_a_, The left ventricle of the heart. _b_, The right auricle. _c_, The superior vena cava. _d_, The root of the pulmonary artery. _e, e_, The aorta, which is seen arching backward over the heart, and passing downward into the abdomen, where it divides into its two great branches, the iliac arteries, through which the blood passes to the lower extremities. _f_, The inferior vena cava, which accompanies the descending aorta and its branches, and returns the blood from the lower extremities. The dotted lines represent the outlines of the kidneys. [Illustration: full body drawing] PRINCIPAL DIVISIONS OF THE AORTA AND VENA CAVA. It should be remembered that most of the branches which spring from the great artery and vein are double—that is, each right branch has a corresponding one at the left side—so that there are, for instance, the right and the left carotid arteries, the right and the left jugular veins, etc. From the arch of the aorta are sent off those arteries which are distributed to the head and arms. The principal ones among these are named as follows: _g_, The carotid artery, which ascends in the side of the neck, and divides into the temporal artery, _h_, which is distributed in the temple, and the facial artery, _i_, which supplies the face; and also sends a branch, called the internal carotid, to the parts within the skull. _j_, The sub-clavian artery, lying beneath the clavicle or collar-bone. That part of the continuation of this artery which passes through the axilla or arm-pit is called the axillary artery, _k_; that which lies in the upper arm, the brachial artery, _l_; and in the fore-arm it divides into the radial and ulnar arteries, _m_, _n_, which are distributed to the hand and fingers in the manner indicated in the plate. The principal branches of the descending aorta are named as follows: The iliac artery, _o_, on passing into the thigh becomes the femoral artery, _p_, and in the leg divides into the tibial and peroneal arteries, _q_, _r_, which form numerous branches for the supply of the leg and foot. Before dividing into the iliac arteries, the descending aorta gives off several important branches, as the cœliac artery, from which the stomach and liver are supplied; the renal artery, which goes to the kidneys, and the mesenteric artery, to the intestines; besides many other sub-divisions in various parts of its course. The branches of the vena cava generally accompany those of the aorta in their distribution, as shown in the figure, and are often called by the same names. The principal divisions of the superior vena cava are: The jugular vein, _s_, which accompanies the carotid artery the sub-clavian vein, _t_, which accompanies the artery of the same name, and receives the blood from the arm and hand.