THE BRAIN 347 



corpus callosum is reached. Hold the brain in your hand 

 while cutting it, and keep both the brain and the razor wet 

 with spirit. 



i. The structure of the hemisphere. 



a. The outer layer or cortex is a superficial in- 

 vestment of grey matter, about an eighth 

 of an inch thick. 



/?. The central or medullary portion consists of 



white matter, i.e. of medullated nerve-fibres. 



y. The fibres of the corpus callosum spread out 



into, and are continuous with, the white 



matter of the hemisphere. 



ii. The lateral ventricle, or cavity of the hemisphere. 



Cut through the corpus callosum on the right side, so as to 

 open the lateral ventricle beneath it. Lift up the edge of 

 the corpus callosum with forceps, and carefully cut it away, so 

 as to expose the ventricle fully, leaving a narrow strip along 

 its inner edge. 



The lateral ventricle, opposite the middle 

 of the hemisphere, is wide from side to side, 

 but shallow dorso-ventrally. Anteriorly, it 

 extends forwards as a narrow vertical slit, 

 the anterior cornu, which lies nearer the inner 

 than the outer surface of the hemisphere. 



Posteriorly, the lateral ventricle curves 

 backwards and inwards towards the hinder 

 end of the hemisphere as the posterior cornu ; 

 and from its outer side, a little behind the 

 middle of the hemisphere, a descending 

 cornu runs outwards and downwards, and 

 then curves round in the temporal lobe to the 

 ventral region, running forwards and inwards 

 almost to the anterior end of the lobe, and 

 ending opposite the infundibulum. 



Open up the descending cornu along its whole length, by 

 carefully cutting away the outer and ventral walls of the 

 temporal lobe. 



