THE BRAIN 



227 



commissure (Fig. 172, and cf. pp. 201 and 220), and the brain of 

 these forms remains at a comparatively low stage of development : 

 the anterior (basal) commissure is comparatively large, whereas 

 in the Eutheria its relative size is in inverse proportion to 

 that of the more important corpus callosum. In Edentates the 

 brain is also of a low type, and the same is true of that of Rodents, 

 Insectivores, and Bats, though a considerable advance is here 

 seen as compared with Marsupials. A large middle commissure 

 connects the two optic thalami. 



In addition to the lobes mentioned above, a central lole of the 

 hemispheres is present in Primates, and increases in development 

 in passing from the Gibbon, Orang, Chimpanzee, and Gorilla, up 

 to Man. But there is no justification for the statement that " the 

 human brain is only an enlarged anthropoid brain," for in the 



JUL 



FIG. 173B. CONVOLUTIONS OF THE HUMAN BRAIN. (After A. Ecker.) 



a,l,r, superior, middle, and inferior frontal gyri ; em, the calloso-marginal sulcus 

 on the dorsal surface ; FS, Sylvian fissure ; HH, cerebellum ; Lj\ frontal 

 lobe ; Lo, occipital lobe ; Lp, parietal lobe ; NH, medulla oblongata ; Po, 

 parieto-occipital fissure ; P, J n , superior and inferior parietal gyri separated 

 from one another by the interparietal fissure (/) ; P,, spinal cord ; T, tem- 

 poral lobe ; A', # I, anterior and posterior central convolutions, separated from 

 one another by the fissure of Rolando (A'); 1 to 3, superior, middle, and 

 inferior temporal convolutions. 



former a number of entirely new regions have been acquired, 

 especially as regards the frontal, temporal, and central lobes, which 

 have consequently undergone extension. 



In correspondence with the division of the hemispheres into 

 lobes, there is a differentiation of the lateral ventricles, 1 so that an 

 anterior, a posterior, and an inferior cornu can be distinguished 

 in each ; the inferior cornu extends into the temporal portion, 

 which corresponds to the hippocampal lobe of Reptiles, and an 

 eminence on its floor, the hippocampus major, 2 formed as an 



1 The ventricles are lined by epithelium (epeudyme), which, strengthened by 

 connective tissue layers derived from the pia mater (tc/a 1 choroidecK), also forms 

 the roofs of the third and fourth ventricles and extends into the lateral ventricles 

 as plexus choroi(l< i. 



"* The hippocampal system has important relations to the olfactory centre. 

 The (/tji-ux dt'-nfatiift (fascia dent at a) and i\\ejiml>ria arise in close connection with 

 the hippocampus, the fimbria having intimate relations to the fornix. 



