ARTERIES OF THE BRAIN. 



487 



circle at the base of the brain, known as the Circle of Willis. 

 They are joined by the posterior communicating branches of the 

 iuternal carotid, and pass outwards round the crura cerebri 

 towards the transverse fissure, anastomosing with the middle 

 cerebrals, and are finally distributed over the posterior part of 

 the cerebrum. In their course, these arteries give off branches 

 which penetrate the interpeduncular space, and a branch dis- 

 tributed over the velum interpo- 

 situm. 



The Internal carotidartery 

 is one of the three terminal 

 branches of the carotid trunk. 

 It enters the cranial cavity 

 through the foramen lacerum 

 basis craiiii, and through the 

 cavernous sinus, where it forms a 

 double curve. The first of these 

 curves, which occupies the carotid 

 fossa in the sphenoid bone, has 

 its convexity looking forwards, 

 and the second has its convexity 

 looking backwards, so that the 

 two curves resemble the letter S. 

 After the second curve, a trans- 

 verse branch joins the right and 

 left trunks, and then the internal 

 carotid pierces the dura mater, 

 and, at the fissure of Sylvius, 

 divides into posterior communi- 

 cating and a branch which fur- 

 nishes the anterior and middle 

 cerebral arteries. 



The Posterior communica- 

 ting artery, which forms the 



lateral boundary of the circle of Willis, passes backwards by the 

 side of the interpeduncular space, and inosculates with the pos- 

 terior cerebral branch of the basilar artery. 



The Anterior cerebral branches are carried forwards to the 

 longitudinal fissure, where they converge and unite ; and, passing- 

 through the fissure, after a short course, the vessel divides into 

 two branches, which subdivide to supply the corpus callosum, the 



Fjg. lS-1. 

 Arteries at the base of the braiu. a, Internal 

 carotid ; a', Its superior ; a'', Its inferior 

 branch , b, Middle cerebral ; b', Artery of the 

 choroid plexus ; c, Transverse communicating ; 

 d, Anterior cerebral ; e e, Anterior meningeal ; 

 /, Branch anastomosing with meningeal brancli 

 of ophthalmic ; g rj, Basilar ; g' g', Posterior 

 cerebrals , h li, Cerebro-spiuals ; i, Middle 

 spinal ; k, Arteries of the medulla ; I I, Pos- 

 terior cerebellar ; m, Artery of internal ear ; 

 n, Anterior cerebellar ; o, Artery supplying 

 optic thalamus, corpora quadrigemina and 

 choroid plexus. 



