THE CEREBRUM 15 



below the fornix. Its free, medial surface (Fig. 5) forms 

 the lateral wall of the third ventricle and is covered by 

 ependyma, which can be traced downwards and backwards 

 to the cerebral aqueduct (of Sylvius). The anterior extremity 

 of the thalamus is separated from the column (anterior pillar) 

 of the fornix by the Interventricular Foramen (of Monro), 

 through which the ependyma of the third ventricle passes 

 to become continuous with the ependyma lining the lateral 

 ventricle. 



A fold of pia mater, termed the tela chorioidea (velum inter- 

 positum), is carried into the interior of the brain below the 

 splenium of the corpus callosum. It is situated between the 

 inferior aspect of the fornix and the superior aspects of the 

 thalami, and extends as far forwards as the interventricular 

 foramen. In the median plane, the' thalami are separated 

 from one another by the third ventricle, the roof of which is 

 formed by the tela chorioidea as it crosses from one side to 

 the other. The large veins which return the blood from the 

 substance of the brain are situated between the two layers 

 of the tela chorioidea, and they emerge at its posterior edge 

 between the splenium above and the dorsal aspect of the 

 mid-brain below (Fig. 7). 



In a median sagittal section of the brain, the mid-brain is 

 divided immediately below the thalamus (Fig. 5). It is 

 traversed, near its dorsal aspect, by the cerebral aqueduct 

 (of Sylvius), which becomes greatly dilated behind the lower 

 part of the pons and the upper part of the medulla oblongata, 

 forming the fourth ventricle. 



The Basal Surface of the Brain 



The Olfactory Tract lies on the inferior aspect of the frontal 

 lobe near the median plane. Its anterior extremity is enlarged 

 to form the olfactory bulb, which is joined by the olfactory 

 nerves from the mucous membrane of the nose. Congenital 



