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 awil/ary 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. 
259. The descending aorta, in its progress along the trunk, 
gives-off several important branches ;—as the celiac, from 
which the stomach, liver, and spleen are supplied ; the renal, 
to the kidneys; and the mesenteric, to the intestines. It 
divides at last into the two ¢liac 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 per- 
formed 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, 
