THE INTERNAL EAR IN VERTEBRATES 585 



basilaris lagence of the amphibian to suggest the great coiled 

 cochlea that dominates the labyrinth of the mammal. But close 

 inspection in the light of a knowledge of this part of the ear in 

 reptiles and birds leaves no doubt that even when it first 

 appears, this pars basilaris has in it in rudiment some of the 

 most essential peculiarities of the cochlea. 



In man and other mammals the cochlea, as every one knows, 

 consists throughout almost its entire length of three fluid-filled 

 channels — a central one {scala media) wedged in between two 

 others {scala vestibuli and 5. tympani) (fig. 1). The central 

 channel is a direct process of the endolymph labyrinth. It 

 is triangular in cross section, with the apex directed to the axis 

 of the cochlear spire, its base applied to the surrounding bony 

 envelope, one side (morphologically the outer) covered by the 

 scala vestibuli and the other (morphologically the mesial) 

 covered by the scala tympani. This third side consists partly 

 of a bony shelf {lamina spiralis) projecting from the axis of the 

 spire and partly of a thin membrane {membrana basilaris) tensely 

 stretched throughout the whole length of the cochlea between 

 the edge of the bony shelf and a corresponding fibrous ridge 

 {ligamcntum spirale) projecting from the wall spoken of above as 

 the base of the triangle. Covering the axial half of the basilar 

 membrane is a strip of sensory epithelium of peculiarly intricate 

 structure, known as the organ of Corti. 



These, from the point of view of comparative anatomy, 

 are the essential characters of the scala media or cochlear canal. 



The two other scalae {S. vestibuli and tympani) are in open 

 communication at the apex of the cochlea. The scala vestibuli 

 is a continuation of the general perilymph space that lies 

 between ;the oval window and the vestibular parts of the 

 endolymph labyrinth. Just short of the tip of the scala media, 

 it passes into the scala tympani, which follows the scala 

 media to its base and there terminates in contact with the 

 membrane-covered round window. Near the round window 

 the scala tympani is connected with the brain cavity by a narrow 

 tube (canalis perilymphaticus). (Fig. 1, Aqued. peril.) 



The fluid in these two continuous perilymph scalse is thus 

 in a position to respond readily to every swing of the stapes in 

 the oval window and to transmit its movements to the sense 

 organ in the scala media. In fact these perilymph scalae are 

 parts of the mechanism for the transmission of vibrations to the 



