ANATOMY OF THE NERVOUS SYSTEM. 167 



bral hemisphere is an olfactory lobe, inconspicuous in man 

 but in many animals larger than the cerebral hemispheres. 

 Buried in the fore-brain on each side are two large gray 

 masses, the corpora striata and optic thalami. The mid- 

 brain forms a connecting isthmus between the two other 

 divisions and presents on its dorsal side four hemispherical 

 eminences, the corpora quadrigemina. On its ventral side 

 it exhibits two semicylindrical pillars (seen under the nerve 

 IV. in Fig. 77), known as the crura cerebri. The hind- 

 brain consists of three main parts : on its dorsal side is the 

 cerebellum, B (Fig. 73), consisting of a rigid, a left, and a 

 median lobe; on the ventral side is the pons Varolii, C 

 (Fig. 73), and behind that the medulla oblonyata, D (Fig. 73), 

 which is continuous with the spinal cord. 



In nature, the main divisions of the brain are not sepa- 

 rated so much as has been represented in the diagram for 



Cbl 



"^^^^ 



Mo 



FIG. 74. The brain from the left side. Cb, the cerebral hemispheres forming 

 the main bulk of the fore-brain; Cbl, the cerebellum; Mo, the medulla oblon- 

 gata; />, the pons Varolii ;* the fissure of Sylvius. 



the sake of clearness, but lie close together, as represented 

 in Fig. 74, only some folds of the membranes extending be- 

 tween them ; and the mid-brain is entirely covered in on its 

 dorsal aspect. Nearly everywhere the surface of the brain 

 is folded, the folds, "known as gyri or convolutions being 

 deeper and more numerous in the brain of man than in that 

 of the animals nearest allied to him; and in the human 

 species more marked in the higher than in the lower races. 

 It should however be added that some species of animals 



