OF HEARING. 35 



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 

 appearance, and which, when examined by the microscope, 

 are found to be 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 membra- 

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

 surmounted by semicircular canals, from one to three in 

 number. The liquid of the vestibule contains chalky con- 

 cretions of irregular forms, which are called Otolites, and 

 whose use 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 antennae. 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 cuttle- 

 fish, the vestibule is a simple excavation of the cartilage of 

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

 auditory nerve terminates. 



103. Finally, some insects, the grasshopper for in- 

 stance, have a sort of ear, 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 been 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, that is to say, this is the essential part of the or- 

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



