XII.J THB COCHLEA. 397 



The cochlear canal, which is often known as the 

 scala media of the cochlea, becomes compressed on the 

 formation of the scalse 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 coch- 

 lear 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; separ- 

 ating 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 membrane is called the tnem- 

 brane of Reissner. The scala tympani is separated from 

 the cochlear canal by a thicker sheet of mesoblast, called 

 the basilar inemhrane, which supports the organ of 

 Corti and the epithelium adjoining it. The upper ex- 

 tremity of the cochlear canal ends in a blind extremity 

 called the cupola, to which the two scales 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. 132, II. L). Subsequently 

 the two scalse 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. 



Accessory auditory structures. The development 

 of the Eustachian tube, tympanic cavity, tympanic 



