AUDITORY ORGANS. 527 



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. Outside 

 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 embedded 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 between 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. 



Accessory structures connected with the organ of hearing- in 



Terrestrial Vertebrata. 



In all the Amphibia, Sauropsida and Mammalia, except the 

 Urodela and a few Anura and Reptilia, the first visceral or hyo- 

 mandibular cleft enters into intimate relations with the organs 

 of hearing, and from it and the adjoining parts are formed the 

 tympanic cavity, the Eustachian tube, the tympanic membrane 

 and the meatus auditorius externus. The tympanic membrane 

 serves to receive from the air the sound vibrations, which are 

 communicated to fluids contained in the true auditory labyrinth 

 by one ossicle or by a chain of auditory ossicles. 



The addition to the organ of hearing of a tympanic membrane 

 to receive aerial sound vibrations is an interesting case of the 



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

 of Corti are more numerous than the outer. 



