THE SENSE OF HEARING 



733 



three structures, the epithelium undergoes a marked change in appearance 

 and structure. It becomes columnar in shape and provided with stiff hair- 

 like processes or threads, which project into the cavity. In the saccule and 

 utricle the hair-like processes are covered by a layer of small crystals of 

 calcium carbonate held together by a gelatinous material. The crystals are 

 known as otoliths (Fig. 321). 



The Vestibular Nerve. The fibers of the vestibular nerve, arising from 

 the cells of the ganglion of Scarpa in the internal auditory meatus, send their 

 peripherally directed branches through the foramina in the inner wall of the 

 vestibule, through the walls of the utricle and semicircular canals near the 

 ampulla. As the fibers approach the maculae acusticae they subdivide into 

 delicate fibrillae, which ultimately become histologically and physiologically 

 related to the neuroepithelium. From the relation of the nerve-fibers to 

 the epithelium, the latter must be regarded as the highly specialized terminal 

 organ of the vestibular portion of the auditory nerve. 



The Cochlea. The cochlea is a closed membranous tube situated between 

 the osseous lamina spiralis and the outer bony wall. A transverse section of 

 the entire cochlea shows the relation of the osseous and membranous portions 

 (Fig. 322). The cochlear tube is triangular in shape. The base is attached 

 to the bony wall, the apex to the edge of osseous lamina spiralis. One side of 

 the tube forms in part the membrane of Reissner, the other side forms in part 

 the basilar membrane. The sides of the cochlea toward the scala vestibuli 

 and scala tympani are covered with epithelium. The triangular cavity of 

 the cochlear tube is known as the scala media. The inner surface of the 

 cochlear tube is lined by epithelium, 

 which becomes extraordinarily modi- 

 fied and specialized along the surface 

 of the basilar membrane, to constitute 

 what is known as 



The Organ of Corti. In Fig. 322 

 this organ is represented as it appears 

 on cross-section of the cochlea. It con- 

 sists 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 colum- FIG. 

 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 ex- 

 ternal 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, 



322. A TRANSVERSE SECTION OF A 

 TURN OF THE COCHLEA. 



