126 



THE LATERAL VENTRICLES. 



plexus in situ it is a deep sulcus, i.e., an invagination only, of the thin hemisphere- 

 wall (here formed of the ventricular epithelium alone). It is known as the choroidal 

 fissure, and appears at an early period of embryonic development. 



Merkel and Mierzejewsky have described an actual cleft in the pia mater along the descend- 

 ing horn which effects a communication between the ventricle and the subarachnoid space, 

 analogous to the foramen of Majendie and the lateral apertures in the fourth ventricle. This 

 observation has not hitherto been confirmed. 



Fig. 90. Two VIEWS OF A PLASTER CAST OP THE CAVITIES OF THE CEREBRAL VENTRICLES. 



(After Welcker.) 



A, from above ; B, from the side. The projections into the cavities of the structures which 

 bound the ventricles are seen as impressions upon the cast. 



At the junction of the descending with the posterior cornu, and occupying the 

 angle between the hippocampus major and the calcar, is a triangular space, the 

 floor of which is mainly occupied by a smooth, somewhat raised surface, which 

 extends backwards into the posterior cornu, and is often continued downwards into 

 a longitudinal eminence which passes for some distance in the inferior horn alongside 



