1318 GENERAL STRUCTURE OF EAR. [BOOK in. 



channel, the external auditory meatus (m.e., m.'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 tympanic cavity (T. (7.), often called the " middle 

 ear," which through the tubular Eustachian tube (E. t.) opens 

 into the pharynx, and which is lined throughout by mucous 

 membrane continuous with that of the pharynx. 



The ' internal ear ' forms the mesial side of the more or less 

 flattened and drum-shaped tympanic cavity opposite to the outer 

 side which is to a large extent formed by the tympanic membrane ; 

 and at two places the osseous tissue of the bony envelope of the 

 internal ear is wanting, the gaps giving rise to what in the dried 

 skull appear as two foramina, but in the fresh state are two 

 membranous fenestra?. One of these, oval in shape, called the 

 fenestra ovalis (Figs. 162, 166, 168 f. o.), lies between the tympanic 

 cavity on the outside and that part of the perilymph space which 

 surrounds the division of the membranous labyrinth known as the 

 utricle on the inside; in the dried bony labyrinth (Fig. 163 F.o.) 

 it appears as a hole in the vestibule. The other, round in shape, 

 called the fenestra rotunda (Figs. 162, 166 f. r.) lies between the 

 tympanic cavity and a part of the perilymph space which enters 

 into the construction of the cochlea ; as we shall see, the perilymph 

 space of the cochlea may be regarded as a peculiar tubular 

 prolongation of that of the vestibule, and the membrane of the 

 fenestra rotunda closes as it were the end of this prolongation. 



Certain bones of the skull, converted by striking developmental 

 changes into a jointed chain of minute bones, the auditory ossicles 

 (Fig. 162 m. i. st.), are by processes of growth thrust into the 

 tympanic cavity in such a way that they eventually seem to lie 

 wholly in the cavity, and to form a bridge across the cavity 

 between the tympanic membrane on the outer side, and the 

 fenestra ovalis on the mesial side. The ossicles are three in 

 number; to the tympanic membrane is attached the malleus', 

 this is joined to the incus, which in turn is joined to the stapes, 

 the end of which is attached to the fenestra ovalis. Into the 

 details of these ossicles we shall enter presently. 



810. The affections of consciousness, which we call sensations 

 of sound, are the result of auditory impulses reaching certain parts 

 of the brain along the auditory nerve ; and these auditory impulses 

 are generated through vibrations, or rhythmically repeated varia- 

 tions of pressure which we call, ' waves of sound,' in some way or 

 other acting upon the terminations of the auditory fibres in the 

 auditory epithelium. The Avaves of sound gain access to the 

 epithelium by means of the perilymph, passing probably in some 

 parts directly through the dermis of the membranous sac to the 

 overlying epithelium, and being in other parts transmitted to the 

 endolymph from the perilymph across the membranous walls, and 

 acting on the epithelium through the endolymph. 



