708 THE EAR 



the position of the organ of Corti, and the fibers of which are too rigid 

 and firmly associated to allow of resonant vibration.' Its tympanic sur- 

 face is clothed by a continuation of the lining membrane of the scala tym- 

 pani, consisting of a mesenchymal epithelium, resting upon a very thin 

 and delicate connective tissue layer. The substantia propria of the bas- 

 ilar 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 fibers of the basilar 

 membrane are found at the base of the cochlea, the longest at its apex. 

 The shortest fibers are also the coarsest. It has been estimated that 

 there are 24,000 distinct fibers or 'auditory strings' in the basilar 

 membrane. 



The substantia propria is covered upon that surface which faces 

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

 tion or exoplasmic derivative, upon which rests the epithelium of the 

 organ of Corti. 



The Organ of Corti. This organ consists of a highly differenti- 

 ated neuro-epithelium whose specialized cells are disposed according to 

 a very regular arrangement. The flattened epithelium of the sulcus 

 spiralis internus is continued for a short distance upon the basilar mem- 

 brane. Suddenly, at the margin of Corti's organ, it alters its char- 

 acter. Here the epithelium becomes abruptly changed to a tall columnar 

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

 parently piled upon one another and resting against the inner hair cells, 

 which form a single row of neuro-epithelium; these, like all the suc- 

 ceeding 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 auditory or hair cells have a broad body which is con- 

 fined to the superficial third of the epithelial layer and which is nu- 

 cleated at its deeper end. Its free surface forms an expanded oval 

 plate from which about twenty stiff cilia project through a cuticular 

 membrane toward the cavity of the scala media. These end plates 

 interdigitate 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 derived 



