368 NERVOUS CENTRES OP MAMMALS. 



convolutions more marked as we rise from the lowest Mam- 

 malia, in which they scarcely exist, towards Man, in whom 

 the furrows are deepest. The two hemispheres are much 

 more closely connected with each other, by means of fibres 

 running across from either side, than they are in the lower 

 tribes ; and in fact, a considerable part of their mass is made 

 up of fibres that pass among their different portions, uniting 

 them with each other. The Cerebellum, also, is divided into 

 two hemispheres (b, fig. 195) ; and the grey matter in its 

 interior has a very complex and beautiful arrangement, which 

 causes it to present a tree-like aspect when it is cut across (d, 

 fig. 196). The Spinal Cord is much reduced in size, when 

 compared with the other parts of the nervous centres ; the 

 motions of the animal now depending more upon its will and 

 being more guided by its sensations, and the simply reflex 

 actions bearing a much smaller proportion to the rest. 



457. The general arrangement of the nervous centres, and 

 distribution of the nervous trunks, of Man, are shown in fig. 

 195. At a are seen the hemispheres of the Cerebrum ; at b 

 those of the Cerebellum ; and at c, the Spinal Cord. The 

 principal motor nerve of the face (the facial) is seen at d; and 

 and at e is seen the brachial plexus, a sort of net- work of 

 nerves, originating by several roots from the spinal cord, and 

 going to supply the arm. From this plexus there proceed the 

 median nerve, // the ulnar nerve, g ; the internal cutaneous 

 nerve, h; and the radial and musculo-cutaneous nerves, t. 

 From the Spinal Cord are given off the intercostal nerves, j, 

 passing between the ribs ; the nerves forming the lumbar 

 plexus, &, from which the front of the leg is supplied ; and 

 those forming the sacral plexus, I, from which the back of the 

 leg is supplied. The latter gives origin to the great sciatic 

 nerve ; which afterwards divides into the tibial nerve, m ; 

 the peroneal oifibular nerve, n ; the external saphenous nerve, 

 o ; and other branches. 



458. We shall now examine the structure of the Brain 

 itself, and the arrangement of the nerves which proceed from 

 it; confining ourselves to the points of most physiological 

 importance, and neglecting those which are interesting only 

 to the professed anatomist. In fig. 196 is represented a per- 

 pendicular section of the Human Brain down its middle ; the 

 two hemispheres forming the Cerebrum having been separated 



