178 A MANUAL OF ANATOMY. 



but is excluded from the ventricular cavity by the remains of 

 the roof of the middle vesicle, which persists as a thin layer 

 of epithelial cells lining its under surface. Further, the 

 lateral margins of the velum become converted into a 

 plexus of capillary vessels, to which the name of choroid 

 plexus is given. These lie within the hollow of the lateral 

 ventricles, but are separated from them by the continuation 

 of the layer of epithelial cells which lines the velum. 

 These cells cover in all the extensions of the capillary loops. 



In an exactly similar manner is the choroid plexus of 

 the fourth ventricle developed from the portion of the 

 primitive vascular membrane which covers in the roof of 

 the hinder portion of this ventricle. At the adult stage, 

 after the cerebellum has been formed, this vascular layer 

 reaches from the under surface of the cerebellum to the 

 upper surface of the medulla. It is lined throughout its 

 extent by the attenuated remains of the original roof of 

 this third cerebral vesicle. The layer of cells is applied to 

 the membrane so as to cover in completely (exceptions 

 below) all the festoons of capillaries which have been 

 formed from it. 



Remember that this portion of the roof of the fourth 

 ventricle is perforated by the foramen of Majendie close to 

 the medulla and in the middle line, and by the foramina of 

 Key and Retzius at its lateral extensions. 



\^ 



The Cerebrum. Figs. 24 to 28. 



The cerebrum consists of the great mass of the brain 

 situated at its anterior and superior part. It is the greatly 

 overgrown forebrain. 



Its shape is ovoid, flattened below, and partially divided 

 in two symmetrical parts by the great longitudinal fissure. 

 These halves are called hemispheres, right and left. 



