302 THE CAT. [CHAP. ix. 



A very peculiar organ lies in the floor of the caualis membranacea, 

 which organ is termed the ORGAN OF CORTI. It lies \ipon the 

 hasilar membrane, and is covered above by a delicate lamina, the 

 membrana tectoria. The latter separates the organ from the cavity 

 of the canalis membranacea with its contained endolymph. 



Thus this organ, with the basilar membrane below it and the 

 membrana tectoria above it, forms a thickened floor to the relatively 

 wide canal of the canalis membranacea. The organ itself consists 

 partly of nucleated cells with stiff hair-like processes, partly of 

 epithelial cells, and partly of two rows (one inner and one outer) of 

 tough rods (compared with cartilage in consistency), so leaning 

 against each other as to enclose beneath them a minute triangular 

 space between them and the basilar membrane. This long double 



Fig. 139. A PAIR, OF RODS OF CORTI IN SIDE VIEW, HIGHLY MAGNIFIED. 



ir. Inner rod. 

 er. Outer rod. 



The nucleated protoplasmic masses at the feet 

 of the two rods are also shown resting on 

 the basilar membrane. 



series of rods has been compared to the keys of a piano, which they 

 to a certain extent resemble. Thus it may be said that each fibre 

 of the organ of Corti consists of two filaments joined together so as 

 to form an angle open downwards. Cells bearing hair-like fibres 

 and epithelial cells are placed on each side of this double range of 

 rods, and beneath the inner cells (between the inner bases of the 

 rods and the basilar membrane) are certain nucleated particles of 

 protoplasm. 



The MEMBRANOUS SEMICIRCULAR CANALS occupy about one-third 

 of the space enclosed by the bony canals in which they are suspended, 

 and they dilate into ampullaB there where the osseous semicircular 

 canals so dilate. The lining of epithelium exhibits cells, which each 

 sends forth hair-like processes projecting into the endolymph. 



The AUDITORY NERVE, after entering the meatus auditorius in- 

 ternus, divides at the bottom of that canal into two bundles of minute 

 fibres, which pass through the cribriform plate and are distributed 

 to the vestibule and cochlea. 



The vestibular nerve sends twigs to definite parts of the utricle, 

 of the saccule, and of each of the three ampulla, and most probably 

 ends by becoming continuous with the cells bearing hair-like pro- 

 cesses, which exist in that part of each of these cavities which is so 

 supplied. 



The cochlear nerve enters the base of the modiolus, and thence 

 radiates to the scala media, and most probably ends by becoming 



