OF THE NEEVOUS SYSTEM. 93 



penetrate the nervous matter. These three membranes invest 

 the entire eucephalon, and present thus a spinal as well as a 

 cranial portion. 



185. Though composed of several parts, the encephalon 

 may be viewed as one ; yet each division seems to perform 

 distinct functions, in a certain sense. 



186. The cranial portion is composed of the brain, cere- 

 bellum, and pons of Varolius ; the spinal portion may be 

 viewed as composed of the medulla oblongata and spinal 

 marrow, properly so called. 



The brain (Fig. 55 a, Figs. 56, 57 a be) (cerebrum) is by 

 much the largest part of the encephalon in man and mammals. 

 The form of the skull, in a general way, represents its shape, 

 more especially in man. The cerebrum proper is subdivided 

 into two hemispheres by a deep mesial fissure, extending quite 

 down to the corpus callosum in the middle, and anteriorly 

 and posteriorly separating the hemispheres completely from 

 each other. The corpus callosum unites the hemispheres, and 

 may be called a transverse commissure. In the fissure descends 

 the falx cerebri, a partition formed by the dura mater. On the 

 surface of these hemispheres may be seen the convolutions 

 and anf ractuo sites ^o the brain so distinct in man, and 

 which have given rise to so much speculation. They are of 

 little depth in very young children and in most animals. 



By turning up the brain and examining its base, it is easy 

 to observe, without much dissection, that the brain admits of 

 being divided into three lobes on each side, an anterior, 

 middle, and posterior; this last is not so distinct as the 

 others. It is on this surface also that the cerebellum may be 

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

 fibres called crura uniting the pons to the hemisphere of the 

 brain ; here also may be seen all the nerves, as they proceed 

 to or from the brain ; the large arteries likewise, which all 

 reach the cerebral portion of the encephalon by the base. The 

 two little rounded eminences seen here are called mam- 

 millary 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. 56) 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 



