SPECIAL PHYSIOLOGY. 



41 



The posterior roots (Fig. 13, P, P, P) contain centripetal 

 or sensory fibres. 



This exact explanation of the function of the spinal roots 

 has been generally attributed to Charles Bell, but to-day it 

 is admitted that the glory belongs to Magendie (Vulpian). 

 This discovery has been the point of departure in all our 

 modern conquests in the physiology of the nervous system. 



Though the anterior roots possess also some sensory fibres, 

 these fibres are sent off to them from the posterior roots. 

 These are the recurrent fibres, and from this fact has arisen 

 the idea of recurrent sensibility 

 (Magendie, Cl. Bernard) ; in 

 fact, these sensory fibres follow, 

 in their progress to the spinal 

 cord, the anterior roots from 

 centre to periphery, and then, 

 either at the anastomosis of the 

 two roots, or, more probably, at 

 the point of the plexus (cervi- 

 cal, thoracic, lumbar, etc.), they 

 turn back towards the posterior 

 roots, entering with them the 

 medullary centre. The recur- 

 rent sensibility of the anterior in- 

 roots is then no exception to 

 the general rule ; all in these is 

 centrifugal, all in the posterior 

 roots centripetal. So, when an 

 anterior root is cut, the periph- 

 eral end only is still sensible. 

 This experiment is a most com- 

 plete demonstration of recur- 

 rent sensibility, especially when 

 we observe that immediate dis- 

 appearance of this recurrent 

 sensibility in the anterior root 

 is caused by section of the pos- 

 terior root. 



In the course of each posterior root, a little before the 



Fig. 13. 

 Origin of the spinal roots.* 



* The anterior surface of the cord is here shown. A, A, A, Anterior spinal 

 roots, with separate origins, uniting afterwards to form the plexus of the root, 

 P, P, P, Posterior roots, c, d, Anastomotic filaments sometimes found between 

 the posterior roots. #, ,9,<7, Ganglions of the posterior roots, m, m. Mixed 

 nerves, formed by the union of two roots. 



