84 NERVOUS SYSTEM. 



a partial explanation ; for there can be no doubt of the fact 

 that violent muscular action, produced independently of vo- 

 lition, is more or less painful ; but it does not explain the 

 great sensibility sometimes observed when the muscular 

 contraction is comparatively feeble. There can be hardly 

 any doubt that the explanation offered by Magendie, and 

 sustained by the ingenious histoiogical observations cited 

 above, is in the main correct. 



Mode of Action of the Motor Nerves. Having estab- 

 lished the anatomical distinction between the motor and 

 sensory nerves, it becomes necessary to study the differences 

 in the mode of action of these two kinds of nervous con- 

 ductors. In the first place, it is evident, taking the nerves 

 and their roots as we find them in, the organism in a normal 

 condition, that certain fibres act from the centres to the pe- 

 riphery, conducting motor stimulus, while others act from 

 the periphery to the centres, conducting sensory impres- 

 sions; but within a few years, certain experiments have 

 raised the question, whether sensory fibres may not be made 

 to conduct the motor stimulus, and vice versa. The experi- 

 ments to which we allude have already been referred to in 

 connection with the regeneration of nerves ; 1 and they show 

 that when a sensory and a motor branch, situated near 

 enough together, be divided, and the peripheral extremity 

 of one be connected with the central extremity of the other, 

 after a time union will take place, and the motor filaments 

 will conduct sensory impressions, and the sensory filaments 

 will conduct the motor stimulus. This is a most curious 

 and interesting experimental fact ; but it is no argument 

 against the distinct seat of motion and sensation in the ner- 

 vous system. 



As regards the motor nerves, the force, whatever it may 

 be, generated in the centres, is conducted from the centres 

 to the peripheral distribution of the nerves in the muscles, 



1 See page 62. 



