292 THE CIRCULATORY SYSTEM. 
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 (@), 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 cere- 
bellum; this is A. cerebelli anterior (¢). Just craniad of this, 
arising from nearly the same point, the smaller A. cerebri 
posterior () 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 com- 
municating 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). 
é. A. mammaria interna (Fig. 118, £).—The internal mam- 
mary artery rises from the ventral surface of the subclavian 
opposite the first rib, and passes in the mediastinum ventro- 
caudad 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 
cat the side of the sternum. It sends off lateral branches to the 
ventral thoracic wall, branches to the mediastinum and peri- 
cardium, 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 anasto- 
moses with the inferior epigastric artery. 
c. Truncus costocervicalis (Fig. 118, g).—The costo- 
cervical axis arises from the subclavian opposite the first rib, 
passes craniodorsad, and divides almost at once into two 
branches. The smaller one of these, the superior intercostal 
