THE EAE. 83 



poi-tions, called scalce, wliiclL only communicate at their 

 apices. The base of the one scala, called scala vestibuli, 

 opens into the cavity which contains the membranous vesti- 

 bule : that of the other, scala tympani, abuts against, and is 

 as it were stopped by, the membrane of the fenestra rotunda. 

 The cavity of the membranous cochlea stretched between, 

 and helping 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 (Ccelo- 

 genys Paca) turns. 



The membranous labyrinth is filled vrith a clear fluid, the 

 endolymph, and usually contains otolithes of various kinds. 

 Between the membranous labyrinth and the walls of the 

 cavity of the periotic mass in which it is contained, lies 

 another clear fluid, the perilymph, which extends thence into 

 the scalce vestibuli and tympani. 



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

 brane gives attachment to a disc, 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 difference of im- 

 portance between stapes and columella, and it is advisable to 

 use the foi-mer name for the bone under all its fonns. 



In the majority of Verteb7-ata of higher organization 

 than fishes, the first visceral cleft does not become wholly 

 obliterated, 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 opposite sides of the canal grow together into a mem- 

 branous partition — the membrana 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 tympa- 

 num in communication with the pharynx. While the outer 



