148 



COMPARATIVE 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, 

 giving rise to gyri separated by sulci, which serve to increase 



J/JT 



Jt 



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

 (Mainly after Reichert.)' 



VH, cerebrum ; To, optic thalamus (thalamencephalon), with the middle commissure 

 (Cm) ; 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 (Cp) ; HH, cerebellum ; NH, medulla oblongata, with the 

 pons Varolii (P) ; R, spinal cord ; B, corpus callosum ; G, foruix, which extends 

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

 the anterior commissure (On), and between the latter and the optic thalami (To) 

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

 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 Monotremes and Marsupials, only the part 

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

 part which is the first to appear in the embryo of the latter. The relative size of the 

 anterior commissure is in inverse proportion to that of the corpus callosum. 



2 Corresponding with this division into definite lobes there is also a ma Ho <! 

 differentiation of the lateral ventricles, so that an anterior, a posterior, and an 

 inferior cornu can be distinguished in each. 



