512 THE ARTERIES 



3. The anterior cerebellar — or anterior inferior cerebellar artery, as it is some- 

 times called — arises from the basilar soon after its origin, passes outwards and 

 l)ackwards across the pons, and then over the crus cerebelli to the front part of the 

 imder surface of the cerebellum. It anastomoses with the posterior inferior cere- 

 bellar artery (fig. 340). 



4. The superior cerebellar comes off from the basilar immediately behind its 

 bifurcation into the posterior cerebral arteries. It courses outwards and backwards 

 over the pons, in a curve roughly corresponding to that of the posterior cerebral 

 artery, from which it is separatecl by the third cranial nerve; but, soon sinking 

 into the groove between the pons and the crus cerebri, it curves round the latter 

 structure on to the upper surface of the cerebellum, lying nearly parallel to the 

 fourth nerve. Here it divides into two branches, an internal and external, (a) The 

 internal or superior vermiform branch courses backwards along the superior 

 vermiform process, anastomosing with its fellow of the opposite side, and at the 

 posterior notch of the cerebellum with the inferior vermiform branch of the poste- 

 rior inferior cerebellar artery. (6) The external or hemispheral branch runs 

 outwards to the circumference of the cerebellum, anastomosing with the external 

 branch of the inferior posterior cerebellar artery. 



Branches are given off from the main trunk of the superior cerebellar artery, or 

 from its internal branch to the valve of Vieussens, the optic lobes, the pineal 

 gland, and the choroid plexus. 



o. The posterior cerebral arteries are the two terminal branches into which 

 the basilar bifurcates at tlie upper border of the pons immediately behind the pos- 

 terior perforated space. Each artery runs at first outwards and a little forwards 

 across the crus cerebri immediately in front of the third nerve, Avhich separates it 

 frc^m the superior cerebellar artery. After receiving the posterior communicating 

 artery, Avhich runs backwards from the internal carotid, the posterior cerebral 

 turns backwards on to the under surface of the cerebral hemisphere, where 

 it breaks up into branches for the supply of the temporo-sphenoidal and occipital 

 lobes. 



The branches of the posterior cerebral artery ma}' be divided into the gan- 

 glionic or central, and tlie cortical or hemispheral. 



(a) The ganglionic or central branches, are divided into the postero-median ; 

 the posterior choroid; and the posterq-lateral. (i) The postero-median 

 branches come off from the posterior cerebral near its origin, and, passing 

 through the cerel)ral substance forming the posterior perforated spot, supply the 

 inner part of the optic thalamus and the walls of the third ventricle; (ii) the 

 posterior choroid branch passes through the transverse fissure to the velum inter- 

 l)ositum and choroid ]ilexus; (iii) the postero-lateral branches arise external to 

 the spot where the posterior cerebral artery is joined by the posterior communicat- 

 ing. They run to the posterior part of the optic thalamus and give branches to 

 the crus cerebri and optic lobes or corpora quaclrigemina. 



(b) The cortical or hemispheral branches are distributed as follows: — (i) the 

 uncinate, a branch to the anterior part of the uncinate convolution; (ii) the 

 temporal, a l)ranch to the inferior part of the temporal lobe; and (iii) the tem- 

 poro-occipital, a branch to the cuneus, lingual convolution, and outer surface of 

 the occipital lobe. 



Here it may be stated, not only in reference to the branches of the jiosterior 

 cerebral artery, l:)ut also with respect to the branches of the anterior and middle 

 cerebral arteries, Avhich are described with the internal carotid (page 503), that 

 there is no anastomosis between the cortical and central branches. The cortical and 

 the central form two distinct and separate systems. The cortical may or may not 

 anastomose with each other, but the comnumication between the neighl>ouring 

 cortical branches is seldom sufficient to maintain the nutrition of an area when the 

 vessel that normally supjJies it is obstructed. The central l)ranches are so-called 

 • ■nd-vessels and do not anastomose with each other. Hence obstruction of the 

 middle cerebral or Sylvian artery leads to softening of the area su])plied by its 

 central l)ranches, l)utnot always to softening of the region supplied by its cortical 

 l)ranches. Indeed the cortical region may escape completely, although the central 

 area is irreparably disorganised. The gross anastomosis of the posterior cerel)ral 



