122 FUNCTIONS OF THE NERVOUS SYSTEM. 



scurity hung 1 over a portion of the phenomena. It was from the first maintained 

 by Magendie, and has been subsequently asserted by other physiologists, that 

 Jhe anterior and posterior roots of the nerves were both concerned in the 

 "reception of sensations and in the production of motions; for that, when the 

 anterior roots were touched, the animal gave signs of pain, at the same time 

 that convulsive movements were performed; and that, on touching the poste- 

 rior roots, not only the sensibility of the animal seemed to be affected, but mus- 

 cular motions were excited. These physiologists were not willing, therefore, 

 to admit more than that the anterior roots were especially motor, and the pos- 

 terior especially sensory. But the recently attained knowledge of the reflex 

 function of the spinal cord, enables the latter portion of these phenomena to 

 be easily explained. The motions excited by irritating the posterior root are 

 entirely dependent upon its connection with the spinal cord, arid upon the 

 integrity of the anterior roots and of the trunks into which they enter ; whilst 

 they are not checked by the separation of the posterior roots from the peri- 

 pheral portion of the trunk. It is evident, therefore, that excitation of the 

 posterior root does not act immediately upon the muscles through the trunk of 

 the nerve, which they contribute to form, but that it excites a motor impulse 

 in the Spinal Cord, which is propagated through the anterior roots to the peri- 

 phery of the system. The converse phenomenon, the apparent sensibility of 

 the anterior roots, has been still more recently explained by the experiments 

 of Dr. Kronenberg ;* who has satisfactorily proved that it is dependent upon 

 a branch of the posterior root passing into the anterior root at their point of 

 inosculation, and then directing itself towards the cord ( 124). 



163. On the other hand, the distinctness of the system of nerves concerned 

 in the simply reflex actions, from those which minister to sensation and voli- 

 tion by their connection with the brain, is by no means so obvious as in the 

 Invertebrated classes. When first pointed out by Dr. Marshall Hall, who had 

 grounded his opinion more upon physiological phenomena than upon anato- 

 mical facts, the statement did not command general assent ; since, while the 

 phenomena were admitted, the inferences which he drew from them by him 

 were not regarded as necessary results. When, however, the anatomy of the 

 nervous centres in Vertebrata was more closely inquired into (by Mr. Grain- 

 ger, who had been partly anticipated by Bellingeri), it was found to present 

 certain phenomena which might be regarded as supporting Dr. M. Hall's 

 views ; and when the inquiry was extended to the Invertebrated classes, the 

 confirmation was found to be still more decisive. In our previous sketch these 

 doctrines have been treated as established ; since they have been found not 

 only to correspond with the facts disclosed by anatomical research, but to be 

 required by them. We shall now apply them to the nervous apparatus of the 

 Vertebrata. 



164. The Spinal Cord consists of two lateral halves; these are partially 

 separated in the higher classes, by the superficial anterior and posterior fis- 

 sures ; and in Fishes by an internal canal, which is continuous with the fourth 

 ventricle.! This canal is evidently the indication of that complete separation 

 of the two columns, which exists in the lower Articulata; and the fourth ven- 

 tricle, which in many fishes remains unclosed (the cerebellum ngt being suffi- 

 ciently developed to overlap it,) corresponds with the passage between the 

 cords uniting the cephalic ganglia with the first subcesophageal, through which 

 the oesophagus passed in all the Invertebrata. The two lateral halves have 

 little connection with each other in Fishes, and the pyramidal bodies at their 



* Mailer's Archiv. 1839, Heft v.; and Brit, and For. Med. Rev., vol. ix. p. 547. 



1 his canal may be traced in the Spinal Cord of Man and other Mammalia; but it is 

 nearly obliterated. 



