THE ORGAN OF CORTI 629 



Corti's organ. Its tympanic surface is clothed by a continuation 

 of the lining membrane of the scala tympani, consisting of an endo- 

 thelium, resting upon a very thin and delicate connective tissue 

 layer. The substantia propria of the basal membrane consists of 

 tendinous bands which, being radially disposed, span the' interval 

 between the margin of the tympanic lip of the osseous spiral lamina 

 and the opposed margin of the spiral ligament. 



Because of the great breadth of the modiolus at the base, and 

 its rapid diminution in thickness toward the apex of the cochlea, 

 this interval is relatively narrow at the beginning of the first turn 

 of the spiral scala media, but progressively widens as the apex of 

 the cochlea is approached. Consequently, the shortest tendinous 

 fibres of the basal membrane are found at the base of the cochlea, 

 the longest at its apex. The progressive increase in length of these 

 fibres, as the scala media ascends its spiral course from the base to 

 the apex of the cochlea, is one of the most significant facts which 

 is disclosed by the minute anatomy of this complicated organ. 



The substantia propria is covered upon that surface which faces 

 the scala media by a thin homogeneous membrane, a cuticular for- 

 mation or exoplasmic derivative, upon which rests the epithelium 

 of the organ of Corti. 



THE ORGAN OF CORTI consists of a highly differentiated 

 neuro-epithelium whose specialized cells are disposed according to 

 a very regular arrangement. The flattened epithelium of the sulcus 

 spiralis is continued for a short distance upon the basal membrane. 

 Suddenly, at the margin of Corti's organ, it alters its character. 

 Here the epithelium becomes abruptly changed to a tall columnar 

 variety, the first cells, known as the inner sustentacular cells, being 

 apparently piled upon one another and resting against the inner 

 hair cells, which form a single row of neuro-epithelium ; these, like 

 all the succeeding rows of cells, can be traced as a continuous line 

 in the spirally wound scala media, from the base to the apex of the 

 cochlea. 



The inner hair cells have a broad body which is confined to the 

 superficial third of the epithelial layer and which is nucleated at its 

 deeper end. Its free surface forms an expanded oval plate from 

 which about twenty cilia project through a cuticular membrane 

 toward the cavity of the scala media. These end plates interdigi- 

 tate with the phalanges of the inner pillar cells, which are to be 

 shortly described. The bases of the inner hair cells are thin and 

 slender, and are in relation with a nerve plexus of fine fibrils 



