CHAP, x.] THE DEVELOPMENT OF THE GAT. 359 



INFLEXION. At first they follow one another in a straight line. 

 Afterwards the fore-brain and mid-brain bend downwards and for- 

 wards over the front end of the notochord, while that part of the 

 hind-brain which forms the cerebellum, bends upwards upon the 

 portion which forms the medulla. 



The various parts which form themselves out of the three primary 

 hollow vesicles, grow at very different rates. The cavities of the 

 three vesicles and the olfactory lobes remain relatively small, but 

 the increase in bulk of the cerebral hemispheres is much greater than 

 that of the other parts. Moreover these cerebral hemispheres become 

 connected together by an outgrowth of nerve substance from their adja- 

 cent sides a little above the fornix, such outgrowth resulting in the 

 formation of a mass of transverse connecting fibres, which become 

 the CORPUS CALLOSUM. This transverse growth comes thus to enclose 

 that space which was originally the deepest part of the great longi- 

 tudinal fissure, which space is, of course, bounded on each side by a 

 part of the inner wall of one of the cerebral hemispheres. These 

 portions of such inner walls become exceedingly thin and form 

 together the septum lucidum, the space included between them and 

 which is bounded above by the corpus callosum, becoming the fifth 

 ventricle. It is plain, therefore, that the fifth ventricle is quite 

 different in nature and origin from all the other ventricles of the 

 brain, since these latter are remnants of, or outgrowths from, the 

 primitive medullary groove and canal of the embryo, while the fifth 

 ventricle, on the contrary, is as it were a portion of outside space 

 which has been enclosed and taken into the brain by means of those 

 inward lateral outgrowths which form the corpus callosum. 



The FORNIX is formed by two sets of longitudinal fibres, which are 

 developed (one bundle on each side,) from the upper margin of the 

 foramen of Monro, while the median junction of these two lateral 

 bundles form the so-called body of the fornix. Thus the fornix is the 

 median part of what was originally the back or underside of the 

 cerebral hemispheres, its body forming part of the outer wall or bag 

 of the cerebral hemispheres enclosing the lateral ventricles. As 

 these hemispheres grow backwards, the two prolongations of the 

 fornix (or its posterior pillars,) extend (following the course of the 

 developing " temporal lobe,") backwards and downwards, one in the 

 wall of each hemisphere, as a bundle of longitudinal fibres, such 

 bundle projecting into the descending course of the lateral ventricle 

 as part of the hippocampus major. Its anterior pillars descend in the 

 lamina terminalis to the corpora inammillaria. 



The cerebral hemispheres, with the fornix as part of their wall, 

 having grown backwards over the mid-brain containing the third 

 ventricle, the space between the roof of that ventricle, the VELUM 

 INTERPOSITUM, and the body of the fornix which comes to be applied 

 to the velum, is thus essentially the outside of the brain, though as 

 a fact it is actually in the very middle of that great complex struc- 

 ture, the brain of the adult. The velum consists only of ependyma, 

 the pia mater and the arachnoid. The vascular membranes called 



