THE INTEENAL EAR 703 



The margin of the osseous spiral lamina is much thickened by 

 the fibrous and epithelial tissues by which it is invested, so that a mem- 

 branous sulcus of considerable depth is formed between the two Tips 

 (vestibular and tympanic lips) of the bony sulcus spiralis internus. 

 This is further thickened by a marked elevation of fibrous tissue cov- 

 ered by columnar cells, from the outer margin of which a delicate mem- 

 brane,, the membrana tectoria, extends outward and overhangs the epi- 

 thelium of Corti's organ. From the inner margin of the elevation of 

 fibrous tissue, the limbus spiralis, which is supported by the vestibular 

 lip of the bony lamina, a delicate membrane, the vestibular membrane 

 (of Reissner), passes obliquely outward to the bony wall of the cochlea, 

 and in transections appears to cut off a corner of the scala vestibuli, 

 thus marking off a triangular space whose base is formed by the outer 

 wall of the cochlea, its sides by the membrane of Eeissner and the 

 basilar membrane upon which rests the organ of Corti; its blunt apex 

 is found at the sulcus spiralis internus. Since these membranes ex- 

 tend the entire length of the bony spiral canal of the cochlea, the 

 space which is thus apparently cut off from the scala vestibuli must 

 form a spiral canal, included between the scala tympani on the one side 

 and the scala vestibuli on the other; this canal is the scala media or 

 cochlear duct. 



The scala media is an endolymphatic canal. At the apex of the 

 cochlea it ends in a blind extremity which is known as the lagena or 

 cecum cupulare; its opposite end forms a blind pouch between the fenes- 

 tra cochleae and the fenestra vestibuli, at the base of the cochlea, which 

 is termed the cecum vestibular e. The scala media is connected with 

 the saccule and utricle by means of the canalis reuniens, as described 

 above. 



The scala tympani and scala vestibuli, on either side of the scala 

 media, extend spirally from the base to the apex of the cochlea. At 

 the apex they are united by the helicotrema, a continuation of these 

 canals which curves around the hamulus. At the base of the cochlea 

 the two canals diverge, the scala tympani ending abruptly at the fenestra 

 cochleae, which is closed by a fibrous membrane, clothed on its tympanic 

 surface by the flattened epithelium of the tympanic mucosa, and on its 

 cochlear surface by the epithelium of the scala tympani. This secondary 

 tympanic membrane serves for the relief, of tension in the cochlea when 

 the perilymph is set into motion by the stapes. The scala vestibuli, on 

 the other hand, is continued backward into the vestibule, where it is 

 in relation with the external surface of the saccule and utricle, and, 



