THE SENSE OF HEARING. 643 



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 consists secondarily of a series of colum- 

 nar 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 

 number 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 articulate with two or three inner rods. 

 The upper edges of the rods are flattened, elongated, and project 

 outward, forming a reticulated membrane through the meshes of 

 which the hair-like processes of the cells project. 



From the connective-tissue thickening on the upper surface of 

 the osseous 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 semicircular 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 pro- 

 cesses of the epithelial cells covering the maculae acusticae and the 

 rods of Corti are consequently bathed by endolymph. 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 endo- 

 lymph 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 termina- 

 tion of these fibers in the hair cells is unknown, but doubtless it is 

 both histologic and physiologic. 



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. 



