536 THE RABBIT CHAP. 



outer wall and a spiral shelf arising from its inner wall. Thus 

 the entire cochlea shows 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 respec- 

 tively, 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 tympanic membrane, and probably correspond- 

 ing morphologically 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. 143, 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. 



