4/8 NERVOUS SYSTEM 



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 auto- 

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

 at the same time scales with both hands, the action being 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 on the whole muscle, and the same is true, probably, of 

 certain restricted sets of muscles ; but the association of movements of 

 corresponding muscles on 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 nerves, are independent of each other in 

 their action ; and in the so-called anastomoses that take place between 

 sensory nerves, the fibres assume no new relations, except as regards 

 contiguity. 



As motor fibres convey to their peripheral distribution impulses pro- 

 duced by a stimulus applied in any portion of their course, so an impres- 

 sion made on a sensory nerve is always referred to the periphery. A 

 familiar example of this is afforded by the very common accident of 

 contusion of the ulnar nerve as it passes between the olecranon and 

 the condyle of the humerus. This is attended with painful tingling of 

 the ring and little finger and other parts to which the filaments of this 

 nerve are distributed, without, necessarily, any pain at the point of 

 injury. More striking examples are afforded in neuralgic affections 

 dependent on disease of or pressure on the trunk of a sensory nerve. 

 In such cases, excision of the nerve is often practised, but no perma- 

 nent relief follows unless the section be made between the affected por- 

 tion of the nerve and the nerve-centres ; and the pain is always referred 

 to the termination of the nerve, even after it has been divided between 

 the seat of the disease and the periphery, leaving the parts supplied by 

 the nerve insensible to direct irritation. In cases of disease it is not 

 unusual to note great pain in parts of the skin that are insensible to 

 direct impressions. The explanation of this is that the nerves are 

 paralyzed near their terminal distribution, so that an impression made 

 on the skin can not be conveyed to the sensorium ; but the trunks of the 

 nerves still retain their conductivity and are the seat of diseased action, 

 producing pain referred by the patient to the periphery. In the very 



