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TEXT-BOOK OF PHYSIOLOGY. 



specialized along the surface of the basilar membrane, to constitute what is 

 known as 



The Organ of Corti. In Fig. 343 this organ is represented as it appears 

 on cross-section of the cochlea. It consists primarily of an arch composed 

 of two modified epithelial cells known as the rods or pillars of Corti, which 

 rest below on the basilar membrane, but meet and interlock above; it con- 

 sists secondarily of a series of columnar epithelial cells provided with hair- 

 like processes which rest upon and are supported by the rods both on the 

 inner and outer aspects of the arch. The space beneath the arch is known 

 as the tunnel. The inner hair cells are not nearly so numerous as the outer 

 hair cells. The epithelial cells external to the outer and inner hair cells are 

 supporting or sustentacular in character. 



The organ of Corti extends the entire length of the cochlea. The num- 

 ber of rods which, standing side by side, form the inner limb of the arch is 

 estimated at 5600; the number which form the outer limb is estimated at 

 3850. The outer rods are broader than the inner and at some places articu- 

 late with two or three inner rods. The 

 upper edges of the rods are flattened, 

 elongated, and project outward, form- 

 ing a reticulated membrane through 

 the meshes of which the hair-like pro- 

 cesses of the cells project. 



From the connective-tissue thicken- 

 ing on the upper surface of the os- 

 seous lamina spiralis there extends 

 outward over the organ of Corti a thin 

 membrane, the membrana tectoria. 

 The common cavity between the walls 

 of the osseous and membranous 

 labyrinth in the vestibule, the semi- 

 circular canals, in the scala vestibuli 

 and scala tympani of the cochlea, is 



filled with a clear fluid the perilymph; the common cavity within the walls 

 of the entire membranous labyrinth is also filled with a similar fluid the 

 endolymph. The hair-like processes of the epithelial cells covering the 

 maculae acusticae and the rods of Corti are consequently bathed by endo- 

 lymph. Both fluids are in relation with the subarachnoid lymph-spaces at 

 the base of the brain, the perilymph through the aqueduct of the vestibule, 

 the endolymph through the endolymphatic duct. 



The fibers of the cochlear nerve, arising from the ganglion cells of the 

 spiral ganglion situated in the osseous lamina spiralis near the modiolus, 

 send their peripheral branches to the saccule and to the organ of Corti. As 

 they approach this structure they lose their medullary sheath and become 

 naked axis-cylinders. The fibers then divide into two parts, of which one 

 passes to the inner hair cells; the other passes between the inner rods and 

 crosses the tunnel between the outer rods to the outer hair cells. The exact 

 method of termination of these fibers in the hair cells is unknown. 



From the relation of the nerve-fibers to the organ of Corti the latter must 

 be regarded as the highly specialized terminal organ of the cochlear division 

 of the auditory nerve. 



FIG. 343 j A TRANSVERSE SECTION OF A 

 TURN OF THE COCHLEA. 



