THE BRAIN. 395 



the corpus callosum and above the fornix, and is 

 bounded laterally by the inner walls of the two 

 hemispheres (septum lucidum). 



It is merely a part of the space between the 

 two hemispheres, partially enclosed by the corpus 

 callosum and the fornix ; and is not, properly speak- 

 ing, a ' ventricle ' at all. 



To expose the fifth ventricle from above, carefully remove 

 the narrow strip of the corpus callosum that has been left along 

 the inner side of the hemisphere, and the fifth ventricle will be 

 seen as a narrow cleft between the two hemispheres. 



b. The fore-brain, or thalamencephalon. 



i. The optic thalami are a pair of large lateral 

 thickenings of the sides of the fore-brain, 

 immediately in front of the optic lobes, 

 and overlapped by the temporal lobes of the 

 hemispheres. They are separated by grooves 

 from the corpora striata in front and the optic 

 lobes behind. 



ii. The optic tracts are a pair of narrow bands, 

 which can be traced backwards and upwards 

 from the outer ends of the optic chiasma, 

 over the surface of the optic thalami, to the 

 anterior borders of the optic lobes. 



Take hold of the anterior end of the right optic tract with 

 the forceps, and strip it off the optic thalamus, so as to see its 

 origin from the optic lobe. 



iii. The pineal body is connected by a tubular stalk 

 with the hinder end of the thin roof. 



Remove the roof of the thalamencephalon so as to open the 

 third ventricle. 



iv. The third ventricle, or cavity of the fore-brain, 

 is a median vertical cleft between the optic 

 thalami. It is very narrow from side to side, 

 and communicates through the foramina of 

 Monro with the lateral ventricles. 



