THE ARTERIES. 291 



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 transver- 

 sarium 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 verte- 

 bral canal through the atlantal I & 



foramen. It passes to the ventral FlG - 121. ARTERIES OF THE VEN- 

 TRAL SURFACE OF THE BRAIN. 

 Side Of the spinal COrd and unites a> A verte bralis; b, A. spinalis an- 



at about the level of the foramen terior ; <r . A - basilaris; d, A. cerebeili 



. , . inferior posterior; e, A. cerebeili an- 



magnum with the vertebral artery terior; /, A. cerebri posterior; g, cut 



of the Opposite side (Fig. I2I, a) ends of the two internal carotid arteries; 



' h, cut ends of branches from the ca- 

 tO form the baSllar artery (A. rotid plexus (g and h are placed within 



basilaris) (Fig. 121, c\ which w e , r c ! rculus A arterio * us or circle of 



Willis); t, A. cerebri media; j, A. 



passes craniad along the ventral cerebri anterior, 

 middle line of the brain. Just before their union the two verte- 

 bral arteries (a) give off each a branch which passes caudo- 

 mediad. These two branches soon unite in the middle line, 

 forming the anterior spinal artery (A. spinalis anterior) (ft), 

 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. 



