( '< SEPARATIVE ANATOMY. 



Mammals. 



While in many cases (e.g. Marsupials, Rodents, and Insecti- 

 vores) the mid-brain lies more or less freely exposed, in the series 

 of the Primates the hemispheres gradually come to cover all the 

 other parts of the brain. The presence of large commissures be- 

 tween the hemispheres the corpus callosum 1 and fornix 

 is very characteristic of Mammals: the hemispheres are also differ- 

 entiated into lobes, which are usually more or less convoluted, 

 to gyri separated by sulci, which serve to increase 



gvng rse 



IfJT 



Jl 



Fto. 123. HUMAN BRAIN. (Median longitudinal vertical section.) 

 (Mainly after Reichert. )<. 



J'ff, cerebrum ; To, optic thalamus (thalamenceplialon), with the middle commissure 

 (''//>); Z, pineal gland; T, infundibulum ; H, pituitary body; MH, corpora 

 bigemina, with the aqueduct of Sylvius (Aq), anterior to which is seen the 

 posterior commissure (Uji) ; HH, cerebellum ; NH, medulla oblongata, with the 

 lions Yarolii (P) ; R, spinal coi-d ; B, corpus callosum ; G, fornix, which extends 

 antero-ventrally to the lamina terminalis (Co!), in the upper part of which is seen 

 the antcri'ii- commissure (Ca), and between the latter and the optic thalarni (To) 

 the foramen of Mouro (FM) ; Teh, tela choroidea ; /, olfactory nerve ; //, 

 optic nerve. 



the superficial area, The amount of convolution varies much 

 in the different orders: thus in the brain of Primates frontal, 

 parietal, occipital, temporal, and central lobes may be dis- 

 tinguished. 2 The central lobes correspond to the region described 

 above as the basal portion of the prosencephalon. 



1 The corpus callosum is very small in Monolivnn^ and Marsupials, only the part 

 corresponding to the anterior geini of higher types being developed, and this is the 

 purl which is the lirst to appear in the embryo ol' the hit lev. The relative size of the 

 aMierinr commissure is in inverse proportion to that of the corpus callo>mn. 



'-' i ''i] -responding with this division into definite lobes there is also a mark id 

 dilleren fiat inn of t lie I ii.-r.-il ventricles, so tint an anterior, a posterior, and an 

 in ferior eornu ean be distinguished in each. 



