OF HEARING. )7 



l.">7. In reptiles, the external ear disappears ; the auditory 

 passage is wanting, and the tympanum becomes external. In 

 some toads, 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 exa- 

 mined by the microscope, are found to be composed of an infi- 

 nite number of crystals. 



[The tympanic cavity is absent in the proteus and salaman- 

 der, and both the skin and muscles are continued over the ex- 

 ternal ear. The foramen ovale is closed by a cartilaginous 

 opercnlum, on which is inserted a style-shaped ossicle, called 

 ciihnnell.a, regarded as the four bones soldered into one. 

 The Eustachian tube is absent : the tympanic cavity is also 

 absent in serpents. In frogs it consists of a membranous 

 chamber, which commences by a funnel-shaped cartilaginous 

 ring, upon which a naked membrana tympani is stretched. 

 The columella rests its oval base on the foramen ovale, and its 

 gristly head on the tympanum. In the crocodile, the rudiment 

 of an external ear exists in the form of a tegumentary fold, 

 containing a bony plate, and which can be made to shut 

 down, like a valve, by a muscle. The internal ear presents nu- 

 merous phases of development in the different groups of rep- 

 tiles : in all it is lined by a membrane, and separated from the 

 cranial cavity. The vestibule varies in form and size, and con- 

 tains crystalline cretaceous masses, or otoliths : the semi- 

 circular canals expand into ampulla : the cochlea is absent 

 in frogs and salamanders, but exists in serpents, tortoises, and 

 lizards, in the form of a hollow cone, with a blunt and dilated 

 apex ; it includes a pair of cartilages, covered by a plicated 

 membrane, turned towards each other, and upon which the 

 auditory nerve expands its delicate fibrils, as upon the lamina 

 spiralis of the human ear. T. \V.] 



lf>8. In fishes, the middle and external ear are both want- 

 ing ; and the organ of hearing is reduced to a membranous 

 bilk', situated in the cavity of the skull, and surmounted 

 by >cmicircular canals, from one to three in number. The liquid 

 of the vestibule contains chalky concretions of irregular forms, 

 the use of which is doubtless to render the vibration of sounds 

 more sensible. 



[The structure of the organ of hearing in this class exhibits 

 an interesting series of gradations, ranging from the simple 

 primitive type of the invertebrata, to the more complicated 



