THE SENSES 841 



In the membranous labyrinth, and in it alone, are contained the end- 

 organs of the auditory nerve. The membranous portion of the 

 cochlea is a small canal of triangular section, cut off from the scala 

 vestibuli by the membrane of Reissner, which stretches from near 

 the edge of the bony spiral lamina to the outer wall (Fig. 325). It 

 has received the name of the scala media, or canal of the cochlea. 

 Below it ends blindly, but communicates by a side-channel with the 

 portion of the membranous vestibule called the saccule, which in its 

 turn communicates with the utricle by the Y-shaped origin of the 

 ductus endolymphaticus. Into the utricle open the three semi- 

 circular canals, the endolymph of which has, therefore, free com- 

 munication with that of the vestibule and cochlea. But although 

 the semicircular canals and vestibule belong anatomically to the 

 internal ear, and are supplied by branches of the auditory nerve, we 

 have no positive proof that in the higher animals, at least, they are 

 in any way concerned in hearing ; and since experiment has assigned 

 them, with a great degree of probability, a definite function of another 

 kind (p. 732), we shall not consider them further in this connection. 

 The scala media contains the organ of Corti, which (Fig. 326) con- 

 sists of a series of modified epithelial cells planted upon the mem- 

 branous spiral lamina or basilar membrane. The most conspicuous 

 constituent of the latter is a layer of parallel transparent fibrils. The 

 epithelial cells are of three kinds: (i) supporting epithelial cells; 

 (2) the pillars or rods of Corti, sloped against each other like the 

 rafters of a roof, and covering in a vault or tunnel which runs along 

 the whole of the scala media from the base to the apex of the 

 cochlea ; (3) the hair-cells, around which the fibres of the auditory 

 nerve arborize. These last are columnar epithelial cells, surmounted 

 by hairs. They are arranged in several rows, one row lying just 

 internal to the inner line of pillars, and several rows external to the 

 outer line of pillars. A thin membrane, the membrana reticularis, 

 covers the pillars and hair-cells of Corti, and is pierced by the hairs ; 

 while a thicker membrane, the membrana tectoria, springing from 

 the edge of the osseous spiral lamina near the attachment of 

 Reissner's membrane, forms a kind of canopy over both pillars and 

 hair-cells. The fact that the hair-cells of Corti's organ are connected 

 with the fibres of the cochlear division of the auditory nerve, and its 

 elaborate structure, suggest that it must play a peculiar part in audi- 

 tory sensation. Comparative anatomy shows us that the cochlea is 

 the most highly-developed portion of the internal ear, the last to 

 appear in its evolution, and the most specialized. It is absent in 

 fishes, which possess only a vestibule and one to three semicircular 

 canals. It first acquires importance in reptiles, but attains its highest 

 development in mammals ; and there is every reason to believe that 

 it is the terminal apparatus of the sense of hearing. 



Function of the Auditory Ossicles. The anatomical arrange- 

 ments of the middle ear suggest that the tympanic membrane 

 and the chain of ossicles have the function of transmitting 



