SENSE OF HEARING. 663 



as the internal fibres of Corti, and which lie immediately next to them 

 in a similar leaning position. The upper extremity of each of these 

 cells bears a tuft of rigid hairs or cilia, which are analogous to those of 

 the hair cells of the sacculus and utricle. On the outside of the arch 

 there are three such rows of hair cells, and in every instance the tufts of 

 cilia project through openings in a sort of fenestrated cuticle which lies 

 above the cells, and extends over them, inward and outward, from the 

 heads of the two bodies forming the arch of Corti. 



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

 distributed to the organ of Corti. The bundles of nerve fibres forming 

 this branch penetrate the cochlea at the base of its central axis, and 

 pass from below upward through its interior, diverging successively 

 from w r ithin outward, to continue their course in a horizontal direction 

 between the two layers of the spiral lamina. At the level of the at- 

 tached border of the spiral lamina there is situated, within the cavity 

 of the osseous canal, a linear collection of bipolar nerve cells, in and 

 among which the nerve-fibres pass, and with many, if not all, of which 

 the nerve fibres are directly connected. This forms the "spiral gan- 

 glion'- of the cochlear nerve. After the bundles of nerve fibres have 

 passed through the ganglion, and while they are contained in the thick- 

 ness of the spiral lamina, they form, by repeated subdivision and re- 

 union, a complicated plexus, the filaments of which continue however 

 to follow a general diverging course toward the outer border of the spi- 

 ral lamina and the attached edge of the membrana basilaris. Arrived 

 at this point, the nerve fibres diminish in diameter and lose their me- 

 dullary layer ; and, in this form, penetrate into the ductus cochlearis, 

 where they continue to radiate toward the organ of Corti. It is at this 

 situation that the final termination of the slender and pale nerve fibres 

 in the substance of the epithelial hair cells has been most positively de- 

 scribed and figured by Waldeyer. 1 There can be no doubt that this 

 structure represents, in the ductus cochlearis, the especial organ of au- 

 ditory sensibility. 



Physiological Action of the Cochlea. The cochlea is undoubtedly that 

 part of the internal ear, which, as compared with the remainder, serves 

 for the more precise discrimination of minute variations in sound. Its 

 elongated and spiral form, the two membranes of uniform tension which 

 inclose the ductus cochlearis above and below, and the remarkable com- 

 plication of structure, with the multiple rows of hair cells belonging to 

 the organ of Corti, all indicate that it is adapted for the distinct percep- 

 tion of particular sonorous impulses. The analogy of its construction 

 in some respects with the mechanism of a musical stringed instrument, 

 the fibres of the membrana basilaris representing its vibrating strings, 

 has induced the belief, in the minds of many eminent physiologists, that 

 it is the organ by which we appreciate the difference in tone or pitch 

 between different sounds. According to this view, the radiating fibres 



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



