536 THE RABBIT CHAP. 



three cavities in transverse section a middle, the membranous cochlea 

 or scala media, and a scala vestibuli and a scala tympani on either side of 

 it respectively, which communicate with one another at the apex of the 

 cochlea and with the perilymphatic cavity surrounding the rest of the 

 membranous labyrinth at its base, where the scala tympani abuts 

 against the membrane of the fenestra rotunda, and the scala vestibuli 

 against that of the fenestra ovalis. On the wall of the scala media which 

 separates it from the scala tympani is a specially modified series of 

 auditory cells forming what is known as the organ of Corti, which 

 receives nerve-fibres from a branch of the auditory nerve extending along 

 the spiral shelf of the cochlea. 



The middle ear (p. 466) is constituted by the tympanic 

 cavity in the tympanic bulla (p. 492), and communicates 

 with the pharynx by the Eustachian tube (Fig. 143, E). 

 The tympanic membrane (M), situated obliquely at the 

 boundary of the bulbous and the tubular portions of the 

 tympanic bone, separates the middle ear from the external 

 ear, consisting of the auditory passage (Ex) and the pinna 

 (p. 486). 



The fenestra ovalis is plugged by a small stirrup-shaped 

 bone, the stapes (Fig. 143. Oj), one of the three auditory 

 ossicles (p. 492) connecting the internal ear with the tym- 

 panic membrane, and probably corresponding morpho- 

 logically to part or the whole of the frog's columella (Fig. 10) : 

 with it is connected a small stapedius muscle, serving to 

 keep the membrane of the fenestra ovalis on the stretch. 

 The middle bone of the chain is the incus (Fig. 133, O 2 ), a 

 short process of which is articulated to the stapes by the 

 intermediation of a small bony nodule, while its body 

 articulates with the outer bone of the series, the malleus 

 (O 3 ). Arising from the body of the malleus is a handle-like 

 process or manubrium, which is attached to the tympanic 

 membrane (M) : this has the form of the roof of a tent, and 

 is kept on the stretch by a small muscle, the tensor tympani, 



