OF THE NEEVOUS STSTEM. 99 



It is on this surface also that the cerebellum may be dis- 

 tinctly seen, the so-called pons, and the large masses of 

 fibres called crura uniting the pons to the hemispheres of the 

 brain; here also may be seen all the cerebral or cranial nerves, 

 as they proceed to or from the brain ; the large arteries like- 

 wise, which all reach the cerebral portion of the encephalon 

 by the base. The two little rounded eminences seen here are 

 called mammillary eminences. 



The grey matter of the brain is found chiefly on the surface, 

 and the white, medullary or fibrous in the interior, but not 

 uniformly so. The ventricles of the brain (Fig. 65) all com- 

 municate with each other, directly or indirectly. 



187. The cerebellum is placed under the posterior part 

 of the brain, and is separated from it by the tentorium, a par- 

 tition formed by the dura mater. It also is composed of two 

 hemispheres, and a mesial portion connecting them together. 

 On the surface of the cerebellum there are no convolutions, 

 but the margins of laminae or plates, of which the cerebellum 

 is composed. It is connected with the medulla spinalis, 

 the pons, and cerebrum by peduncles of medullary fibres 

 called crura. In volume it is about one-third of the cere- 

 brum, and is larger comparatively in the child than in the 

 adult. 



188. Optic Lobes. By removing the upper portion of 

 the hemispheres and the corpus callosum, the ventricles of the 

 brain are exposed ; also certain rounded masses, forming as it 

 were the base of the section. These masses, named in suc- 

 cession from before backwards, are the corpora striata, the 

 thalami nervorum opticorum, and the tubercula quadri- 

 gemina ; and on the back and somewhat lower part of the 

 thalami may be seen, by raising the thalami upward, certain 

 rounded elevations of a greyish colour ; these are the corpora 

 geniculata. On all these structures important experiments 

 have been made in living animals. 



189. Spinal Marrow ; Medulla Spinalis. The 

 medulla spinalis (Fig. 64 <?; Fig. 66/ 1 ), may be viewed as a 

 continuation of the medulla oblongata and that of the brain 

 itself. A median fissure of no great depth divides it into 

 two lateral portions, anteriorly and posteriorly. Its upper 

 extremity, which is enlarged, is usually called the bulb, by 

 others the medulla oblongata : here are to be seen various 

 swellings called olivary, pyramidal, and restiform bodies ; and 

 at the sides of this medulla oblongata and of the spinal 



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