CHAP, iv.] 



HEARING. 



1317 



of the rest of the sac, and is known as the cochlea. The bony 

 envelope surrounding the parts of the membranous sac known as 

 the utricle and saccule does not follow closely the contour of those 

 parts but remains as an undivided part called the vestibule 

 (Fig. 163); the parts of the membranous sac called the semi- 

 circular canals are however followed somewhat closely by the bony 

 envelope. The whole bony envelope may be dissected out from 



FIG. 163. THE BONY LABYRINTH. LEFT EAR. (Schwalbe.) 

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



Vb. vestibule. Chi. cochlea. Chi', the beginning of the first turn of the cochlea. 

 F.o. fenestra ovalis, /.?. fenestra rotunda, s.s.c. superior, p.s.c. posterior, 

 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. 



the spongy bone surrounding it, and may be obtained as a separate 

 mass (Fig. 163), known by the name of the labyrinth, or bony 

 labyrinth to distinguish 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. 163 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 'internaj. 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 (Fig. 162 t. m.) or tympanic 

 membrane. On the outer side of the membrane lies a tubular 



842 



