could be readily measured upon a scale attached to the side of the glass tube. Volkmann found, as the average result of his observations, that the blood moves in the carotid arteries of warm-blooded quadrupeds with a velocity of 12 inches per second.

VENOUS CIRCULATION.

The veins, which collect the blood from the tissues and return it to the heart, are composed, like the arteries, of three coats; an inner, middle, and exterior. In structure, they differ from the arteries in containing a much smaller quantity of muscular and elastic fibres, and a larger proportion of simple condensed areolar tissue. They are consequently more flaccid and compressible than the arteries, and less elastic and contractile. They are furthermore distinguished, throughout the limbs, neck, and external portions of the head and trunk, by being provided with valves, consisting of fibrous sheets arranged in the form of festoons, and so placed in the cavity of the vein as to allow the blood to pass readily from the periphery toward the heart, but to prevent altogether its reflex in an opposite direction.

The flow of blood through the veins is less powerful and regular than that through the arteries. It depends on the combined action of three different forces.

1. The force of aspiration of the thorax.-When the chest expands, by the lifting of the ribs and the descent of the diaphragm, it has the effect of drawing into the thoracic cavity all the fluids which can gain access to it. The expanded cavity is principally filled by the air, which passes in through the trachea and fills the bronchial tubes and pulmonary vesicles. But the blood in the large veins is also drawn into the chest at the same time and by the same force. It can readily be seen, when the jugular and subclavian veins are exposed in the living animal, that these vessels collapse with every inspiration, and fill out again at the moment of expiration. During inspiration, the blood is drawn forward into that part of the vein which occupies the cavity of the chest; and during expiration, the flow being momentarily checked by the compression of the thorax, the vein fills up from behind, and again becomes distended. This force does not act efficiently at any great distance from the chest, owing to the flaccidity of the venous parietes, which collapse at a short distance from the entrance to the thoracic cavity, as the vein becomes emptied by inspiration. It is active, however, in the neighborhood of the chest, and the respiratory movements exert, therefore, a certain degree of influence on the venous circulation.

2. The contraction of the voluntary muscles.-The veins which convey the blood through the limbs, and the parietes of the head and trunk, lie among voluntary muscles, which are more or less constantly in a state of alternate contraction and relaxation. At every contraction these muscles become swollen laterally, and, of course, compress the veins which are situated between them. The blood, driven out from the vein by this pressure, cannot regurgitate toward the capillaries, owing to the valves, already described, which shut back and prevent its reflux. It is accordingly forced onward toward the heart; and when the muscle relaxes and the vein is liberated from pressure, it again fills up from behind, and the circulation goes on as before. This force is a very efficient one in producing the venous circulation; since the voluntary muscles are more or less active in every position of the body, and the veins constantly liable to be compressed by them. It is on this account that the veins, in the external parts of the body, communicate so freely with each other by transverse branches; in order that the current of blood, which is momentarily excluded from one vein by the pressure of the muscles, may readily find a passage through others, which communicate by cross branches with the first. (Figs. 96 and 97.)

3. The force of the capillary circulation.--This last cause of the motion of the blood through the veins is the most important of all, as it is the only one which is constantly and universally active. In

fish, for example, respiration is performed altogether by gills; and in reptiles the air is forced down into the lungs by a kind of deglutition, instead of being drawn in by the expansion of the chest. In neither of these classes, therefore, can the movements of respiration assist mechanically in the circulation of the blood. In the splanchnic cavities, again, of all the vertebrate animals, the veins coming from the internal organs, as, for example, the cerebral, pulmonary, portal, hepatic, and renal veins, are unprovided with valves; and the passage of the blood through them cannot therefore be effected by any lateral pressure. The circulation, however, constantly going on in the capillaries, everywhere tends to crowd the radicles of the veins with blood; and this vis a tergo, or pressure from behind, fills the whole venous system by a constant and steady accumulation. So long, therefore, as the veins are relieved of blood at their cardiac extremity by the regular pulsations of the heart, there is no backward pressure to oppose the impulse derived from the capillary cir culation; and the movement of the blood through the veins continues in a steady and uniform course.

With regard to the rapidity of the venous circulation, no direct results have been obtained by experiment. Owing to the flaccidity of the venous parietes, and the readiness with which the flow of blood through them is disturbed, it is not possible to determine this point for the veins, in the same manner as it has been determined for the arteries. The only calculation which has been made in this respect is based upon a comparison of the total capacity of the arterial and venous systems. As the same blood which passes outward through the arteries, passes inward again through the veins, the rapidity of its flow in each must be in inverse proportion to the capacity of the two sets of vessels. That is to say, a quantity of blood which would pass in a given time, with a velocity of x, through an opening equal to one square inch, would pass during the same time through an opening equal to two square inches, with a velocity of; and would require, on the other hand, a velocity of 2 x, to pass in the same time through an opening equal to onehalf a square inch. Now the capacity of the entire venous system, when distended by injection, is about twice as great as that of the entire arterial system. During life, however, the venous system is at no time so completely filled with blood as is the case with the arteries; and making allowance for this difference, we find that the entire quantity of venous blood is to the entire quantity of arte

rial blood nearly as three to two. The velocity of the venous blood, as compared with the arterial, is therefore as two to three; or about 8 inches per second. It will be understood, however, that this calculation is altogether approximative, and not exact; since the venous current varies, according to many different circumstances, in different parts of the body; being slower near the capillaries, and more rapid near the heart. It expresses, however, with sufficient accuracy, the relative velocity of the arterial and venous currents, at corresponding parts of their course.

THE CAPILLARY CIRCULATION.

Fig. 98.

The capillary blood vessels are minute inosculating tubes, which permeate the vascular organs in every direction, and bring the blood into intimate contact with the substance of the tissues. They are continuous with the terminal ramifications of the arteries on the one hand, and with the commencing rootlets of the veins on the other. They vary somewhat in size in different organs, and in different species of animals; their average diameter in the human subject being a little over 3 of an inch. They are composed of a single, transparent, homogeneous, somewhat elastic, tubular membrane, which is provided at various intervals with flattened, oval nuclei. As the smaller arteries approach the capillaries, they diminish constantly in size by successive subdivision, and lose

first their external or fibrous (a), breaking up into capillaries. From the pia

tunic. They are then composed

mater.

only of the internal or homogeneous coat, and the midde or muscular. (Fig. 98, a.) The middle coat then diminishes in thickness, until it is reduced to a single layer of circular, fusiform, unstriped, muscular fibres, which in their turn disappear altogether as the artery merges at last in the capillaries; leaving only, as we have