450 THE BRAIN 



of the fornix will be seen, descending in the side wall. At 

 the anterior end of the side wall lies the interventricular 

 foramen which is bounded in front by the corresponding 

 column of the fornix. At the lower margin of the inter- 

 ventricular foramen the column of the fornix turns laterally 

 and disappears in the anterior part of the hypothalamus to 

 reach the mamillary body. 



The roof 'of the third ventricle is formed by a thin epithelial 

 layer which stretches across the median plane, from the one 

 taenia thalami to the other. It is applied to the under 

 surface of the tela chorioidea, which overlies the ventricle, 

 and is invaginated into the cavity by the chorioid plexuses 

 which hang down from the under surface of that fold of pia 

 mater. In the removal of the tela chorioidea the thin 

 epithelial roof was torn away (Figs. 175, 187). 



The third ventricle communicates with the lateral ven- 

 tricles, through the interventricular foramina, and it communi- 

 cates with the fourth ventricle by the aquceductus cerebri 

 (Sylvius], a narrow channel which tunnels the mesencephalon. 

 The opening of this canal will be seen in the posterior wall 

 of the third ventricle, immediately below the posterior com- 

 missure. The interventricular foramina, which put the third 

 into communication with the two lateral ventricles, lie at the 

 anterior part of the third ventricle, one on each side. Each 

 passes laterally and slightly upwards, between the most pro- 

 minent part of the corresponding column of the fornix and 

 the anterior tubercle of the thalamus. Through the foramina 

 the epithelial lining of the third ventricle becomes continuous 

 with that of the lateral ventricles (Figs. 177, 165, 166). 



From each interventricular foramen a distinct groove passes 

 backwards, on the side wall of the ventricle, to the mouth of 

 the aquaeductus cerebri. It is termed the sulcus hypothalamicus, 

 and it separates the ventral part of the boundary of the third 

 ventricle, which is called the hypothalamus, from the more 

 dorsally placed thalamus. 



The outline of the third ventricle is seen to be very irregular when 

 it is viewed from the side in a median section through the brain (Fig. 177), 

 or .as it is exhibited in a plaster cast of the ventricular system of the brain 

 (Fig. 165). It presents several diverticula or recesses. Thus, in the anterior 

 part of the floor there is a deep funnel-shaped recess, recessus infundilndi , 

 leading down, through the tuber cinereum, into the infundibulum of the 

 hypophysis. Another recess, recessus opticus, lies above the optic chiasma. 

 Posteriorly, two additional recesses are present. One, the recessus pinealis, 



