390 



COMPARATIVE ZOOLOGY 



As complete in man, it consists of the following parts : 

 i. The external ear (which is peculiar to mammals 146 ); 

 the auditory canal, about an inch long, lined with hairs 

 and a waxy secretion, and closed at the bottom by a 

 membrane, called tympanum, or " drum of the ear." 



2. The middle ear, con- 

 taining three little bones 

 (the smallest in the 

 body), malleus, incus, 

 and stapes, articulated 

 together. The cavity 

 communicates with the 

 external air by means 

 of the Eustachian tube, 



FIG. 349- - Section of Human Ear : a, external which Opens at the back 



part of the mouth. 



3. The internal ear, or 

 labyrinth, an irregular 

 cavity in the solid part 



of the temporal bone, and separated from the middle 

 ear by a bony partition, which is perforated by two 

 small holes. - The labyrinth consists of the vestibule, or 

 entrance ; the semicircular canals or tubes ; and the 

 cochlea, or spiral canal. While the other parts are full 

 of air, the labyrinth is filled with a liquid, and in this 

 are the ends of the auditory nerve. The vibrations of 

 the air, collected by the external ear, are concentrated 

 upon the tympanum, and thence transmitted through 

 the chain of little bones to the fluid in the labyrinth. 



The essential organ of hearing is the labyrinth, which 

 is, substantially, a bag filled with fluid and nerve fila- 

 ments. Fishes generally have but little more. In 

 amphibians and reptiles there are added a tympanum, a 

 single bone, connecting this with the internal ear, the 

 cochlea, and the Eustachian tube, the tympanum being 



ear, with auditory canal ; b, tympanic cavity 

 containing the three bones; c, hammer, and its 

 three muscles, d, e,f; g, tympanic membrane, 

 or head of the drum ; h, Eustachian tube lead- 

 ing to the pharynx ; z, labyrinth, with semi- 

 circular canals and cochlea visible. 



