CHAPTER XV. 



SPECIAL SENSES. 



TACTILE SENSE. 



THE organs of special sense constitute the peripheral portion of 

 the centripetal part of the nervous system. The nervous system is 

 open to receive the impressions from the external world according 

 to the nature of the .different agents which must impress the organs 

 of the special senses. 



The various kinds of sense-organs have each a different con- 

 struction. They are always adapted to receive an impression of a 

 given agent. Thus, the eye is an organ that is particularly adapted 

 to receive impressions from rays of light; the ear receives sound- 

 waves; the skin is responsive to touch, etc. 



Man is endowed with five senses. That is, he possesses five kinds 

 of organs which are destined to give him notice of the impressions 

 upon his nervous system from five different agents. To these agents 

 man has assigned special names which recall their relations to the 

 organs of sense, and without which they could not he conceived of. 

 These agents, with the corresponding organs of senee, are (I) contact, 

 which is perceived through the sense of touch, whose highest devel- 

 opment is in the skin; (2) taste, a modification of touch is perceived 

 through the sense of taste embodied in the tongue; (3) odor is recog- 

 nized through the sense of smell as located in the nose; (4) sound- 

 waves are made known to the economy through the sense of hearing, 

 whose peripheral organ is the ear; and (5) light is perceived through 

 sight by reason of the response produced in the eye from the excita- 

 tion of rays of light. 



Miiller's law of the specific energy of sensory nerves is that irri- 

 tation of the nerves of special sense always causes sensations of the 

 same kind. An induction current upon the skin will produce un- 

 pleasant tactile sensations. Upon the eye it provokes luminous sensa- 

 tions, upon the ear, noise sensations, and upon the tongue there is pro- 

 duced a sensation of taste. Yet in each case the stimulus is always 

 the same. 



In order that the impressions caused by the external excitants 

 may be able to reach the consciousness of the individual, it becomes 

 necessary that each organ of sense be furnished with centripetal 

 (722) 



