00 NERVOUS SYSTEM. 



fibres. That it is the axis-cylinder which conducts and the 

 medullary tube which insulates, it is impossible to say with 

 positiveness ; but, as we have already seen, it is move than 

 probable that the central band is the only conducting ele- 

 ment. , 



We have incidentally noted the fact that direct stimula- 

 tion applied to the centres, even when the connection between 

 these and the muscles is perfect, is incapable of inducing the 

 generation of nerve-force ; but the generation of a motor 

 stimulus may be induced by an impression made upon sen- 

 sitive nerves and conveyed by them to the centres. If, for 

 example, we isolate a certain portion of the central nervous 

 system, as the spinal cord, and leave its connections with 

 the motor and sensitive nerves intact, these phenomena may 

 be readily observed : An impression made upon the -sensi- 

 tive nerves will be conveyed to the gray matter of the cord 

 and will induce the generation of a motor stimulus by the 

 cells of this part, which will be conducted to the muscles 

 and give rise to contraction. As the stimulus, in such ob- 

 servations, seems to be reflected from the cord through the 

 motor nerves to the muscles, this action has been called 

 reflex. These phenomena constitute an important division 

 in the physiology of the nervous system, and will be fully 

 considered by themselves. 



Associated Movements. It is well known that the action 

 of certain muscles is with difficulty isolated by an effort of 

 the will. This applies to sets of muscles on one side of the 

 body and to corresponding muscles upon the two sides. 

 For example, it is almost impossible, without great practice, 

 to move some of the fingers, restraining the movements of 

 the others ; and the action of certain sets of muscles of the 

 extremities is always simultaneous. The toes, which are but 

 little used as the foot is confined in the ordinary dress, are 

 capable of very little independent action. It is difficult to 

 move o?ie eye without the other, or to make rapid rotary 



