THE COCHLEA 625 



from the base to the apex, is subdivided into two parallel longitu-' 

 dinal divisions, which are respectively known as the scala vestibuli 

 and the scala tympani. They are so disposed that in a given turn 

 of the canal the former is always nearer the apex, the latter nearer 

 the base of the cochlea. 



The bony lamina spiralis presents a grooved margin or sulcus, 

 from the basal or tympanic lip of which the lamina basilaris is con- 

 tinued to the opposing surface of the bony wall. The bony lamina 

 spiralis is hollowed out in a diploic manner for the transmission of 

 the branches of the cochlear nerve, which are continuously given 

 off all the way from the base to the apex of the spiral lamina, and 

 which pass outward upon the basal membrane to be distributed to 

 the epithelium of the organ of Corti. This organ is a peculiar 

 spiral group of neuro-epithelial cells which extends the whole length 

 of the basal membrane from the base to the cupola of the cochlea. 



The margin of the osseous lamina spiralis is much thickened by 

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

 membranous sulcus of considerable depth is formed between the 

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

 This is further thickened by a marked elevation of fibrous tissue 

 covered by columnar cells, from the outer margin of which a deli- 

 cate membrane, the membrana tectoria, extends outward and over- 

 hangs the epithelium of Corti's organ. From the inner margin of 

 this elevation, which is supported by the vestibular lip of the bony 

 lamina, a delicate membrane, the membrane of Reissner, passes 

 obliquely outward to the bony wall of the cochlea, and in transec- 

 tions appears to cut off a corner of the scala vestibuli, thus mark- 

 ing off a triangular space whose base is formed by the outer wall of 

 the cochlea, its sides by the membrane of Reissner and the basal 

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

 found at the sulcus spiralis. Since these membranes extend 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 ; its opposite end forms a blind pouch between the fenestra 

 rotunda and the fenestra ovalis, at the base of the cochlea, which 

 is termed the ccecum vestibular e. The scala media is connected 

 41 



