ACTION OF MOTOR AND SENSORY NERVES 477 



of the anterior roots near the cord produces contraction in the muscles 

 to which the fibres of these roots are distributed ; but the same effect 

 follows stimulation of the nerve going to these muscles, in any parts of 

 its course. 



So far as their physiological action is concerned, the individual 

 nerve-fibres are independent ; and the relations which they bear to each 

 other in nervous fasciculi and in the so-called anastomoses of nerves 

 involve simple contiguity. Comparing the nerve-force to electricity, 

 each individual fibre seems completely insulated ; and a stimulus con- 

 ducted by it to muscles does not extend to the adjacent fibres. That it 

 is the axis-cylinder which conducts and the medullary tube which in- 

 sulates, it is impossible to say with positiveness ; but it is more than 

 probable that the axis-cylinder is the only conducting element. 



Generation of motor impulses may be induced by an impression 

 made on sensory nerves and conveyed by them to the centres. If, for 

 example, a certain portion of the central nervous system, as the spinal 

 cord, is isolated, leaving its connections with the motor and sensory 

 nerves intact, these phenomena may readily be observed. An impres- 

 sion made on the sensory nerves will be conveyed to the gray matter of 

 the cord and will induce the generation of a motor impulse by the cells 

 of this part, which will be conducted to the muscles and give rise to 

 contraction. As the impulse, in such observations, 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 

 of 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 

 on the two sides. For example, it is almost impossible, without great 

 practice, to move some of the fingers, at the same time restraining the 

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

 the extremities is simultaneous. The toes, which are but little used as 

 the foot is confined in the ordinary dress, are capable of little indepen- 

 dent action. It is difficult to move one eye without the other, or to 

 make rapid rotary movements of one hand while a different order of 

 movements is executed with the other ; and instances of this kind might 

 be multiplied. 



Many associated movements may be influenced in a remarkable 

 degree by education, of which no better examples can be found than 

 in skilful performers on certain musical instruments, such as the piano, 

 harp, violin and other stringed instruments. In the technical study of 



