CIRCULUS ARTERIOSUS 



555 



and communicates with it by a short transverse trunk, about five mm. lon^, known 

 as the anterior communicating artery [a. communicans anterior] (fig. 456). On- 

 ward from this point it runs side by side with its fellow in the longitudinal fissure 

 round the genu of the corpus callosum; then, turning backward, it continues along 

 the upper surface of that commissure, and, after giving off large branches to the 

 frontal and parietal lobules, anastomoses with the posterior cerebral artery. 



6. THE MIDDLE CEREBRAL ARTERY 



The middle cerebral artery [a. cerebri media] (figs. 456, 460), the larger of the 

 terminal divisions of the internal carotid, supplies the basal ganglia and a part of 

 the cortex of the frontal and parietal lobes. It passes obliquely upward and 

 lateralward into the lateral (Sylvian) fissure, and opposite the insula divides into 

 cortical branches. 



CiRcuLus Arteriosus 



The four arteries which supply the brain, namely, the two internal carotid 

 arteries and the two vertebrals (which unite to form the basilar), form a remark- 



FiG. 456. — The Arteries of the Brain. 

 (The cerebellum has been cut away on the left side to show the posterior part of the cere- 

 brum.. From a preparation in the Museum of St. Bartholomew's Hospital.) 



Anterior cerebral, 

 artery 



Middle cerebral 



artery 



Internal carotid, 



artery 



Postero-median 



perforating 

 Posterior cere- 

 bral artery 

 Superior cerebel- 

 lar artery 

 Anterior inferior 

 cerebellar artery 



Vertebral artery 



Posterior inferior, 

 .cerebellar artery 



Anterior commu- 

 nicating artery 

 Antero-lateral 



perforating 

 Chorioid 



Posterior com- 

 municating artery 



Posterior chorioid 



Basilar artery 



Cerebellum, cut 



Anterior spinal 

 artery 



able anastomosis at the base of the brain known as the circle of Willis [circulus 

 arteriosus (Willisi)]. This so-called circle, which has really the form of a heptagon, 

 is formed, in front, by the anterior communicating artery uniting the anterior 

 cerebral arteries of opposite sides; laterally, by the internal carotids and the 

 posterior communicating arteries stretching between these and the posterior 

 cerebrals; behind, by the two posterior cerebrals diverging from the bifurcation of 

 the basilar artery (fig. 456). 



This free anastomosis between the two internal carotid and the two vertebral arteries 

 serves to equalise the flow of blood to the various portions of the brain; and, should one or more 

 of the arteries entering into the formation of the circle be temporarily or permanently obstructed, 

 it ensures a flow of blood to the otherwise deprived part through some of the collateral arteries. 

 Thus, if one carotid or one vertebral is obstructed, the parts supphed by that vessel receive 

 their blood through the circle from the remaining pervious vessels. Indeed, one vertebral 

 artery alone has been found equal to the task of carrying sufficient blood for the supply of the 



