ORGAN OF HEARING IN MAMMALIA. 



237 



Osseous labyrinth and otosteals, Human ; magn. xcvui" 



Of the three semicircular canals the shortest, r, has a nearly hori- 

 zontal position : the other two arc more vertical : the upper one 

 rises at the convexity of its curve, d, above the level of the upper 

 surface of the petrosal : it is that which, with its arch-area, is 

 most free in many lower Mammals. The lower vertical canal, e, 

 unites by its upper extre- 

 mity with the contiguous 

 one at I ; the common 

 opening of which is shown 

 at m, fig. 163. Each of 

 the semicircular canals ex- 

 pands at one extremity ; 

 but this is more marked 

 in the membranous canals, 

 fig. 165, where the dilata- 

 tions, a, b, c, are termed 

 * ampulla? : ' the bony ca- 

 nals are wider in propor- 

 tion to the membranous 

 ones in Man than in most Mammals, and consequently the peri- 

 lymph is more abundant. This is seen in fig. 179, which repre- 

 sents the osseous labyrinth 

 laid open, with the mem- 

 branous labyrinth in situ 

 of the human ear. Of the 

 latter the part occupying 

 the vestibule is divided 

 into the ( common sinus,' 

 i, and the ' sacculus,' I ; 

 each contains a mass of 

 otolithic powder, k, m, re- 

 ceiving filaments of the 

 acoustic nerve : other 

 brushes of nerve filaments 

 go to the ampullary ends 

 of the semicircular canals : 

 the opposite non-dilated 

 ends communicate with 

 the c common sinus ' either 

 singly, at h, or by the 

 conjoint termination q. 



J t t J Left osseous labyrinth laid open, with membranous 



The different positions of labyrinth and nerves; magnified. Human, xcvn". 



the three canals and the different directions in which their 



