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 {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 peduncuii cerebri to the cranial part of the cere- 

 bellum ; this is A. cerebelli anterior (c). 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 (^). 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). 



b. A. mamraaria interna (Fig. 1 18, h). — 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 

 at 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-y — 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 



