THE BRAIN 361 



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 

 will he seen as a narrow cleft between the two hemispheres. 



xi. The hippocampal sulcus is a deep groove on the 

 surface of the temporal lobe. It gives rise to 

 the inwardly projecting fold of the hemisphere 

 which forms the hippocampus major. 



Bemove the temporal lobe completely, and the hAppocampal 

 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, 

 placed 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 geniculatuju. 



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 thaUmius, 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 

 its upper surface is a plexus of blood-vessels, 

 continuous with the choroid plexuses of the 

 lateral ventricles. 



