CHAP, iv.] 



HEARING. 



983 



the membranous sac called the semicircular canals are however 

 followed somewhat closely by the bony envelope. The whole 

 bony envelope may be dissected out from the spongy bone sur- 

 rounding it, and may be obtained as a separate mass (Fig. 167), 



,s.s,c 





B 



FIG. 167. THE BONY LABYRINTH. LEFT EAR. (Schwalbe.) 

 A. seen from the outside. B. seen from the median side. Both magnified twice. 



Vb. vestibule. Chi. cochlea. Chl f . the beginning of the first turn of the 

 cochlea. F.o. fenestra ovalis, f.r. fenestra rotunda, s.s.c. superior, p. s.c. pos- 

 terior, h.s.c. horizontal semicircular canals, m.i. meatus auditorius internus, 

 canal for the auditory nerve. VII. opening of the canal containing the seventh 

 nerve. 



known by the name of the labyrinth, or "bony labyrinth to distin- 

 guish it from the membranous labyrinth which lies within it, 

 separated from it by the perilymph space. The bony labyrinth 

 consists of cochlea, vestibule and semicircular canals, but the 

 part of the membranous labyrinth corresponding to the vestibule 

 is divided into utricle and saccule. The auditory nerve pierces 

 the bony labyrinth at the so-called meatus auditorius internus 

 (Fig. 167 m.i.) on its way to be distributed to the walls of the 

 membranous sac. 



All these structures, lying at first not far beneath the skin 

 and forming together the ' internal ear,' as they grow come into 

 close connection with a passage on the side of the head leading 

 from the exterior into the pharynx and known as the " first " or 

 " hyomandibular visceral cleft." By a series of changes, which 

 we need not describe here, and indeed about which there is 

 some divergence of opinion, this simple primitive passage is 

 replaced in the adult by two passages separated from each other 

 by a partition known as the membrana tympani (Figs. 166 t.m., 

 168), or tympanic membrane. On the outer side of the mem- 

 brane lies a tubular channel, the external auditory meatus (Figs. 

 166 wi.e., m'.e'., 169 w.e.), lined by skin, and opening on to the 

 exterior by an orifice guarded with the " pinna " or " auricle." 

 On the inner side of the membrane lies the drum-shaped 



