14 THE MENINGE3 OF THE BRAIN. 



THE BLOOD SUPPLY OF THE BRAIN. 



The brain is furnished with blood by the internal carotid and 

 vertebral arteries (Fig. 9). The internal carotid artery (a. 

 carotis interna] gives origin to the anterior and the middle cere- 

 bral, 'the anterior chorioidal and the posterior communicating; 

 the vertebral artery (a, vertebralis) gives off the anterior and 

 posterior spinal and the posterior inferior cerebellar and then 

 unites with its fellow at the inferior border of the pons and forms 

 the basilar artery. The basilar artery (a. basilaris] runs upward 

 the length of the pons and terminates in the two posterior cerebral 

 . arteries and, furthermore, gives off the following collateral branches, 

 viz., the anterior inferior cerebellar, the pontal, the internal audi- 

 tory and the superior cerebellar. Certain of these arteries form 

 a wonderful circular anastomosis at the base of the brain, called 

 the arterial circle and the distribution of that circle is to the cere- 

 brum (Figs. 9 and 10); while the rhombencephalon (pons, cere- 

 bellum and medulla) is supplied by the remainder of the arteries 

 above enumerated. It is therefore convenient to describe the 

 circulation of the brain under two heads: (A) The circulation 

 of the cerebrum, and (B) The circulation of the rhomben- 

 cephalon. 



A. THE CEREBRAL CIRCULATION, ARTERIES. 



The Arterial Circle. (Circulus arteriosis, Willisi). The arter- 

 ies which supply the cerebrum freely communicate in the arterial 

 circle, which is really a heptagon extending from a point in the 

 longitudinal fissure anterior to the optic chiasma, back to the pons 

 (Fig. 9). It is about an inch and a half long, and from a half 

 to one inch in transverse diameter. In front are the anterior 

 cerebral arteries converging forward from the internal carotids 

 and, through the anterior communicating artery (a. communicans 

 anterior], uniting just as they enter the longitudinal fissure of the 

 cerebrum. These vessels form three sides of the heptagon and 

 the front of the circle. On either side, the posterior communic- 

 ating artery (a. communicans posterior] which connects the internal 

 carotid with the posterior cerebral artery, forms the lateral bound- 



