3G4 



THE CEREBRAL HEMISPHERES. 



of the descending cornu has become transformed into a simple 



f, 



FIG. 201. TKANSVEKSE SECTION THROUGH THE BKAIN OF A SHEEP'S EMBRYO 



OF 2 - 7 CM. IN LENGTH. (From Kolliker.) 



The section is taken a short distance behind the section represented in fig. '200, 

 and passes through the posterior part of the hemispheres and the third ventricle. 



at. corpus stiiatum; tli. optic thalamns; to. optic tract; t. third ventricle; d. roof 

 of third ventricle; c. fibres of cerebral peduncles; c'. divergence of these fibres into the 

 walls of the hemispheres; e. lateral ventricle with choroid plexus pi; h. cornu am- 

 monis; /. primitive falx; am. alisphenoid; . orbito-sphenoid; sa. piesphenoid; p. 

 pharynx; ink. MeckeFs cartilage. 



process of the lateral ventricle the hippocampus major forms a 

 prominence upon its floor. 



The wall of the lower fold becomes very thin, and a vascular 

 plexus, derived from the connective-tissue septum between the hemi- 

 spheres, and similar to that of the roof of the third ventricle, is formed 

 outside it. It constitutes a fold projecting far into the cavity of the 

 lateral ventricle, and together with the vascular connective tissue in 

 it gives rise to the choroid plexus of the lateral ventricle (figs. 200 

 and 201 pi). 



It is clear from the above description that a marginal fissure 

 leading into the cavity of the lateral ventricle does not exist in the 

 sense often implied in works on human anatomy, in that the epi- 

 thelium covering the choroid plexus, which forms the true wall of the 

 brain, is a continuous membrane. The epithelium of the choroid 

 plexus of the lateral ventricle is quite independent of that of the 

 choroid plexus of the third ventricle, though at the foramen of Munro 

 the roof of the third ventricle is of course continuous with the inner 

 wall of the lateral ventricle (fig. 200 s). The vascular elements of 

 the two plexuses form however a continuous structure. 



