CHAPTER VIL 



^STHESIOLOGY. 



This section of the subject deals with the organs of sensation. 

 When sensation is peculiar to a certain organ, and received 

 therein by the medium of a complex anatomical mechanism, it is 

 known as sensation of the higher order, or special sense; the 

 special senses are hearing, siglit, smell, and taste, localised re- 

 spectively in the ear, the eye, the nose, and the tongue. 



On the other hand, when the organs of sensation merely trans- 

 mit the impressions of pleasxire and pain, such impressions are 

 termed common sensation ; and sensitiveness to such is shared by 

 nearly all parts of the body, though in a much greater degree in 

 som'e than in others. 



Portions of the surface of the body which are more sensitive 

 than the rest, and prominent, are known as the orga^as of touchy 

 such as the end of the finger in man, the nose in the horse, &c. 

 The skin being a very sensitive structure, we place it and its 

 appendages under the heading of organs of common sensation. 



We describe first the organs of special sense, then those of 

 •common sensation. 



The Ear. 



The apparatus of hearing is composed of three parts — ^the 

 external, middle, and internal ear ; the two first being accessoiy, 

 for the collection and transmission of sounds, and the latter the 

 essential organ which receives the impressions thus conveyed. 



EXTERNAL EAR. 



The external ear consists of the concha, or projecting shell-like 

 orifice, and the meatus auditorius externus, or passage which 



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