368 
NERVOUS CENTRES OF 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 (6, 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, f; the ulnar nerve, g ; the internal cutaneous 
nerve, h; and the radial and musculo-cutaneous nerves, i. 
From the Spinal Cord are given off the intercostal nerves, j, 
passing between the ribs; the nerves forming the lumbar 
plexus, 1c , from which the front of the leg is supplied; and 
those forming the sacral plexus, l, 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 or fibular 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 
