ARTERIAL SYSTEM OF MAN.

225

direction; and then descends along the front of the spinal column, behind the heart, as far as the lower part of the

226

ARTERIAL SYSTEM OF MAN.

trunk, where it divides into two great branches, which proceed to the lower extremities. From the arch of the aorta are given off the arteries which supply the head and upper extremities. These are, the two carotids, which ascend on either side of the neck; and the two subclavian, which pass outwards beneath the clavicles, so as to arrive at the arms, becoming successively in their course the axillary and brachial arteries, as they pass through the axilla or arm-pit, and along the arm. The subclavian and carotid arteries of the right side arise together from the aorta, in Man, by a common trunk; but this arrangement varies much in different Mammals. Thus in the Elephant, the two carotids arise by a common trunk,-the two subclavians separately. In some of the Whale tribe, all four are separate. In the Bat, the subclavian and carotid of the left side arise from a common trunk, like those of the right. And in those Ruminating animals which possess a long neck, all four arteries come off from the aorta together, by a large trunk, which first gives off the subclavians on either side, and then divides into the carotids, All these varieties occasionally present themselves in Man, a fact of no small interest.

It

259. The descending aorta, in its progress along the trunk, gives-off several important branches; as the coeliac, from which the stomach, liver, and spleen are supplied; the renal, to the kidneys; and the mesenteric, to the intestines. divides at last into the two iliac arteries; which, after giving off branches for the supply of the lower bowels, pass into the thighs, where they become the femoral arteries; and these again subdivide into branches for the supply of the leg.

260. For the sake of comparison, a figure of the arterial system of a Bird is introduced; from which it will be seen that by far the larger proportion of its blood is distributed to its upper extremities. In Man, the descending aorta is evidently the continuation of the aortic arch; and the parts which it supplies receive far more blood than the head and upper extremities, the locomotion of biped man being performed almost entirely by his lower limbs. In Quadrupeds, which require nearly as much strength in their fore feet as in their hind, the subclavian arteries bear a larger proportion to the iliac. But in Birds, the function of locomotion is almost entirely performed by the wings; and their powerful muscles,

ARTERIAL SYSTEM OF BIRD.

227

which constitute the mass of flesh lying on the breast, are supplied with blood by the arteries of the upper extremities, which here possess a manifest predominance. The aorta, soon after its origin, subdivides into three large branches; of which the first two (one on either side giving-off the subclavian and carotid arteries) convey the blood to the head, the wings, and

Fig. 125.-ARTERIAL SYSTEM OF BIRD.

the muscles lying on the thorax; whilst the middle one curves backwards and downwards, and becomes the descending aorta. Now that which is here the continuation of the great side

228

DISTRIBUTION OF ARTERIES.

branch, is neither the carotid nor the subclavian, both of which are subordinate branches given-off from it; but it is the trunk which distributes the blood to the muscles of the breast, and which in Man is a subordinate branch of the subclavian artery (the mammary). The descending aorta is seen to lose itself almost entirely in supplying the viscera of the trunk; so that the branches into which it divides at last for the supply of the legs, are very small. These limbs, in birds, are usually required only for the support of the body at times of rest, and are seldom much concerned in locomotion; so that they possess little muscular power, and require but a small supply of blood.

261. It is very interesting to trace such differences in the arrangement of the vascular system, corresponding with variations in the general plan of structure, yet not exhibiting any actual departure from the general type. Thus, there is probably not a single large artery in Man, to which a corresponding branch might not be found in the Bird; on the other hand, there is perhaps not a single large artery in the Bird, to which there is not an analogous branch in Man. The chief difference consists in the relative sizes of the several trunks; and these correspond closely with the amount of tissue they have respectively to supply. Here, then, we have one example, out of many that might be adverted-to, of that Unity of Design which we see everywhere prevalent throughout nature; manifesting itself in the close conformity of a great number of apparently-different structures to one general plan, whilst there is, at the same time, an almost infinite variety in the details.

262. There is a very interesting peculiarity in the distribution of the arteries, by which the due circulation of blood in their branches is provided for, even though there should be an obstruction in the main trunk. The branches which are given-off from it at different points, have frequent communications or anastomoses with each other; so that blood may pass from an upper part of a main artery into the lower, by means of these lateral communications, even though its flow through the trunk itself should be completely stopped.

263. These anastomoses are very numerous in the arteries of the limbs, and particularly about the joints; and it is well that they are so; for, by relying on the maintenance of the

ANASTOMOSES OF ARTERIES

-ANEURISM.

229

circulation through them, the Surgeon is often able to save a limb, or even a life, which would otherwise be sacrificed. Arteries are liable to a peculiar disease, termed aneurism, which consists in a thinning-away, or rupture, of the tough fibrous coat, and a great dilatation of the other coats, so that a pulsating tumour is formed. This change takes place most frequently at the bend of the thigh, the ham, the shoulder, and the elbow; where the artery, in the working of these joints, often has to undergo sudden twists. The result of the disease would be generally fatal, in consequence of the gradual thinning-away of the walls of the tumour, which at last bursts, allowing the blood to escape from the arterial trunk with such rapidity as, if unchecked, to cause almost instantaneous death. In order to prevent this, the surgeon ties the artery at some little distance above the aneurism,—that is, he puts a thread round it, which is drawn so tight as to prevent the passage of any blood to the aneurism. The circulation in the lower part of the limb is at first retarded; its temperature falls; and it becomes more or less insensible. But after the lapse of a few hours, the circulation becomes quite vigorous, the pulsations strong, the temperature rises, and the numbness passes off; and as the main trunk still continues completely obstructed, this can only have been brought about by the flow of blood through the anastomoses, which must in that short period have undergone considerable enlargement. Examination of the vessels after death shows that this has been actually the case. Even the aorta has thus been tied in dogs, without causing death; the anastomoses of the branches given-off from its upper part, with those proceeding from the lower, being sufficient to maintain the circulation in the latter, when the current through the main trunk is obstructed.

264. A very complex series of anastomoses, forming a complete network of large tubes, is found in several situations, where it seems desirable that the flow of blood to a particular organ should be retarded, whilst a large amount is to be allowed to pass through. Thus in animals which keep their heads near the ground for some time together, as in grazing, the arteries which supply the brain suddenly divide, on their entrance within the skull, into a great number of branches, by the anastomoses of which a complex network is formed; and from this network, by the reunion of its small vessels,