592 NERVOUS SYSTEM. 



pable of inducing the generation of nerve-force ; but the generation of a motor stimulus 

 may be induced by an impression made upon sensitive 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 sensitive 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 gives rise to 

 contraction. As the stimulus, in such observations, seems to be reflected from the cord 

 through the motor nerves to the muscles, this action has been called reflex. These phe- 

 nomena 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 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 restrain- 

 ing the movements of the others ; and the action of certain sets of muscles of the extrem- 

 ities 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 associated movements, the question arises 

 as to how far they are due to the anatomical relations of the nerves to the centres and 

 their connections with muscles, and how far they depend upon habit and exercise. We 

 can imagine that 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 

 anatomical arrangement that might render a separate action of these cells impossible. 

 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 affords a ready explanation of the fact that we cannot, as a rule, by an effort of 

 the will, cause only a portion of a single muscle to contract ; yet some of the larger mus- 

 cles receive an immense number of motor nerve-fibres which are probably connected 

 with gray matter composed of numerous anastomosing cells. 



Many of the associated movements are capable of being influenced to a surprising 

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

 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 frequently pianists execute at the same time scales with 

 both hands, the action being entirely opposed to the natural association of movements. 

 Feats of sleight of hand also show how wonderfully the muscles may be educated, and to 

 what an extent the power of association and disassociation of movements may be acquired 

 by long practice. 



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 stimulus, or an effort of the will, cannot be conducted to a portion only 

 of a muscle, but must act upon the whole muscle, and the same is true, probably, of cer- 

 tain restricted sets of muscles ; but the association of movements of corresponding muscles 

 upon the two sides of the body, with the exception, perhaps, of the muscles of the eyes, 

 is due mainly to habit and may be greatly modified by education. 



Mode of Action of the Sensory Nerves. The sensory nerve-fibres, like the fibres of the 

 motor system, are entirely independent of each other in their action ; and, in the so-called 



