THE BRAIN. 389 



the two hemispheres, partially enclosed by the 

 corpus callosum and the fornix ; and is not, 

 properly speaking, 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 

 luill be seen as a narrow cleft between the two hemisplieres. 



xi. The hippocampal sulcus is a deep groove on that 

 surface of the temporal lobe which lies upon 

 the thalamencephalon. It lies within the 

 inwardly projecting fold of the hemisphere 

 which forms the hippocampus major. 



Bemove the temporal lobe completely, and the hippocampal 

 sulcus will be seen on its inner surface. 



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. 



On the outer and ventral surface of each 

 optic thalamus, near its anterior end, is a 

 rounded elevation, the corpus geniculivtum. 



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 velum interpositum is a thin membrane 

 forming the roof of the third ventricle. On 



