THE NERVOUS SYSTEM 21$ 



middle of the pons into the two posterior cerebral arteries, 

 which supply the posterior cerebral lobes. At the pons 

 the basilar gives off the auditory artery, which enters the 

 internal auditory meatus together with the auditory nerve. 

 It also gives off the superior and inferior cerebellar arteries 

 for the supply of the upper and lower surface of the cere- 

 bellum, and eventually enters the circle of Willis. 



The circle of Willis provides a free supply of blood from 

 the several arteries of which it is formed the anterior 

 communicating in front; the two posterior communicating 

 behind; the former running between the two anterior 

 cerebral arteries, the latter from the posterior cerebral to 

 the carotid. The tortuosity of the vessels before entering 

 the brain provides a means to divert the excessive force 

 of the blood in the small vessels ; the four vessels that enter 

 the skull break up and form minute branches, and it is 

 for this reason that congestion of the brain is relieved only 

 with difficulty and recovery therefrom is slow. 



In considering the cerebral circulation, mention must 

 be made of the important part played by the meningeal 

 arteries. 



The meningeal arteries ramify between the dura mater 

 and the skull. Their most important branch is the 

 middle meningeal, a branch of the internal maxillary artery, 

 which enters the skull through the foramen spinosum, 

 passes through a groove in the sphenoid bone and anterior 

 angle of the parietal bones to the top of the skull, giving 

 off branches in every direction. The arteries are accom- 

 panied by corresponding veins. The other branches are of 

 less importance the anterior is given off from the internal 

 carotid within the cranial cavity; the posterior is a branch 

 of the occipital, and enters the skull through the jugular 

 foramen. 



The sinuses of the brain are venous channels; 

 they run in pairs and as single sinuses ; there are five pairs 



