32 



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, they are generally quite con- 

 spicuous externally, and as they are at the same time 

 quite movable, they become one of the prominent 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 superior 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). 



Fig. 19. 



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

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

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



