HEARING IN ANIMALS. 418 



are two slightly twisted cartilaginous folds, which represent the spiral lamina 

 of the Mammalia. 



Reptiles, generally, are unprovided with an external ear, the crocodile alone 

 possessing a rudimentary pinna, in the form of two folds of skin, the upper 

 one of which incloses a plate of cartilage and possesses a muscular valve. All 

 reptiles except serpents, possess a membrana tympani, tympanic cavity, and 

 Eustachian tube. In the serpents, however, these parts are absent, and the 

 columella, which is represented by a small rod-like bone, is embedded in the 

 flesh. In some, the membrana tympani is visible externally ; in others, it is 

 covered by the skin. The columella consists of a row of little bones ; the 

 first, corresponding to the stapes, closes in the fenestra ovalis ; the second 

 represents the incus ; and the third, a cartilaginous portion connected with the 

 membrana tympani, the malleus. The labyrinth contains a rudimentary 

 cochlea, consisting of a short, conical, straight, or slightly curved canal, 

 divided by an internal septum into two scalse. This simple form of cochlea illus- 

 trates very well the formation and structure of the more complex spiral coch- 

 lea in the Mammalia and Man ; for by imagining such a double conical tube 

 or canal to be rolled upon a central axis, the shell-like organ of the higher 

 Yertebrata would be produced. There are three semicircular canals provided 

 with ampullae. The sac of the vestibule contains otoliths, which form a fri- 

 able mass. 



In Amphibia, the cochlea is absent, and there is no fenestra rotunda. 

 Some possess a tympanum, others do not. The triton has merely a vestibule 

 with a single otoiith and three semicircular canals ; the vestibule approaches 

 the exterior by the fenestra ovalis, which is not occupied by the stapes, but is 

 closed by a small lid. The flat plate of the stapes alone represents the audi- 

 tory ossicles ; it lies in the muscles. In the fully developed frogs, with but few 

 exceptions, there is a membrana tympani, and a tympanum, from which a 

 short Eustachian tube passes into the throat. The pipa has a cartilaginous 

 membrana tympani, and its two Eustachian tubes open, by a common orifice, 

 in the middle of the palate. The walls of the labyrinth are partly cartilagin- 

 ous, partly bony, and this cavity ends externally in the fenestra ovalis, from 

 which three ossicles, in part cartilaginous, pass across the tympanum to the 

 membrana tympani. 



In Fishes generally, the external ear and tympanum never exist. The 

 cochlea also is absent, so that even the internal ear is incomplete. Some 

 osseous fishes, however, present rudiments of a tympanic cavity. The vesti- 

 bule always contains a utricle, and generally a saccule as well, each with its 

 included otoiith ; connected with these, are either some imperfectly developed 

 semicircular canals, or one, two, or more commonly, three perfect canals of 

 large size, ending in the vestibule. In certain cartilaginous fishes, viz., the 

 rays and skates, the cavity of the vestibule is prolonged to the surface of the 

 back part of the head, where a membrane, which may be said to correspond 

 with that closing the fenestra ovalis, is seen. The walls of the vestibule and 

 semicircular canals, which are either cartilaginous or bony, according to the 

 character of the skeleton, usually project into the cranial cavity ; in the higher 

 forms only are they partly contained in the temporal bone. No auditory 

 organ has yet been discovered in the amphioxus. 



From the preceding account it will be seen, that a fully developed spiral 

 cochlea exists only in the Mammalia ; that this part is comparatively simple 

 and slightly curved in Birds, is quite rudimentary in Reptiles, or altogether 

 absent, as in the aquatic chelonia, and is wanting in Amphibia and Fishes. 

 The tympanic chamber and apparatus, together with the Eustachian tube, 

 are also simplified in Birds and Reptiles, below which a tympanum does not 

 exist, except in the most highly developed amphibia, in which a small tym- 

 panum is present, but there is no fenestra rotunda. The above-named struc- 

 tures, including also the fenestra ovalis and fenestra rotunda, are therefore 

 proper to animals which live entirely in air. This general fact must be re- 

 garded as a proof of the special office of these parts, as conductors of atmos- 

 pheric sounds. In those which inhabit water exclusively, the semicircular 

 canals and vestibule alone are present, and even the former disappear in the 

 lowest organized fishes, leaving the vestibule only as the representative of the 



