76 THE ANATOMY OF YERTEBRATED ANIMALS. 



cavity whicb contains the membranous vestibule : that of the 

 other, scala tympard^ abuts against, and is as it were stopped 

 by, the membrane of the fenestra rotunda. The cavity of the 

 membranous cochlea stretched between, and helj)ing to divide, 

 these two scalce, is called the scala media. 



In Reptiles, Birds, and Ornithodelphous Mammals, the 

 cochlea is only slightly bent or twisted upon itself. But, in 

 the higher Mammalia^ it becomes coiled in a flat or conical 

 spiral of one and a half (^Cetacea, Erinaceus) to five {^Gcelo- 

 genys Pacd) turns. 



The membranous labyrinth is filled with a clear fluid, the 

 endolymph^ and usually contains otolithes of various kinds. 

 Between the membranous labyrinth and the walls of the cav- 

 ity of the periotic mass in which it is contained, lies another 

 clear fluid, the perilymph^ which extends thence into the scalce 

 vestihull and tympani. 



In all animals which possess a fenestra ovalis, its mem- 

 brane gives attachment to a disk, whence an ossified rod, or 

 arch, proceeds. Where the former structure obtains, as in 

 Birds, most Reptiles, and some Amphibia^ the bone is com- 

 monly called columella auris ; when the latter, as in most 

 Mammals, stapes. But there is really no difibrence of impor- 

 tance between stapes and columella^ and it is advisable to use 

 the former name for the bone under all its forms. 



In the majority oiVertehrata of higher organization than 

 fishes, the first visceral cleft does not become wholly obliter- 

 ated, but its upper part remains as a transversely elongated 

 cavity, by means of which the pharynx would be placed in 

 communication with the exterior, were it not that the oppo- 

 site sides of the canal grow together into a membranous par- 

 tition — the memhrana tympani. So much of the canal as lies 

 external to this is the external auditory meatus ; while what 

 lies internal to it, is the tympanum,^ or drum of the ear, and 

 the Eustachian tube^ which places the tympanum in communi- 

 cation with the pharynx. While the outer wall of the tym- 

 panum is the tympanic membrane, its inner wall is the periotic 

 mass with its fenestrm ; and, in all Vertebrata below Mam- 

 mals, the outer end of the stapes is either free, or, more com- 

 monly, is fixed to the tympanic membrane, and thus the latter 

 and the membrane of \\i<d fenestra ovalis become mechanically 

 connected. In all these animah the mandible is connected 

 with the skull by the intermediation of an as quadratum. 



But, in the JMammalia^ the mandible is articulated directly 

 with the squamosal, and the quadratum is converted into one 



