428 



AUDITORY ORGAN OF THE MAMMALIA. 



is imbedded in the mesoblast at the side of the hind-brain. It soon 

 becomes triangular in section, with the apex of the triangle pointing in- 

 wards and downwards. This apex gradually elongates to form the rudi- 

 ment of the cochlear canal and sacculus hemisphericus (fig. 302, CC). 

 At tlie same time the recessus labyrinth! {R.L) becomes distinctly 

 marked, and the outer wall of the main body of the vesicle grows out 

 into two protuberances, which form the rudiments of the vertical 

 semicircular canals {V.B). In the lower forms (fig. 305) the cochlear 

 process of the vestibule hardly reaches a higher stage of development 

 than that found at this stage in Mammalia. 



The parts of the auditory labyrinth thus established soon increase 

 in distinctness (fig. 303); the cochlear canal {GO) 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 



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 A'^ with the hind-brain are also 

 shewn. 



Below the otic vesicle on either side lies the jugular veiu. 



