524 ORGAN OF CORTI. 



parts of the embryonic mucous tissue between the perichondrium 

 and the walls of the membranous labyrinth. 



The scala vestibuli is formed before the scala tympani, and 

 both scalse begin to be developed at the basal end of the cochlea : 

 the cavity of each is continually being carried forwards towards 

 the apex of the cochlear canal by a progressive absorption of the 

 mesoblast At first both scalae are somewhat narrow, but they 

 soon increase in size and distinctness. 



The cochlear canal, which is often known as the scala media 

 of the cochlea, becomes compressed on the formation of the 

 scalae so as to 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 cochlear 

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

 bordered on the cochlear aspect by flat epiblast cells. This mem- 

 brane is called the membrane ofReissner. The scala tympani 

 is separated from the cochlear canal by a thicker sheet of meso- 

 blast, 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 scalae 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). Subse- 

 quently the two scalae 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 a still later 

 period the scala vestibuli, growing further round, separates 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 



