THE BRAIN 



159 



lutions (gyri) separated by grooves (sulci), the number of these being 

 greatest in man. By this a greater amount of cortical gray matter is 

 provided for, the cortex covering, not only the tops of the gyri, but 

 dipping beneath every sulcus. 



By the bending there are differentiated several regions or lobes in 

 the cerebrum (fig. 164). In front is the frontallobe, the occipital 

 lobe is at the posterior end of the brain, the parietal is between frontal 

 and occipital, and the temporal lobe is on the lower outer side, be- 

 hind and below the lateral fissure. This bending also involves the 

 lateral ventricles, portions (horns) entering each lobe. 



The two cerebral hemispheres are connected by commissures. In 

 all vertebrates there is one of these transverse bands (anterior com- 

 missure) in the lamina terminalis (fig. 158, ac), but as low as the am- 



I 



Fig. 165. — Diagram of the development of the corpus callosum and septum pellu- 

 cidum in man, after G. Elliott Smith. In A the hemisphere is outlined, ac, anterior 

 commissure; cc, corpus callosum; ep, epithelial roof of tnird ventricle; he, hippocampal 

 commissure; //, lamma terminalis; 0, olfactory lobe; oc, optic chiasma; p, paraterminal 

 body; s, septum pellucidum; vh, vestigial pre- and supracallosal hippocampus. 



phibia a second tract, the pallial commissure, appears dorsal to this, 

 which increases in size in the sauropsida and monotremes (fig. 181, 

 EC) . This serves, in the lower groups chiefly as a connexion between 

 the hippocampal areas of the two sides. In the higher mammals it 

 becomes differentiated into two commissures (fig. 165), a more dorsal 

 and anterior corpus callostun, and a more posterior and ventral hip- 

 pocampal commissure, the callosum being the connexion between the 

 cortical layers of the two sides of the cerebrum. As the cortex is 

 the seat of mentality, it follows that the callosum reaches its greatest 

 development in man. 



The development of the corpus callosum brings about a stretch- 

 ing of the gray matter (paraterminal body) immediately adjacent to 

 the terminal lamina, eventually expanding it, in the higher mammals, 



