The =left subclavian= (Fig. 118, _c_) arises from the convexity of the aortic arch just distad of the origin of the innominate artery, and about two or three centimeters from the heart. It passes craniad and slightly to the left, and turns into the left arm just craniad of the first rib. The =right subclavian= (Fig. 115, _n_) is a direct continuation of the innominate, the artery receiving the name subclavian after the right common carotid is given off, usually at about the level of the second or third intercostal space. The subclavian has the following branches: the =vertebral artery= (Fig. 118, _g′_), the =internal mammary= (Fig. 118, _h_), the =costocervical axis= (Fig. 118, _g_), the =thyrocervical axis= (Fig. 118, _j_). Beyond the last-named branch it continues into the arm as the =axillary= artery (Fig. 118, _i_; Fig. 122, _g_). [Illustration: FIG. 121.--ARTERIES OF THE VENTRAL SURFACE OF THE BRAIN. _a_, A. vertebralis; _b_, A. spinalis anterior; _c_, A. basilaris; _d_, A. cerebelli inferior posterior; _e_, A. cerebelli anterior; _f_, A. cerebri posterior; _g_, cut ends of the two internal carotid arteries; _h_, cut ends of branches from the carotid plexus (_g_ and _h_ are placed within the circulus arteriosus or circle of Willis); _i_, A. cerebri media; _j_, A. cerebri anterior.] _a._ =A. vertebralis= (Fig. 118, _g_′).--The vertebral artery arises from the dorsal surface of the subclavian opposite the first rib. It passes craniad and dorsad at the side of the thoracic portion of the longus colli muscle, and enters the foramen transversarium of the sixth cervical vertebra. It passes thence craniad through the foramina transversaria (which together form the =vertebrarterial canal=) and gives off at the intervertebral foramina branches to the muscles of the neck and branches which pass across the ventral surface of the spinal cord to join A. spinalis anterior. Craniad of the foramen transversarium of the atlas the vertebral artery turns dorsad in the groove on the lateral surface of the atlas. Here it gives off a large branch which passes laterodorsad to the muscles of the neck and may anastomose with a branch of the occipital artery. The vertebral artery then passes into the vertebral canal through the atlantal foramen. It passes to the ventral side of the spinal cord and unites at about the level of the foramen magnum with the vertebral artery of the opposite side (Fig. 121, _a_) to form the =basilar artery (A. basilaris)= (Fig. 121, _c_), which passes craniad along the ventral middle line of the brain. Just before their union the two vertebral arteries (_a_) give off each a branch which passes caudomediad. These two branches soon unite in the middle line, forming the =anterior spinal artery= (=A. spinalis anterior=) (_b_), which passes caudad the entire length of the spinal cord, lying on its ventral middle line and receiving many communicating branches from the vertebral, intercostal, and lumbar arteries.