THE CEREBRAL ARTERIES 121 



this artery gives rise to the condition of homonymous 

 hemianopia (p. 52). 



The Arterial Circle (of Willis) brings the six cerebral 

 arteries into communication with one another. As the two 

 anterior cerebrals lie side by side in the longitudinal fissure, 

 they are connected to one another by the anterior communi- 

 cating artery, which thus links up the two carotid systems. 

 Each internal carotid artery is connected to the posterior 

 cerebral of its own side by the posterior communicating artery, 

 and thus the two carotid systems are linked up with the 

 basilar system. This arterial anastomosis provides a means 

 for the re-establishment of the circulation when any of the 

 great cerebral blood-vessels is obstructed outside the skull. 



When emboli are carried along the internal carotid artery 

 into the skull, they usually pass into the middle cerebral 

 artery, owing to the abrupt bend which the anterior cerebral 

 makes at its origin. In cortical lesions due to this cause, 

 the prognosis is good and almost complete recovery may be 

 expected, because the cortical branches of the cerebral arteries 

 anastomose with one another, although not with any degree 

 of freedom On the other hand, the central branches are all 

 end-arteries, i.e. they do not establish any anastomoses with 

 one another, and, on this account, obstruction to the blood- 

 supply of any central area will, unless very transient, always 

 be followed by necrosis of practically the whole of that area. 



The Veins of the Brain are divided into a superficial and 

 a deep group. The superficial veins lie in the subarachnoid 

 space and terminate in the various cranial blood-sinuses. 

 They drain the cerebral cortex and communicate very freely 

 with one another. The deep veins drain the substance of the 

 brain and eventually enter the internal cerebral veins or the 

 great cerebral vein (of Galen) (p. 27). 



Classification of Sensory Nerves. Head and Sherren 

 have pointed out that the afferent fibres of cerebro-spinal 

 nerves may be subdivided into three groups : (a) Those which 



