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CHAPTER XIV. 



GENERAL REMARKS OF SENSATION. OF THE SPECIAL SENSES. OF 



THE SENSE OF TOUCH. ANATOMY OF THE SKIN, AND ITS AP- 

 PENDAGES. 



SENSATION is an affection of the mind occasioned by an impres- 

 sion made on certain parts of the nervous system, hence called 

 sensitive. A state of the sensitive organs, and a corresponding per- 

 ception by the mind, must concur to produce sensation: either con- 

 dition may exist alone, but then the phenomenon is not a true 

 sensation, in the acceptation here given to the word. Thus, light 

 falling on the eye in sleep excites the whole visual sensitive ap- 

 paratus, while the organ of perception is inactive : on the other 

 hand, in dreams, vivid pictures of objects float before the mind, and 

 are referred by it to the external organ, which may be all the 

 while entirely quiescent. 



The organs of sensation are those parts of the nervous system, with 

 their dependencies, which, when stimulated, occasion in the mind a 

 perception of the impression. Hence certain parts of the cerebro- 

 spinal centre, as well as certain nerves and their peripheral expan- 

 sions, are comprehended under this term. It is remarkable that 

 the organ of the mind itself does not appear capable of thus under- 

 going sensory excitement. Little is known of the central parts oi 

 the organs of sensation, in consequence of their deep seat, and oi 

 their ill-defined limits. The peripheral parts are more conspicu- 

 ously placed, and on several accounts are commonly styled, par 

 excellence) the organs of sensation : they are specially adapted to 

 receive in the most advantageous manner the impressions to which 

 their excitability is adapted to respond. The intervening nei 

 are, more properly speaking, media of transmission. 



Sensations excited by a stimulus originating in the body itself, 

 especially if it act rather on the intermediate or central part of the 

 sensitive apparatus than on the peripheral, are termed subjective : 

 on the other hand, they are styled objective if the stimulus be de- 

 rived from without. 



Under the name of common or general sensibility may be in- 

 cluded a variety of internal sensations, ministering for the mosl 

 part to the organic functions and to the conservation of the body. 



