Touch. 427 



body is about eight times as great as the heat energy of the visible ra- 

 diation, the thermo-electric pile shows that in no two spots of the spec- 

 trum is it the same in energy, and the fact that the length of the invisi- 

 ble part of the spectrum is almost double that of the colored part, shows 

 that the wave length of the invisible rays regularly increases, and the 

 capacity for refraction as regularly decreases with the distance ( down- 

 ward ) from the red end of the color spectrum. Now, it is evident that 

 the skin is sensitive to the energy of all the rays of the dark part of the 

 spectrum, and a good many of those in the color part of it also, regard- 

 less of their greatly differing refracting capacities. 



Each of the minute optic organs situated upon the retina is competent 

 to receive and transmit a stimulation from only a single radiant ray, but 

 the sensation produced in the brain by that rendering is inconceivably 

 different from another rendering of the very same ray which may be 

 made by the skin. . We find here an illustration of the difference in the 

 result of the action of an external stimulus upon a completely differen- 

 tiated organ, as compared with the action of the same stimulus upon an 

 organ not specialized. The skin is the unspecialized organ, and ac- 

 cording to the laws of differentiation it responds in an indifferent man- 

 ner ( comparatively ) to a vast number of phases of radiant stimulation. 

 The little optic organ is a highly specialized piece of that very skin, dif- 

 ferentiated to respond to a single one of the many stimuli that con- 

 stantly beat on the great unspecialized skin. These terms, however, are 

 only comparative. Our skin is specialized and can be set in motion by 

 several octaves of radiant energy. It is evident that the various heat 

 stimuli, striking upon the skin, are elaborated by it, all reduced to a 

 common form and projected upon the brain as a uniform sensation vary- 

 ing only in intensity. This quality which I infer resides in the skin, 

 calls to mind that property of certain substances, which is called fluor- 

 escence, "by which the high rate vibrations of the rays above the color 

 spectrum are reduced to rates which render them visible. ( See chap. 41.) 

 From whatever cause it may come, we are forced to the conclusion that 

 the skin is the common organ for a wide range of tones of heat, and the 

 probability that its composition may include constituents able to equalize 

 these tones, affords a satisfactory explanation. In regard to the other 

 two methods of stimulation, viz. , friction and pressure, it appears prob- 

 able that separate papillae and nerve connections are devoted to each. The 

 epidermis is the common organ in which the external stimulus is arrested 

 and reduced to one of three modes or tones of nervous electrical motion. 

 Each tone flows to its own differentiated vehicle and reaches its own cor- 

 responding brain cells, the erethism of which completes the sensation. 

 That is, the waves of ether arrested by the skin become current motion 

 up the different nerves to the brain cell, where such motion ceasing, it is 



