364 NERVOUS SYSTEM OF VERTEBRATA. 



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 

 Fig. i9i. PORTION op roots contain fibres that connect them with 



THE SPIKAL CORD, the brain ag well ag with the g -^ CQrd 

 Showing the origin of ,-, , ,, , ,, , .., 



the nerves : a, spinal so that, through the same trunk, either of 



c^n g n fupon Us theS6 CentreS ma J act U P n tlie P ar > We 



course; d, anterior shall presently find that there is good 

 b^ne'unTon^Sht reason to believe the Brain to be the seat 

 /, branch. 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 Vertebrata. 

 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 



