386 NERVOUS SYSTEM. 



It will now be understood, that an arterial circle or link en- 

 closes the chiasm of the optic nerves and the corpora albican- 

 tia. The fore and lateral parts of the circle are formed by the 

 internal carotids and their branches; while the hind part is 

 formed by the Basilar Artery and its bifurcation. This is the 

 circle of Willis, and establishes a very free communication be- 

 tween the vessels of the two sides of the brain. 



The veins of the Brain have been sufficiently alluded to in 

 the account of the Pia Mater, and of the sinuses of the Dura 

 Mater. 



