THE SENSES. 565 



inner and outer " fibres of Corti," are separated below, where they rest 

 upon the membrana basilaris, by a considerable interval; but their 

 upper extremities lie in contact with each other, thus forming a roof- 

 like connection, the "arch of Corti." Near the arch, the epithelium 

 cells increase in length ; and at its inner border there is a row of cells 

 nearly as long as the innermost fibres of Corti, and in a similar leaning 

 position, bearing upon their upper extremity a tuft of rigid hairs or 

 cilia. On the outer border of the arch there are three such rows of hair 

 cells ; and in every instance the cilia project through openings in a sort 



FIG. 152. 



li 



DIAGRAMMATIC SECTION OF THE ORGAN OF CORTI. 1. Membrana basilaris. 2, 3. Internal and 

 external fibres of the arch. 4. Epithelium cells near its inner and outer borders. 5. Hair cells 

 lying in contact with the arch. Magnified 500 diameters. 



of fenestrated cuticle extending above the cells, inward and outward, 

 from the middle of the arch. 



The fibres of the cochlear branch of the auditory nerve are distributed 

 to the organ of Corti. The bundles forming this branch penetrate the 

 base of the cochlea, and thence pass upward, through its axis, diverging 

 successively in a horizontal direction between the two layers of the 

 spiral lamina. At the attached border of the lamina, within the osseous 

 canal, there is a linear collection of bipolar nerve cells, in and among 

 which the fibres pass, and with many of which they are connected. 

 This forms the " spiral ganglion " of the cochlear nerve. After the 

 fibres have passed through this ganglion, they diverge toward the 

 outer border of the spiral lamina and the membrana basilaris. At this 

 point they diminish in size and lose their medullary layer ; after which 

 they penetrate into the ductus cochlearis, and reach the organ of Corti. 

 In this organ their termination in the epithelial hair cells has been 

 most positively described and figured by Waldeyer.* It evidently 

 represents, in the ductus cochlearis, the especial apparatus of auditory 

 sensibility. 



Physiological Action of the Cochlea. The cochlea, no doubt, as com- 

 pared with the rest of the internal ear, serves for the precise discrimi- 

 nation of minute variations in sound. Its elongated and spiral form, 

 the two membranes of uniform tension which inclose the ductus coch- 

 learis, and the multiple rows of hair cells in the organ of Corti, all 

 indicate its capacity for the distinct perception of sonorous impulses. 

 Its analogy of construction, in some respects, with stringed musical 

 instruments, has induced the belief, in many physiologists, that it is the 



* Strieker's Manual of Histology, Buck's edition. New York, 1872, p. 1040. 



