THE SENSE OF HEARING. 



681 



ing from the central canal, pass through the osseous lamina spiralis and 

 transmit fibers of the auditory nerve. The interior of the bony labyrinth is 

 lined by periosteum covered by epithelium and in communication with lymph- 

 spaces at the base of the skull by means of the aqueduct of the vestibule. 



The membranous labyrinth, lying within the osseous labyrinth, cor- 

 responds with it in form, though it is smaller in size. It may be subdivided 

 into vestibule, semicircular canals, and cochlea (Fig. 341). 



The vestibular portion consists of two small sacs, the utricle and the saccule, 

 which communicate with each other by means of the two branches of a duct 

 passing through the aqueduct of the vestibule the ductus endolymphaticus. 



The semicircular canals communicate with the utricle in the same manner 

 as the bony canals communicate with the vestibule. The saccule communi- 

 cates with the membranous cochlea by a short canal, the canalis reuniens. 

 The walls of the utricle, saccule, and semi- 

 circular canals are composed of connective- 

 tissue lined by epithelium. At the points of 

 entrance of the auditory nerve, the macula 

 acusticcB, in all three structures, the epithe- 

 lium undergoes a marked change in appear- 

 ance 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. 342). 



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 semi- 

 circular 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 re- 

 garded as the highly specialized terminal organ of the vestibular portion of 

 the auditory nerve. 



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. 343). 

 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 modified and 



FIG. 342. SECTION OF WALL OP 

 UTRICLE OF THE INTERNAL EAR, 

 through macular region, from 

 rabbit, showing otoliths (o), em- 

 bedded within granular substance 

 (g)~ h. Ciliated cells with proc- 

 esses. (/>), extending between 

 sustentacular elements (s). m. 

 Basement membrane, n. Nerve- 

 fibers within fibrous tissue (/) 

 passing toward hair-cells and 

 becoming non-medullated at base- 

 ment-membrane. (After Piersol.) 



