CHAPTER VII. 



.ESTHESIOLOGY. 



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, sight, 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 pleasure 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 some than in others. 



Portions of the surface of the body which are more sensitive 

 than the rest, and prominent, are known as the organs of touch, 

 such as the end of the ringer in man, the ' muzzle ' in the horse, &c. 

 The skin being an exposed and 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 first two being accessory, 

 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 auditorium externus, or passage which 



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