THE BRAIN. 161 



very small or wanting, and the other quite large to make 

 up for the deficiency. When both are present the arterial 

 circuit called the circle of Willis is formed. The arteries 

 which form it are the two anterior cerebrals, which are 

 united by the anterior communicating, the two middle 

 cerebrals (or internal carotids), which are joined by the two 

 posterior communicating vessels to the two posterior cere- 

 brals. Within this circle are found the following structures 

 at the base of the brain. The posterior perforated space, third 

 and fourth nerves, the corpora albicantia or mammillaria, 

 the tuber cinereum, infundibulum and pituitary body, the 

 optic chiasm tracts and nerves, and the lamina cinerea. 



The function of the circle of Willis is to maintain an 

 even supply of blood to all parts of the brain. The 

 anastomoses of the opposite carotids through the anterior 

 cerebrals and anterior communicating, and of the posterior 

 cerebrals with each other, is so free that no trouble would 

 arise from interference with one carotid or one vertebral. 



But the anastomosis of these two systems through the 

 posterior communicating is not so free and constant, owing 

 to the smallness of these arteries, or even the absence 

 of one. As a rule, we find both of these arteries small, 

 rarely find both large, often one may be quite good sized 

 and the other very small or wanting entirely. In cases 

 where there is not a free communication between the 

 anterior and posterior parts of the circle of Willis it would 

 evidently be dangerous to ligate both carotids at one 

 operation. 



The cerebral blood-supply is divided into the vessels 

 distributed to the base of the brain ganglionic system, 

 and those to the outer surface of the cortex cortical 

 system. 



The blood supply to the brain differs from the blood 

 ii 



