524 NERVOUS SYSTEM. 



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 mus- 

 cles is with difficulty isolated by an effort of the will. This applies to sets of 

 muscles upon 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, at the same time 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 one eye without the other, or to make rapid rotary movements of one 

 hand while an entirely different order of movements is executed by the other ; 

 and instances of this kind might be multiplied. In studying these associ- 

 ated movements, the question arises as to how far they are due to the ana- 

 tomical relations of the nerves to the centres and their connections with 

 muscles, and how far they depend upon habit and exercise. There may be 

 certain sets of nerve-cells connected with each other by commissural fibres 

 and giving origin to motor nerves distributed to sets of muscles, an anatomi- 

 cal arrangement that might render a separate action of these cells impossi- 

 ble. The anatomy of the nerve-centres and their connection with fibres are 

 so difficult of investigation, that demonstrative proof of the existence of 

 such systems is impracticable ; but this would afford a ready explanation of 

 the fact that it is impossible, as a rule, by an effort of the will, to cause only a 

 portion of a single muscle to contract ; yet some of the larger muscles receive 

 a considerable number of motor nerve-fibres which are probably connected 

 with gray matter composed of many anastomosing nerve-cells. 



Many of the associated movements may be influenced to a remarkable 

 degree by education, of which no better example can be found than in the 

 case of skillful performers upon certain musical instruments, such as the 

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

 of such instruments, not only does one hand become almost independent of 

 the other, but very complex associated movements may be acquired. An 

 accomplished pianist or violinist executes the different scales automatically 

 by a single effort of the will, and pianists frequently execute at the same 

 time scales with both hands, the action being entirely opposed to the natural 

 association of movements. 



Looking at the associated movements in their relations to the mode of 

 action of the motor nerves, it seems probable that as a rule, the anatomical 

 relations of the nerves are such that a motor impulse or an effort of the 

 will can not be conducted to a portion only of a muscle, but must act upon 

 the whole muscle, and the same is true, probably, of certain restricted 

 sets of muscles ; but the association of movements of corresponding mus- 

 cles upon the two sides of the body, with the exception, perhaps, of the mus- 



