=A. basilaris= (_c_).--This arises by the union of the two vertebral arteries in the manner just described. It passes craniad along the ventral middle line of the medulla and pons, giving numerous small branches to these structures. A large branch, =A. cerebelli inferior posterior= (_d_), passes on each side to the caudal surface of the cerebellum, on which it ramifies. At the cranial margin of the pons the basilar artery divides. From each division a very large branch passes on each side across the pedunculi cerebri to the cranial part of the cerebellum; this is =A. cerebelli anterior= (_e_). Just craniad of this, arising from nearly the same point, the smaller =A. cerebri posterior= (_f_) passes laterad to the caudal part of the cerebrum; it is joined by the internal carotid artery (_g_). The small continuations of the two halves of =A. basilaris= then pass craniad at the side of the hypophysis to join the posterior communicating branches from the carotid plexus. As other branches of the carotid plexus unite across the middle line craniad of the optic chiasma, an arterial circle is formed on the base of the brain, surrounding the hypophysis and the optic chiasma. This is known as the =circulus arteriosus=, or circle of Willis (see page 289). _b._ =A. mammaria interna= (Fig. 118, _h_).--The internal mammary artery rises from the ventral surface of the subclavian opposite the first rib, and passes in the mediastinum ventrocaudad to reach the sternum opposite the third intercostal space. It sends two or three small branches craniad to the midventral part of the thoracic wall and then extends caudad at the side of the sternum. It sends off lateral branches to the ventral thoracic wall, branches to the mediastinum and pericardium, a branch to the diaphragm which anastomoses with the phrenic; and finally it passes out of the thoracic cavity caudad of the last costal cartilage, extends caudad in a zigzag course at the lateral border of the rectus muscle and anastomoses with the inferior epigastric artery.