The parts of the auditory labyrinth thus established soon increase in distinctness ([fig. 303]); the cochlear canal (CC) becomes longer and curved; its inner and concave surface being lined by a thick layer of columnar epiblast. The recessus labyrinthi also increases in length, and just below the point where the bulgings to form the vertical semicircular canals are situated, there is formed a fresh protuberance for the horizontal semicircular canal. At the same time the central parts of the walls of the flat bulgings of the vertical canals grow together, obliterating this part of the lumen, but leaving a canal round the periphery; and, on the absorption of their central parts, each of the original simple bulgings of the wall of the vesicle becomes converted into a true semicircular canal, opening at its two extremities into the auditory vesicle. The vertical canals are first established and then the horizontal canal.
Fig. 302. Transverse section of the head of a fœtal Sheep (16 mm. in length) in the region of the hind-brain. (After Böttcher.)
HB. the hind-brain.
The section is somewhat oblique, hence while on the right side the connections of the recessus vestibuli R.L., and of the commencing vertical semicircular canal V.B., and of the ductus cochlearis CC., with the cavity of the primary otic vesicle are seen; on the left side, only the extreme end of the ductus cochlearis CC, and of the semicircular canal V.B. are shewn.
Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC; on the left side the auditory nerve G and its connection N with the hind-brain are also shewn.
Below the otic vesicle on either side lies the jugular vein.
Shortly after the formation of the rudiment of the horizontal semicircular canal a slight protuberance becomes apparent on the inner commencement of the cochlear canal. A constriction arises on each side of the protuberance, converting it into a prominent hemispherical projection, the sacculus hemisphericus ([fig. 304], S.R).
Fig. 303. Section of the head of a fœtal Sheep 20 mm. in length. (After Böttcher.)
R.V. recessus labyrinthi; V.B. vertical semicircular canal; H.B. horizontal semicircular canal; C.C. cochlear canal; G. cochlear ganglion.
The constrictions are so deep that the sacculus is only connected with the cochlear canal on the one hand, and with the general cavity of the auditory vesicle on the other, by, in each case, a narrow though short canal.
The former of these canals ([fig. 304], b) is known as the canalis reuniens. At this stage we may call the remaining cavity of the original otic vesicle, into which all the above parts open, the utriculus.