432 ORGAN OF CORTI. 



be triangular in section, with the base of the triangle outwards. 

 This base is only separated from the surrounding cartilage by a 

 narrow strip of firm mesoblast, which becomes the stria vascularis, etc. 

 At the angle opposite the base the canal is joined to the cartilage by 

 a narrow isthmus of firm material, which contains nerves and vessels. 

 This isthmus subsequently forms the lamina spiralis, separating 

 the scala vestibuli from the scala tympani. 



The scala vestibuli lies on the upper border of the cocnlear canal, 

 and is separated from it by a very thin layer of mesoblast, bordered 

 on the cochlear aspect by flat epiblast cells. This membrane is called 

 the membrane of Reissner. The scala tympani is separated from 

 the cochlear canal by a thicker sheet of mesoblast, called the basilar 

 membrane, which supports the organ of Corti and the epithelium 

 adjoining it. The upper extremity of the cochlear canal ends in a 

 blind extremity called the cupola, to which the two scala; do not for 

 some time extend. This condition is permanent in Birds, where the 

 cupola is represented by a structure known as the lagena (fig. 305, 

 II. L). Subsequently the two scalas join at the extremity of the 

 cochlear canal; the point of the cupola still however remains in 

 contact with the bone, which has now replaced the cartilage, but at 

 n still later period the scala vestibuli, growing further round, sepa- 

 rates the cupola from the adjoining osseous tissue. 



The ossification around the internal ear is at first confined to the 

 cartilage, but afterwards extends into the thick periosteum between the 

 cartilage and the internal ear, and thus eventually makes its way into the 

 lamina spiralis, etc. 



The organ of Corti. In Mammalia there is formed from the epi- 

 thelium of the cocblear canal a very remarkable organ known as the organ 

 of Corti, the development of which is of sufficient importance to merit a 

 brief description. A short account of this oi-gan in the adult state may 

 facilitate the understanding of its development. 



The cochlear canal is bounded by three walls, the outer one being the 

 osseous wall of the cochlea. The membrane of Reissner bounds it towards 

 the scala vestibuli, and the basilar membrane towards the scala tympani. 

 This membrane stretches from the margin of the lamina spiralis to the 

 ligamentum spirale ; the latter being merely an expanded portion of the 

 connective tissue lining the osseous cochlea. 



The lamina spiralis is produced into two lips, called respectively the 

 labium tympanicum and labium vestibulare; it is to the former 

 and longer of these that the basilar membrane is attached. At the margin 

 of the junction of the labium tympanicum with the basilar membrane the 

 former is perforated for the passage of the nervous fibres, and this region is 

 called the habenula perforata. 



The labium vestibulare, so called from its position, is shorter than the 

 labium tympanicum and is raised above into numerous blunt teeth. Partly 

 springing out from the labium vestibulare, and passing from near the inner 

 attachment of the membrane of Reissner towards the outer wall of the 

 cochlea, is an elastic membrane, the menibrana tectoria. Resting on 

 the basilar membrane is the organ of Corti. 



