170 CLINICAL APPLIED ANATOMY. 



owing to variations in the degree of collateral circulation which 

 is possible. When perforating arteries are occluded, softening of 

 the parts they supply is inevitable ; on the other hand the 

 anastomosis of cortical branches in the pia mater may be sufficient 

 to maintain the integrity of the cortex. 



The position of the anterior communicating artery at the 

 commencement of the anterior cerebral arteries explains how 

 softening of the head of the caudate nucleus may be the only 

 result of a plug in the first part of one of these vessels. When 

 the middle cerebral artery is occluded near its origin softening 

 may be expected in the central ganglia and at the anterior 

 perforated spot. The amount of cortical softening is rarely 

 co-extensive with the distribution of the vessel, and in some 

 cases there may be none. The cortical collateral circulation 

 appears to vary within considerable limits. The distribution of 

 the cortical branches of the middle cerebral artery to Broca's 

 convolution, to the precentral or ascending frontal convolution 

 which contains the motor centres, to the higher visual cortex and 

 to the auditory centre render this artery of great importance. 

 The central or perforating branches of the same vessel supply 

 the neighbourhood of the internal capsule. It may here be noted 

 that the leg centre is in part supplied by the anterior cerebral 

 artery and the posterior part of the internal capsule receives 

 some branches from the posterior cerebral trunk. 



The collateral circulation of the cortical branches of the 

 posterior cerebral trunks is usually sufficient to prevent much 

 softening in their area of distribution. These important vessels 

 supply the optic radiation and the occipital visual cortex. They 

 also supply the visual portion of the thalamus, the anterior and, 

 in part, the posterior corpora quadrigemina. The relation of 

 their branches to the posterior limb of the internal capsule has 

 been mentioned. The crus cerebri receives part of its supply 

 from the same source. 



The optic tract is supplied by the anterior choroidal branch of 

 the internal carotid. The cerebellar arteries anastomose so freely 

 that, even if they are occluded, any extensive softening is rare. 



