58 SPECIAL SENSES. 



97. Three parts a.e to be distinguished in the labyrinthj 

 namely, the vestihule, 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, therefore, 

 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 auditort/ 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 

 tympanum. The tympanum, by its delicate elasticity, aug- 

 ments the vibrations, and transmits them 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, (^,) 

 and produces vibrations there, corresponding to those of the 

 tympanum. 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, tlie middle 

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

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

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

 behind the eyes, is merely surrounded by a circle of peculi- 

 arly formed feathers. The bones of the middle (sar are alio 

 ^esv'- n "merous, there being generally but one. 



