OF HEARING. 59 



100. In reptiles, the whole exterior ear disappears ; the 

 auditory passage is always wanting, and the tympanum be- 

 comes external. In some toads, even the middle ear also is 

 completely wanting. The fluid of the vestibule is charged 

 with salts of lime, which frequently give it a milky appear- 

 ance, and which, when examined by the microscope, are 

 found to bo composed of an infinite number of crystals. 



101. In fishes, the middle and external ear are both 

 wanting; and the organ of hearing is reduced to a mem- 

 branous vestibule, situated in the cavity of the skull, and 

 surmounted by semicircular canals, from one to three in 

 numoer. The liquid of the vestibule contains chalky con- 

 cretions of irregular forms, which are called Otolites, the 

 use of which is doubtless to render the vibration of sounds 

 more sensible. 



102. In crabs, the organ of hearing is found on the lower 

 face of the head, at the base of the large antennse. It is a 

 bony chamber closed by a membrane, in the interior of 

 which is suspended a membranous sac filled with water. On 

 this sac, the auditory nerve is expanded. In the cuttlefish, 

 the vestibule is a simple excavation of the cartilage of the 

 head, containing a little membranous sac, in which the audi- 

 tory nerve terminates. 



103. Finally, some insects, the grasshopper for instance, 

 have an auditory apparatus, no longer situated in the head, 

 as with other animals, but in the legs ; and from this fact, we 

 may be allowed to suppose, that if no organ of hearing has 

 yet been found in most insects, it is because it has beeii 

 sought for in the head only.. 



104. It appears from these examples, that the part of the 

 organ of hearing which is uniformly present in all animals 

 furnished with ears, is precisely that in which the auditory 

 nerve ends. This, therefore, is the essential part of the or- 

 gan. The other parts of the apparatus, the tympanum, 

 audit .iry passage, a- id even the semicircular canals, have for 



