THE BRAIN 225 



The cerebellum consists of a well-developed and folded median 

 lobe, and of two lateral portions (flocculi), which vary much both 

 in form and size. Posteriorly it completely covers the fourth 

 ventricle. The two optic lobes, in which, as in certain Reptiles, a 

 subdivision is indicated, are separated from one another and 

 pressed downwards, so. as to lie at the sides of the brain in the 

 angle between the hemispheres, cerebellum, and medulla oblongata, 

 and they are connected by a broad commissure. The ventral side 

 of the short medulla shows a marked flexure, bending upwards to 

 the spinal cord. 



Mammals. The brain in embryo Mammalia is very similar 

 to that of the Sauropsida, but the subsequent differentiation 

 of its parts, and more particularly that of the pallium, gives it a 



epi 



C.<j 



cbl 



FIG. 172. LONGITUDINAL SECTION OF BRAIN OF ROCK WALLABY (Petrogale 

 penicillata). (From Parker and Has well's Zoology.) 



anf.com, anterior commissure ; ell, cerebellum ; c.mam, corpus mammillare ; c.qu, 

 optic lobes ; critr, crura cerebri ; epi, epiphysis, with the posterior com- 

 missure immediately behind it; f.mon, position of foramen of Monro ; 

 hijj.com, hippocampal commissure, consisting here of two layers, continuous 

 at a posterior bend, the splenium, somewhat divergent in front where the 

 septum lucidum extends between them ; hypo, hypophysis ; med, medulla 

 oblongata ; mid.com, middle commissure ; olf, olfactory lobe ; opt, optic 

 chiasma ; vent. 3, third ventricle. 



very special character. The cerebral cortex becomes much more 

 highly developed and in many Mammals is more or less markedly 

 convoluted l (Figs. 171 and 173, B). In others, again, the 

 surface of the hemispheres remains smooth (Fig. 170), but a 

 subdivision into lobes (frontal, parietal, occipital, and temporal), as 

 well as certain fissures (e.g. rhinal, hippocampal, callosal) can 

 always be recognised to a greater or less extent, and the 

 hemispheres are relatively so large as to cover over the more 

 posterior parts of the brain ; in some of the lower forms, the mid- 

 brain can still be seen from above, while in the higher types 



1 It is only possible to homologise the main sulci to a greater or less extent 

 amongst the various types of convolution seen in the Mammalian brain (cf. p. 228). 



