306 INNERVATION. [CHAP. XI. 



which contribute more or less to supply with nerves the parts 

 paralysed. 



Functions of the Roots of Spinal Nerves. The application of 

 anatomical investigation, and of experiment, to determine the func- 

 tions of the anterior and posterior roots of spinal nerves respec- 

 tively, was the first important step towards a right understanding 

 in the physiology of the nervous system. This was undoubtedly 

 taken by Sir C. Bell ; and, although there were other labourers 

 in the same field not unworthy claimants of some share in the 

 merit of this important investigation, it cannot be denied that the 

 endowments of the roots were discovered by Bell. 



The original experiments of Bell, in which he was assisted by the 

 late Mr. John Shaw, consisted in laying open the spinal canal in 

 rabbits, and irritating or dividing the roots of the spinal nerves. . 

 Bell distinctly affirmed that irritation of the anterior roots caused 

 muscular movement, and that the posterior roots might be irritated 

 without giving rise to any muscular action. Destruction of the 

 posterior roots did not impair the voluntary power over the 

 muscles. Hence it was inferred that the anterior roots were mo- 

 tor, and the posterior roots not motor ; but, from the violence of 

 the operation, and the pain produced in performing it, the experi- 

 ments having been tried on rabbits, it was impossible to deter- 

 mine what degree of sensibility remained in parts supplied from the 

 divided roots. 



Numerous subsequent experimenters arrived at similar results to 

 those of Bell ; but no one obtained such satisfactory conclusions as 

 Miiller, who adopted the expedient of experimenting on frogs in- 

 stead of mammalia, with which latter the experiments involved the 

 necessity of a tedious and painful operation, and much bloodshed. 

 In frogs, on the contrary, from the great width of the lower part of 

 the spinal canal, the roots of the nerves can be exposed with facility, 

 and their excitability lasts sufficiently long to yield every result. 

 These experiments we have repeated frequently, with results pre- 

 cisely similar to those which Miiller obtained. 



In the experiments on frogs, irritation, mechanical or galvanic, of 

 the anterior root always provokes muscular contraction. No such 

 effect follows irritation of the posterior root. Section of the anterior 

 root causes paralysis of motion ; that of the posterior, paralysis of 

 sensation. This latter effect is evinced by the utter insensibility to 

 pain shewn on pinching a toe, whilst in the limb in which the poste- 

 rior root is entire such an irritation is evidently acutely felt. If the 

 anterior roots of the nerves to the lower extremity be cut on one 



