ARTERIAL CIRCULATION. 



487 



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

 They are joined by the posterior communicating branches of. the 

 internal carotid, and pass outwards round the crura cerebri 

 towards the transverse fissure, anastomosing with the middle 

 <;erebrals, 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 carotid artery 

 is one of the three terminal 

 Isranches of the common carotid. 

 It enters the cranial cavity 

 through the foramen lacerum 

 basis cranii, 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 



FiQ. 184. 

 Arteries at the base of the brain, a, Internal 

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

 branch ; 6, Jliddle cerebral ; b', Artery of the 

 choroid plexiis ; c, Transverse communicating ; 

 d, Anterior cerebral ; e e, Anterior meningeal ; 

 ./.Branch anastomosing with meningeal branch 

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

 cerebrals ; h h, Cerebro-spinals ; 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 qnadrigetnina and 

 choroid plexus. 



