520 



AUDITORY ORGANS OF THE MAMMALIA. 



lined by a thick layer of columnar epiblast. The recessus laby- 

 rinthi 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 semi- 



V.B 



FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN 

 LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Bottcher.) 



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 semi- 

 circular 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. 



circular canal. At the same time the central parts of the walls 

 of the flat bulgings of the vertical canals grow together, oblit- 

 erating 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. 



