ix.] THE NERVOUS SYSTEM. 375 



The hind-brain sends out an outgrowth above, which is the 

 cerebellum. Its upper wall becomes excessively thin, a mere 

 delicate layer of epithelium, which roofs its cavity the fourth 

 ventricle. 



The anterior end of the first vesicle (or fore-brain) becomes 

 the lamina terminalis. On each side of it another vesicle 

 grows out, which is one of the cerebral hemispheres, and the 

 aperture of communication is the future foramen of Munro. 



From the anterior part of the floor of each cerebral hemi- 

 sphere yet another vesicle buds forth, which is the future 

 olfactory lobe (or nerve), the cavity of which becomes ob- 

 literated in the adult. 



The three original hollow vesicles and the olfactory lobes 

 remain small, but the cerebral hemispheres grow out of all 

 proportion to the other parts. They also become united 

 together by an outgrowth of transverse connecting fibres (the 

 corpus callosum), which outgrowth, by this mode of develop- 

 ment, comes to enclose what was originally the deepest part 

 of the great longitudinal fissure. The space thus enclosed is, 

 of course, bounded on each side by part of the inner wall of 

 one of the cerebral hemispheres. These parts of the inner 

 cerebral walls become however excessively thin, and the two 

 parts together form the septum lucidum, while the space 

 enclosed between them becomes the fifth ventricle. Thus 

 the fifth ventricle is quite different in its nature from all the 

 other ventricles of the brain, it being taken in as it were 

 from outside space, while all the others are either remnants of 

 the primitive embryonic dorsal groove and canal, or (as the 

 lateral and primitive olfactory ventricles) outgrowths irom 

 and extensions of such. 



9. We may now better understand the nature of some ot 

 the parts before noticed. 



The fornix is the median part of what was originally the 

 back of the hemispheres. It (together with the lyra which 

 joins the two diverging and posterior portions of the fornix), 

 really forms part of the outer wall or bag of the cerebrum, 

 enclosing the lateral ventricles each half of the fornix 

 belonging to one of the hemispheres. As these grow back- 

 wards, the fornix looks more and more downwards, following 

 the course of the developing " temporal lobe." 



Beneath the fornix is the roof of the third ventricle, i.e. 

 the velum interpositum, and the space between the upper sur- 

 face of this velum and the under surface of the closely applied 

 lyra is morphologically the outside of the brain, though in 



