906 



THE VASCULAE SYSTEM. 



emerges from the foramen in the transverse process of the atlas, between the anterior 

 division of the sub-occipital nerve medially and the rectus capitis lateralis laterally, 

 and runs almost horizontally backwards and medially, round the lateral and posterior 

 aspects of the corresponding superior articular process of the atlas. In this part of 

 its course it enters the sub-occipital triangle, where it lies in the groove on the 

 upper surface of the posterior arch of the atlas (sulcus arteriae vertebralis). It is 

 separated from the bone by the sub-occipital nerve, and is overlapped superficially 

 by the adjacent borders of the superior and inferior oblique muscles. Finally, this 



Anterior communicating artery 



Olfactory tract 



Anterior cerebral artery 



Optic chiasma 



Infundibulum 



Oculomotor nerv 



Glossopharyn 

 geal nerve 



Vagus nerve 

 Accessory ner 



Basilar artery 



Anterior 

 inferior cere- 

 bellar artery 



Posterior inferior 

 cerebellar artery 



Vertebral artery 



Hypoglossal nerve Anterior spinal artery 



FIG. 764. THE ARTERIES OF THE BASE OF THE BRAIN. THE CIRCULDS ARTERIOSUS (WILLIS). 



part of the artery passes anterior to the oblique ligament of the atlas and enters 

 the vertebral canal. 



The fourth part pierces the spinal dura mater and runs upwards into the cranial 

 cavity. It passes between the roots of the hypoglossal nerve, posteriorly, and the 

 first dentation of the ligamentum denticulatum, anteriorly, pierces the arachnoid, and, 

 gradually inclining to the front of the medulla oblongata, reaches the lower border 

 of the pons, where it unites with its fellow of the opposite side to form the basilar 

 artery. 



Branches. From the first part. As a rule there are only a few small muscular 

 twigs from this portion of the artery. 



From the second part. (1) Muscular branches which vary in number and size, 

 supply the deep muscles of the neck, and anastomose with the profunda cervicis, the 

 ascending cervical, and the occipital arteries. 



