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.
Fig. 10.—The Heart cut in the Plane of its Long Axis, and the Vessels which open into and out of it.
Chordæ tendineæ attach the margins of the auriculo-ventricular valves to musculi papillares which project from the inner aspect of each ventricle.
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.