ORGAN OF HEARING IN MAMMALIA. 



237 



Osseous labyrinth aiid otosteals, Human ; inagn. xcvui" . 



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

 zontal position : the other two are 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- 178 



mity with the contiguous 

 one at / ; the common 

 opening of which is shown 



at m, fig. 163. Each of 



o 



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 1 79 

 laid open, with the mein- 

 branous labyrinth in situ 

 of the human ear. Of the 

 latter the part occupying 

 the vestibule is divided 

 into the f common sinus,' 

 i, and the ( sacculus,' / ; 

 each contains a mass of 

 otolithic powder, k, m, re- 

 ceiving filaments of the 

 acoustic nerve : other 

 brushes of nerve filaments 

 go to the ampul lary ends 

 of the semicircular canals : 

 the opposite non-dilated 

 ends communicate with 

 the ( common sinus ' either 

 singly, at h, or by the 

 conjoint termination y. 

 The different positions of 

 the three canals and the different directions in which their 



Left osseous labyrinth laid open, with membranous 

 labyrinth and nerves ; magnified. Human, xcvu". 



