$26 ORGAN OF CORTI. 



canal only one cell deep is being dealt with, the organ of Corti will be found 

 to consist of a central part composed of two peculiarly shaped rods widely 

 separated below, but in contact above. These are the rods or fibres of 

 Corti. On their outer side, i.e. on the side towards the osseous wall of the 

 canal, is a reticulate membrane which passes from the inner rod of Corti 

 towards the osseous wall of the canal. With their upper extremities fixed in 

 that membrane, and their lower resting on the basilar membrane are three 

 (four in man) cells with auditory hairs known as the outer 'hair cells,' 

 which alternate with three other cells known as Deiters' cells. Between 

 these and the outer attachment of the basilar membrane is a series of cells 

 gradually diminishing in height in passing outwards. On the inner side of 

 the rods of Corti is one hair cell, and then a number of peculiarly modified 

 cells which fill up the space between the two lips of the lamina spiralis. 



It will not be necessary to say much in reference to the development of 

 the labium tympanicum and the labium vestibulare. 



The labium vestibulare is formed by a growth of the connective tissue 

 which fuses with and passes up between the epithelial cells. The epithelial 

 cells which line its upper (vestibular) border become modified, and remain 

 as its teeth. 



The labium tympanicum is formed by the coalescence of the connective 

 tissue layer separating the scala tympani from the cochlear canal with part 

 of the connective tissue of the lamina spiralis. At first these two layers are 

 separate, and the nerve fibres to the organ of Corti pass between them. 

 Subsequently however they coalesce, and the region where they are 

 penetrated by the nervous fibres becomes the habenula perforata. 



The organ of Corti itself is derived from the epiblast cells lining the 

 cochlear canal, and consists in the first instance of two epithelial ridges or 

 projections. The larger of them forms the cells on the inner side of the 

 organ of Corti, and the smaller the rods of Corti together with the inner and 

 outer hair cells and Deiters' cells. 



At first both these ridges are composed of simple elongated epithelial 

 cells one row deep. The smaller ridge is the first to shew any change. The 

 cells adjoining the larger ridge acquire auditory hairs at their free extremities, 

 and form the row of inner hair cells ; the next row of cells acquires a broad 

 attachment to the basilar membrane, and gives origin to the inner and outer 

 rods of Corti. 



Outside the latter come several rows of cells adhering together so as to 

 form a compact mass which is quadrilateral in section. This mass is 

 composed of three upper cells with nuclei at the same level, which form the 

 outer hair cells, each of them ending above in auditory hairs, and three 

 lower cells which form the cells of Deiters. Beyond this the cells gradually 

 pass into ordinary cubical epithelial cells. 



As just mentioned, the cells of the second row, resting with their broad 

 bases on the basilar membrane, give rise to the rods of Corti. The breadth 

 of the bases of these cells rapidly increases, and important changes take 

 place in the structure of the cells themselves. 



