34 SPECIAL SENSES. 



97. Three parts are to be distinguished in the labyrinth, 

 namely, the vestibule, which is the part at the entrance of the 

 cavity ; the semicircular canals (d), which occupy its upper 

 part, in the form of three arched tubes ; and the cochlea, 

 which is a narrow canal placed beneath, at the lower part of 

 the vestibule, having exactly the form of a snail-shell (e). 

 The entire labyrinth is filled with a watery fluid, in 

 which membranous sacs or pouches float. Within these sacs, 

 the auditory nerve (f) terminates. These pouches, there- 

 fore, are the actual seat of hearing, and the most essential 

 parts of the ear. The auditory nerve is admitted to them 

 by a long passage, the internal auditory canal. 



98. By this mechanism, the vibrations of the air are first 

 collected by the external ear, whence they are conveyed 

 along the auditory passage, at the bottom of which is the tym- 

 panum. The tympanum, by its delicate vibrations, augments 

 the sound, and transmits it to the internal ear, partly by means 

 of the little bones in the chamber, which are disposed in such 

 a manner that the stapes exactly fits the oval aperture, 

 (foramen ovale) ; and partly by means of the air which 

 strikes the membrane covering the round aperture (g),and 

 produces vibrations there, analogous to those of the tympa- 

 num. After all these modifications, the sonorous vibrations 

 at last arrive at the labyrinth and the auditory nerve, 

 which transmits the impression to the brain. 



99. But the mechanism of hearing is not so complicated 

 in all classes of animals, and is found to be more and more 

 simplified, as we descend the series. In birds, the middle 

 and interior ears are constructed on the same plans as in the 

 mammals ; but the outer ear no longer exists, and the au- 

 ditory passage, opening on a level with the surface of the head 

 behind the eyes, is surrounded only by a circle of peculiarly 

 formed feathers. The bones of the middle ear are also 

 jass numerous, there being generally but one. 



