Fig. 304. Section through the internal ear of an embryonic Sheep 28 mm. in length. (After Böttcher.)
D.M. dura mater; R.V. recessus labyrinthi; H.V.B. posterior vertical semicircular canal; U. utriculus; H.B. horizontal semicircular canal; b. canalis reuniens; a. constriction by means of which the sacculus hemisphericus S.R. is formed; f. narrowed opening between sacculus hemisphericus and utriculus; C.C. cochlea; C.C´. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate; Ch. notochord.
The cavities just spoken of are formed by an absorption of 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 scalæ 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 scalæ 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 scalæ 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 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 scalæ 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 scalæ 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 epithelium of the cochlear 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 organ in the adult state may facilitate the understanding of its development.
Fig. 305. Diagrams of the Membranous labyrinth. (From Gegenbaur.)
I. Fish. II. Bird. III. Mammal.
U. utriculus; S. sacculus; US. utriculus and sacculus; Cr. canalis reuniens; R. recessus labyrinthi; UC. commencement of cochlea; C. cochlear canal; L. lagena; K. cupola at apex of cochlear canal; V. cæecal sack of the vestibulum of the cochlear canal.