216 A MANUAL OF ANATOMY. 



portion into the fornix. See Development, page 173 and 

 Diag. 1 6. 



The septum lucidum consists of a double layer for some 

 extent ; the small, oval, slit- like cavity between the two layers 

 is the fifth ventricle. This ventricle is not, in all probability, 

 a portion of the foetal longitudinal fissure which has become 

 included beneath the corpus callosum, but is formed within 

 the originally solid septum lucidum by a process of vacuola- 

 tion. 



The fifth ventricle has no communication with, nor is it 

 formed in any like manner to, the rest of the ventricular 

 cavities of the brain. 



The Pornix. Figs. 29, 31, 32, 35, 38, 39, 40, 43 to 47 

 inclusive. 



The fornix is a double band of fibres, one on each side 

 of the median line, separated in front and behind but coming 

 together in the middle, where they fuse for a short distance 

 and form the body. The anterior and posterior separated 

 portions are called the anterior and posterior pillars re- 

 spectively. 



The fornix begins at the base of the brain, as the anterior 

 pillars, in the corpora albicantia. The pillars ascend nearly 

 vertically and at the anterior extremity of the third ventricle 

 until they nearly reach the under surface of the corpus 

 callosum, then they turn suddenly backward (the genud) 

 (leaving an aperture extending into the third ventricle, 

 which is the foramen of Monro), become united to each 

 other, forming the body of the fornix, and to the under 

 surface of the corpus callosum. After a short distance 

 they again diverge, and, as the posterior pillars, pass outward 

 and downward to the outer surface of the hippocampus 

 major, with which they become blended. 



