ANATOMY OF THE NERVOUS SYSTEM 



125 



the two other divisions and presents on its dorsal side four hemi- 

 spherical eminences, the corpora quadrigemina or colliculi. On 

 its ventral side it exhibits two semicylindrical pillars (seen under 

 the nerve IV in Fig. 64), known as the-crwra cerebri. The hind- 

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

 bellum, B (Fig. 60), consisting of a right, a left, and a median lobe; 

 on the ventral side is the pans Varolii, C (Fig. 60), and behind 

 that the medulla oblongata, D (Fig. 60), which is continuous with 

 the spinal cord. 



In nature, the main divisions of the brain are not separated so 

 much as has been represented in the diagram for the sake of clear- 

 ness, but lie close together, as represented in Fig. 61, only some 



FIG. 62. Diagram of the right half of a vertical median section of the brain. 

 H, H, convoluted inner surface of right cerebral hemisphere; Cc, corpus callosum; 

 pt, the pituitary body; the mass on which the figure 3 is placed is the inner side 

 of the right optic thalamus; o, d, the anterior and posterior corpora quadriaemina 

 of the right side; .!/, the medulla oblongata; Cr, right crus cerrbri; P, pans Varolii; 

 Cb, cerebellum; where it is divided in the middle line the radial arrangement of its 

 central white matter forming the so-called arbor vitae is seen; op, right optic nerve 

 proceeding from the optic chiasma; oc, the third cranial nerve arising from the 

 cms cerebri. 



folds of the membranes extending between them; and the mid- 

 brain is entirely covered in on its dorsal aspect. Nearly every- 

 where 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 



