THE ORGAN OF HEARING. 



399 



The semicircular canals next form as tubular projections from 

 the vesicle and rapidly assume great prominence ; the superior 

 vertical canal appears first, and the external or horizontal 

 canal last. The growth of the epithelial diverticula is later accom- 

 panied by a condensation of the surrounding mesoderm, which 

 differentiates into an external layer, the future cartilaginous and 

 later bony capsule, and an inner layer of fibrous tissue. The latter 

 suffers partial atrophy and absorption, in consequence clefts appear 

 among the delicate bundles, an arrangement permanently represented 

 by the fibrous walls and intervening trabeculae of the spaces occu- 

 pied by the perilymph surrounding the membranous canals. Within 

 the ampullae, which early develop, the epithelial lining undergoes 

 specialization, accompanied by thickening of the mesodermic wall 

 within circumscribed areas to form the cristae acusticae. 



Coincidently with the development of the semicircular canals a 

 diverticulum the cochlear canal appears at the lower anterior 

 end of the membranous sac ; this tube, oval in section, grows for- 

 ward, downward, and inward, and represents the future cochlear 

 duct, or scala media. After attaining considerable length, further 

 elongation is accompanied by coiling and the assumption of the 

 permanent disposition of the tube. 



The epithelium of the cochlear tube early exhibits a distinction, 

 the cells of the upper surface of the somewhat flattened canal be- 

 coming attenuated, while those on the lower wall undergo thick- 

 ening and further differentiation ; the flattened cells form the 

 covering of Reissner's membrane and the outer wall, and 

 the taller elements are converted into the complicated structures of 

 the tympanic wall of the scala media, including the crista, the 

 sulcus, and the organ of Corti. 



The development of these structures includes the differentiation 

 of two epithelial ridges ; from the inner and larger of these is 

 derived the lining of the sulcus spiralis and the overhanging mem- 

 brana tectoria, and from the outer and smaller ridge is produced 

 the elaborate and complicated organ of Corti. The crista appears 

 between the sulcal cells and the cochlear axis as a thickening of the 

 spiral lamina. 



The cochlear outgrowth of the primary otic vesicle forms the 

 membranous cochlea, or scala media, alone, the walls of the 

 adjacent divisions, the scala vestibuli and scala media, resulting 

 from the changes within the surrounding mesoderm. The latter 

 differentiates into two zones, an outer, which becomes the car- 

 tilaginous, and finally osseous, capsule, and an inner, lying 

 immediately around the membranous canal, which for a time consti- 

 tutes a stratum of delicate connecting tissue between the denser 



