BLOOD-VESSEIS OF THE BRAIN. 427 



little above the condyles of the occipital bone. In the 

 cranium they are seen on the under surface of the medulla 

 oblongata, approaching each other till they reach the pos- 

 terior part of the pons. At this point they unite into a 

 common trunk called the basilar artery. 



The vertebral arteries in their course send off small ar- 

 teries to the membranes of the spinal marrow and adjacent 

 muscles, and at their superior extremity they give off three 

 important branches, the anterior and posterior spinal, and 

 inferior cerebellar arteries. The anterior and posterior 

 spinal, as elsewhere stated, are two long and delicate 

 branches, the one in front, the other behind the spinal 

 cord, running the entire length of this organ and giving 

 off branches to the several spinal nerves. 



The basilar artery, formed by the junction of the two 

 vertebrals, is situated on the median line of the pons, send- 

 ing many fine branches into its substance, and at its upper 

 edge giving off four branches, two to each side, the superior 

 cerebellar and posterior cerebral arteries. The former wind 

 round the crura cerebri, send a branch with the seventh 

 pair of nerves into the internal auditory foramen and 

 finally distribute themselves upon the upper surface of the 

 cerebellum, anastomosing with the inferior cerebellar arte- 

 ries. 



The posterior cerebral are much larger branches, and 

 separated from the latter at their origin by the third pair 

 of nerves. They receive the posterior communicating branch 

 of the carotids, wind round the crura cerebri, to which 

 branches are supplied, and are finally spent upon the pos- 

 terior lobes, anastomosing with the middle and anterior 

 arteries of the cerebrum. 



The circle of Willis will now be understood to be formed 

 in front and laterally by the internal carotids and their 

 posterior communicating branches, while the back part of 

 the circle comes from the basilar. This circle surrounds 

 the commissure of the optic nerve. 



The veins of the brain have already been noticed with the 

 sinuses of the dura mater. 



