434 ORGAN OF CORTI. 



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. 



The nucleus of each cell divides ; so that there come to be two nuclei 

 or sometimes three which lie close together near the base of the cell. 

 ( hitside the nuclei on each side a fibrous cuticular band appears. The 

 two bands pass from the base of the cell to its apex, and there meet 

 though widely separated below. The remaining contents of the cell, 

 between the two fibrous bands, become granular, and are soon to a 

 great extent absorbed ; leaving at first a round, and then a triangular 

 space between the two fibres. The two nuclei, surrounded by a small 

 amount of granular matter, come to lie, each at one of the angles between 

 the fibrous bands and the basilar membrane. 



The two fibrous bands become, by changes which need not be described 

 in detail, converted into the rods of Corti each of their upper ends 

 growing outwards into the processes which the adult rods possess. 



Each pair of rods of Corti is thus (Bottcher) to be considered as the 

 product of one cell ; and the nuclei imbedded in the granular mass between 

 them are merely the remains of the two nuclei formed by the division of 

 the original nucleus of that cell 1 . The larger ridge is for the most part not 

 permanent, and from being the most conspicuous part of the organ of 

 Corti comes to be far less important than the smaller ridge. Its cells 

 undergo a partial degeneration ; so that the epithelium in the hollow be- 

 tween the two lips of the lamina spiralis, which is derived from the larger 

 ridge, comes to be composed of a single row of short and broad cells. In 

 the immediate neighbourhood however of the inner hair cell, one or two of 

 the cells derived from the larger ridge are very much elongated. 



The membrana reticularis is a cuticular structure derived from the 

 parts to which it is attached. 



1 It is not clear from Bottcher's description how it conies about that the inner rods 

 of Corti are more numerous than tLe outer. 



