574 TACTILE SENSATIONS. [BOOK in. 



our skin or how hot it is. But we can through touch derive 

 no other perceptions and form no other judgments. An electric 

 shock sent through the skin will give rise to a sensation, but 

 the sensation is an indefinite one, because the electric current acts 

 not on the terminal organs of touch, but on the fine nerve-branches 

 of the skin. We cannot distinguish the sensation so caused from a 

 mechanical prick of similar intensity, we cannot perceive that the 

 sensation is caused by an electric current. Similarly certain 

 chemical substances such as a strong acid will give rise to a 

 sensation, but we cannot perceive the acid, we can form no judg- 

 ment of its nature such as we could if we tasted it ; and if the acid 

 does not permeate the skin so as to act directly and chemically on 

 the fine nerve-fibres, we cannot distinguish the acid from any 

 other liquid giving rise to the same simple contact impressions. 

 The terminal organs of the skin are such as are only affected 

 by pressure or by temperature. Conversely pressure or a variation 

 in temperature brought to bear on a nerve-trunk, instead of on the 

 terminal organs, produces no specific tactile sensations of pressure 

 or temperature, but merely general sensations of feeling rapidly 

 rising into pain. 



