THE FIFTH VENTRICLE. Ill 



the stalk of the pineal body to the opposite nucleus habenulae 

 and constitute the commissura habenularum. The columna 

 of the fomix is joined by a small fasciculus from the medial root 

 of the olfactory tract, which runs backward to the hippocampus 

 and uncus. 



The upper surface of the body of the fornix is convex from 

 before backward (Figs. 29 and 36). It forms the postero-medial 

 part of the floor of the lateral ventricle. Along the median line, 

 it is joined to the corpus callosum by the septum pellucidum. 



The septum pellucidum (Figs. 29, 35, 40 and 73), a double- 

 walled median partition, divides the superior chamber of the 

 fore-brain cavity into lateral halves, the lateral ventricles. The 

 septum pellucidum is crescentic in outline. Its convex border 

 fits into the concave surface of the body, genu and rostrum of the 

 corpus callosum. Its concave border rests upon the fornix. 

 Between the rostrum of the corpus callosum and the anterior 

 commissure, the septum pellucidum is continuous \\ith the gyms 

 subcallosus with which it is associated in development and func- 

 tion. 



The septum pellucidum, like the anterior commissure, corpus 

 callosum and fornix, is developed from the thickened upper border 

 of the lamina terminalis and the adjacent medial wall of the 

 cerebral hemisphere in front of the interventicular foramen. 

 These several structures extend upward and backward with the 

 development and rotation of the hemispheres and, together, 

 roof over the inter- brain. A lymph space, the cavum septi pellucidi, 

 appears in the septum and is commonly called the fifth ventricle. 

 The fore-brain cavity thus embraces four ventricles, viz.: 



Two lateral ventricles (the ventricles of the hemispheres), 



Fifth ventricle (the ventricle of the septum), and 



Third ventricle (ventricle of the inter-brain). 



THE FIFTH VENTRICLE. 



(Cavum Septi Pellucidi.) 



This is the ventricle of the septum (Figs. 35 and 73). The 

 fifth ventricle is a very narrow, antero-posterior cleft between 

 the walls of the septum pellucidum, with which it coincides in 



