AUDITORY ORGANS. 



429 



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 be- 

 comes 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. 



Shortly after the formation of the rudiment of the horizontal semi- 

 circular canal a slight protuberance becomes apparent on the inner 



c.c 



FIG. 303. SECTION OF THE HFAD OF A FCETAL SHEEP 20 MM. IN LENGTH. 



(After Bottcher.) 



E.V. recessus labyrinth! ; V.B. vertical semicircular canal; HB. horizontal semi- 

 circular canal; C. C. cochlear canal ; G. cocklear ganglion. 



commencement of the cochlear canal. A constriction arises on each 

 side of the protuberance, converting it into a prominent hemisphe- 

 rical projection, the sacculus hemisphericus (fig. 304, S.R). 



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 caualis 

 reuniens. At this stage we may call the remaining cavity of the 



