364 
NERVOUS SYSTEM OF VERTEBRATA. 
Fig. 19L— Portion op 
the Spinal Cord, 
Showing the origin of 
the nerves : a, spinal 
cord; b ,posterior root; 
c, ganglion upon its 
course; d, anterior 
root; e , trunk formed 
by the union of both ; 
/, branch. 
Charles Bell, that the posterior set of roots consists of those 
fibres that bring impressions from the body in general to the 
Spinal Cord; which impressions, if carried-on to the Brain, 
become sensations. On the other hand, 
the anterior roots consist of fibres which 
convey motor influence from the Spinal 
Cord and Brain, to the muscles of the body. 
Thus if the spinal cord of an animal be 
laid bare, and the posterior set of roots be 
touched, acute pain is obviously produced ; 
whilst, if the anterior roots be irritated, 
violent motions of the muscles supplied by 
that nerve are occasioned. Both these 
roots contain fibres that connect them with 
the brain as well as with the spinal cord; 
so that, through the same trunk, either of 
these centres may act upon the part. We 
shall presently find that there is good 
reason to believe the Brain to be the seat 
of sensibility and of voluntary power ; whilst 
the Spinal Cord is the instrument of those reflex actions which 
take place automatically, as it were, without direction on the 
part of the animal, and which are concerned in the mainte¬ 
nance of the organic functions of the body, and in its preser¬ 
vation from injury. 
452. The relative proportions which these different parts 
present, are very different in the several classes of Yertebrata. 
We find that among the lower, the Sensory Ganglia , or gan¬ 
glionic centres immediately connected with the organs of sense 
(which are analogous to the cephalic ganglia of thelnvertebrata), 
are very large, and occupy a considerable part of the cavity of the 
skull; whilst the Cerebrum and Cerebellum are comparatively 
small.—The Cerebrum increases, as we ascend the scale, in 
proportion to the development of the intelligence , and the 
predominance which it gradually acquires over blind unde¬ 
signing instinct (Chap. xiv.). Its greatest development is 
seen in Man.—The Cerebellum seems to be connected with 
muscular motion, and to bear a proportion in size with the 
variety and complexity of the movements which the animal 
performs, serving to harmonise these and blend them together 
(§ 480). On the other hand, the Spinal Cord, and the nerves 
