56 



SPECIAL SENSES. 



the acoustic or auditory nerve, which arises from the poste- 

 rior part of the brain. (Fig. 21, c.) 



92. The ears never exceed two in number, and are 

 placed, in all the vertebrates, at the hinder part of the head. 

 In a large proportion of animals, as the dog, horse, rabbit, 

 and most of the mammals, the external parts of the ear 

 are generally quite conspicuous ; and as they are, at the 

 same time, quite movable, they become one of the promi- 

 nent features of physiognomy. 



93. These external appendages, however, do not consti- 

 tute the organ of hearing, properly speaking. The true seat 

 of hearing is deeper, quite in the interior of the head. It is 

 usually a very complicated apparatus, especially in the supe- 

 rior animals. In mammals it is composed of three parts, the 

 external ear, the middle ear, and the internal ear ; and its 

 structure is as follows : (Fig. 19.) 



./f 



Fig. 19. 



94. The external ear, which is popularly regarded as the 

 ear, consists of the conch, (a,) and the canal which leads 

 from it the external auditory passage, (&.) The first is a 



