CHAP, ir.] THE BRAIN. 935 



median organ ; and attached to the ventral surface of the 

 ventricle, at the apex of a funnel-shaped projection, the infun- 

 dibulum, lies the pituitary body, also a remnant of important 

 ancestral structures. 



603. We may then divide the whole brain into a series of 

 parts corresponding to the main divisions of the embryonic brain. 

 At the front lie the cerebral hemispheres, with the lateral ventricles, 

 developed out of the cerebral vesicles ; and with these are asso- 

 ciated the corpora striata, the term cerebral hemisphere being 

 sometimes used so as to include these bodies, and sometimes so 

 as to exclude them. Next come, corresponding to the original 

 fore-brain, the parts forming the walls of the third ventricle, 

 conspicuous among which are the optic thalami ; for these 

 bodies though they appear to intrude into the lateral ventricles 

 belong properly to the third ventricle. In the mid-brain which 

 follows, the cavity, now the tubular passage of the aqueduct, is 

 roofed in by the two pairs, anterior and posterior, of corpora quad- 

 rigemina, the dimensions of which are not very great ; but a 

 thick floor is furnished by the crura cerebri. In each crus we 

 must distinguish between a dorsal portion called the tegmentum, 

 in which a large quantity of grey matter is present and in Which a 

 great complexity in the arrangement of fibres exists, and a ventral 

 portion, the pes or crusta, which is a much more uniform mass of 

 longitudinally disposed fibres. As the crura passing forward diverge 

 into the cerebral hemisphere on each side, the tegmentum ceases 

 at the hinder end and ventral parts of the optic thalamus ; it 

 is the pes which supplies the mass of fibres radiating into each 

 cerebral hemisphere. In a view of the ventral surface of the 

 brain, the base of the brain as it is frequently called, the crura 

 may be seen emerging from the anterior border of the pons. This 

 we have spoken of as the thickened floor of the front part of the 

 hind-brain, but in reality, it encroaches a little on the mid-brain, 

 the hind part of the corpora quadrigemina being in the same 

 dorso ventral plane as the front part of the pons (see Fig. 108). 

 In the main, however, the pons belongs to the fore part of the 

 hind-brain, the roof and sides of which are developed as we have 

 said into the cerebellum. This superficially resembles the cerebral 

 hemispheres in its large size, and in the special development of its 

 surface, which is formed of grey matter folded in a remarkable 

 manner and often spoken of as cortex. The cerebellum, though 

 the lateral portions, called the hemispheres, project above the 

 median portion, called the vermis, is, unlike the cerebrum, a 

 single mass ; each lateral half however sends down ventrally a 

 mass of fibres which, running transversely, partly end in the 

 pons and partly are continued across the pons into the other 

 lateral half; this mass of fibres, thus constituting as we have 

 said a considerable part of the pons, forms on each side, just as 

 it leaves the cerebellum to enter the pons, a thick strand, called 



