2-_>6 



COMPARATIVE ANATOMY 



(Primates) even part of the cerebellum is hidden (Figs. 173, A 

 and B), although this is to a greater extent the case in some of 

 the lower Apes, with smooth hemispheres (e.g. Hapale, Chrysothrix), 

 than in Man. No satisfactory explanation has so far been given 

 for the different degrees of convolution seen amongst Mammals: 

 as a general rule, the brain in lower and smaller types (except, e.g. 

 in Echidna) is less convoluted than in higher and larger ones. 



The number of fibres radiating from the cortex (corona radiata) 

 is very small in lower types (e.g. Rodents), and largest in Man. A 

 complex network of fibres in the cortex itself connects its various 

 parts together, and other strong bundles extend through the 



MH 



Jiff 



FIG. 173A. HUMAN BRAIN. Median longitudinal vertical section. 

 (Mainly after Reichert. ) 



corpus callosum ; G, fornix, which extends antero-ventrally to the lamina 

 terminalis (Col), in the upper part of which is seen the anterior commissure 

 (Ca), and between the latter and the optic thalami (To) the foramen of 

 Monro (FM) ; H, pituitary body ; ////, cerebellum ; MH, corpora bigemina, 

 with the iter (Aq], anterior to which is seen the posterior commissure (Cp) ; 

 NH, medulla oblongata, with the pons Varolii (P) ; -ff, spinal cord; T, 

 infundibulum ; Teh, tela choroidea ; To, optic thalamus (diencephalon), with 

 the middle commissure (Cm); VH, cerebral hemisphere; Z, pineal bod}'; 

 /, olfactory nerve ; //, optic nerve. 



hemispheres connecting individual regions of the pallium with one 

 another. The commissures between the hemispheres known as 

 the corpus callosum and fornix (Fig. 173, A) are also much more 

 highly developed than in other Vertebrates. The former is an 

 important structure in the higher Mammalia, its development 

 corresponding to that of the pallium: ifc extends upwards and then 

 backwards from the region of the lamina terminalis in the form of 

 a thin plate, and reaches its highest development in Primates. 

 A corpus callosum is apparently wanting in Monotremes and 

 Marsupials, in which a hippocampal commissure is present in 

 the position of the body of the fornix, just above the anterior 



