THE EVOLUTION OF THE BRAIN 99 



(Fig. 3a). On closer study, however, the brain of man reveals 

 in all particulars the same ground plan of structure as that 

 seen in the frog, the chief point of difference being the 

 relative development of its several parts. If the cerebellum 

 and the hemispheres of a human brain are cut off, the 

 stem that is left reproduces in many respects the essentials 

 of the frog's brain (Fig. 3 b and c). The human spinal cord 

 enlarges at its front end to form in the brain stem of man 

 the medulla oblongata, as it does in the frog. In place of the 

 small tongue-like cerebellum in the frog man possesses a 

 complex and much convoluted cerebellum of relatively 

 large size. The medulla and the cerebellum in man together 

 represent a positional brain as they do in the frog. Above 

 this section of the brain in man but quite hidden from view 

 is a pair of optic lobes forming a part of the so-called corpus 

 quadrigeminum of human anatomy. These lobes mark the 

 terminations of many of the optic nerve fibers and represent 

 the visual brain of the frog. In front on the underside of the 

 hemispheres of the human brain are the olfactory tracts 

 and lobes connected by nerves with the nose. They 

 correspond to what has been called the olfactory brain in the 

 lower animals and are entirely overshadowed in man by 

 his enormous hemispheres. 



Thus all the important parts in the brain of the frog recur 

 in proper relations in the stem of the human brain, but the 

 human brain differs from that of the frog in the very con- 

 siderable development of its cerebellum and particularly 

 of its hemispheres. This excessive growth of these two 

 parts can be traced step by step in the animals intermediate 

 in position between the frog and the higher mammals. 

 In the frog and its relatives the stem of the brain and the 

 three functional regions already pointed out are all clearly 

 open to view from above. In reptiles the cerebellum and the 

 hemispheres are relatively larger than in the frog, but they 

 do not cover up in any important way the stem of the brain. 

 In the lower mammals, such as the rabbit, the cerebellum 

 and the hemispheres have enlarged sufficiently to cover 

 most of the stem so that from above little of the medulla 

 oblongata and none of the optic lobes can be seen. Finally 

 in man the hemispheres have so far exceeded in growth 



