504 



THE ARTERIES 



(h) The hemispheral or cortical branches come off from the middle cerel)ral 

 opposite the insula. They are four in number: — (i) The inferior, or orbito- 

 frontal, to the inferior frontal convolutions, (ii) The ascending frontal, to the 

 lower part of the ascending frontal convolution, the upper part being sui^plied by 

 the margino-frontal, a branch of the anterior cere})ral. (iii) The parietal, to the 

 •whole of the ascending parietal and the adjacent part of the inferior ])arietal convo- 

 lution, (iv) The parieto-temporal, to the convolutions around the posterior 

 limb of the fissure of Sylvius — viz. the supra-marginal, the angular, and the 

 posterior part of the inferior i»arietal above, and the first and the greater part of the 

 second and third temporal Ijelow. 



The Circle of "Willis 



The anastomosis of the l)ranches of the internal carotid and vertebral arteries 

 at the base of the brain is known as the circle of Willis (fig. 340). This so-called 



Fig. 340.— The Arteries of the Brain. 



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

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



Anterior cerebral 

 artery 



Middle cerebral 



artery 

 Internal carotid 



artery 

 Postero-median 

 per/oratitig 

 Posterior cerebral 

 artery 

 Superior cerebellar 

 artery 



Anterior inferior 

 cerebellar artery 



Vertebral artery 



Posterior inferior 

 cerebellar artery 



A n te.rior com- 

 municating 

 artery 



Antero-lateral 

 jierforalinrj 



Anterior choroid 



Po.iterior 

 commimicaling 

 artery 



I'osterior choroid 



Basilar artery 

 Cms cerebelli, cut 



Anterior sj)inal 

 artery 



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, Ijy the trunk of the internal carotid and the posterior communicating 

 arteries stretching between it and the posterior cerebral; behind, by the two 

 posterior cerebrals diverging from the bifurcation of the basilar artery (page 543). 

 Tliis free anastomosis l>etween the two internal carotid and the two vertebral 

 arteries serves to equalise the flow of blood to the various ] tortious of the brain; 

 and, should one or more of the arteries entering into the formation of the circle l)e 

 temi)orarily 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 ol)structed, the i)arts supplied by that vessel receive their l)lood 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 



