258 ZOOTQMY. 



LVI. Separate the hemispheres from one another by 

 gently pressing them outwards, noting the absence 

 of any connection between them : make a small 

 incision in the inner wall of one of them near its 

 postero-dorsal corner, so as to lay bare its cavity 

 ( 357) : carefully cut away enough of the inner 

 and posterior walls of the hemisphere to expose 

 the whole of this cavity : also remove the cere- 

 bellum by cutting through its lateral attachments 

 to the medulla oblongata ( 354) : note : 



354. The attachment of the cerebellum to the medulla by 

 strong, paired pillars of white matter, the peduncles (Fig. 

 59, A, p) : from its attachments the cerebellum extends 

 backwards over the medulla and forwards between the 

 optic lobes. 



355. The optic commissure (<?.*), a flat, horizontal 

 band of white fibres uniting the optic lobes beneath the 

 anterior prolongation of the cerebellum. 



356. The thalamencephalon (the), lying immediately 

 in front of the optic commissure, and hidden in the entire 

 brain by the meeting of the cerebral hemispheres with the 

 cerebellum : it consists of small paired masses of grey 

 matter, the optic thalami, which, if the dissection has 

 been carefully conducted, will be seen to be covered by a 

 thin sheet of pia matter full of blood-vessels, the velum 

 interpositum : this is best seen in a fresh brain. 



357. The lateral ventricle, or cavity of the cerebral 

 hemisphere, an irregular chamber with very thin inner, 

 upper, and posterior walls, but having its floor thickened 

 and its dimensions greatly diminished by a large mass of 

 white matter, the corpus striatum (c.s). 



358. The third ventricle (e'-3), a vertical cleft between 

 the optic thalami, roofed over by the velum interpositum, 



