OF THE NEBVOUS SYSTEM. 97 



penetrate the nervous matter. These three membranes invest 

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

 cranial portion. 



185. Though composed of several parts, the encephalon 

 may he 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 Varolii ; the spinal portion may be viewed 

 as composed of the medulla oblongata and spinal marrow, 

 properly so called. 



The brain (Fig. 64 a, Figs. 65, 66 a b c) (cerebrum) is by 

 much the largest part ot 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 anfractuosites of 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. 



[In point of fact there is no very obvious distinction between 

 the middle and posterior lobes, and it would have been much 

 better had anatomists divided the inferior surface of each hemi- 

 sphere into two lobes only, an anterior and posterior. Thus 

 would have been avoided those silly disputes which have lately 

 occupied the attention of the public about the size and form of 

 the brain in apes as compared with the human brain. The great 

 distinction between the latter and other vertebrate animals is 

 that in man, the posterior lobe of the cerebrum overlaps the 



* a, cerebrum ; b, cerebellum ; e, spinal marrow ; d , facial nerve ; e, brachial 

 plexus caused by the union of several nerves coming from the spinal mar- 

 row ; f, median nerve ; g, cubital nerve ; h, internal cutaneous nerve of the 

 arm; i, radial and musculo- cutaneous nerve of the arm; ^intercostal nerves ; 

 k, femoral plexus; I, sciatic plexus; m t tibial nerve; n, external peroneal 

 nerve ; o, external saphenous nerve. 



H 



