THE SKIN AS AN ORGAN OP SENSE. 523 



suiting sensation. Thus, the temperature of a fluid does not 

 seem the same to a finger and the entire hand. This fact is not 

 irreconcilable with the existence of the various kinds of ther- 

 mal spots, referred to above, but it does re-enforce the view we 

 are urging of the complexity of those sensations which seem to 

 us to form simple wholes — as, indeed, they do — just as a piece 

 of cloth may be made up of an unlimited number of different 

 kinds of threads. 



TACTILE SX:NSIBII,ITY. 



As a matter of fact, one may learn, by using a pair of com- 

 passes, that the different parts of the surface of our bodies are 

 not equally sensitive in the discrimination between the contact 

 of objects — i. e., the judgment formed as to whether at a given 

 instant the skin is being touched by one or two points is de- 

 pendent on the part of the body with which the points are 

 brought into contact. 



Certain it is that exercise of these and all the senses greatly 

 improves them, though it is likely that such advance must be 

 referred rather to the central nerve-cells than to the peripheral 

 mechanism. 



We practically distinguish between a great many sensations 

 that we can neither analyze nor describe, though the very 

 • variety of names suffices to show how much our interpretation 

 of sense depends on past experience. 



Mammals are always able to define the part of their bodies 

 touched, and with great accuracy, no doubt, owing to the simul- 

 taneous use in the early months and years of life of vision and 

 the senses resident in the skin. 



An impression made on the trunk of a nerve is referred to 

 the peripheral distribution of that nerve in the skin ; thus, if 

 the elbow be dipped in a freezing mixture, the skin around the 

 joint will experience the sensation of cold, but a feeling of pain 

 will be referred to the distribution of the ulnar nerve in the 

 hand and arm. The same principle is illustrated by the com- 

 mon experience of the effects of a blow over the ulnar nerve, 

 the pain being referred to the peripheral distribution ; also by 

 the fact that pain in the stump of an amputated limb is thought 

 to arise in the missing toes, etc. 



