STRUCTURE OF THE INTERNAL EAR. 
409 
which lines the vestibule; each passes-off from the vestibule 
and returns to it again. The cochlea , n, also is a cavity exca¬ 
vated in the hard hone, and lined by a continuation of the 
same membrane; its form is almost precisely that of the in¬ 
terior of a snail-shell (whence its name), being a spiral canal 
which makes about two turns and a half round a central pillar. 
This canal is divided into two, however, by a partition that 
runs along its whole length; which partition is partly formed 
by a very thin lamina of bone, and partly (in the living state) 
by a delicate membrane. The two passages do not communi¬ 
cate with each other except at the top or centre; at their 
lower end (corresponding to the mouth of the snail-shell) they 
terminate differently; for whilst one freely opens into the 
vestibule, the other communicates with the cavity of the 
tympanum, by an aperture termed the fenestra rotunda (round 
window), which is closed by a membrane. 1 Thus the internal 
ear communicates with the cavity of the tympanum by two 
minute orifices only,—the fenestra ovalis and the fenestra 
rotunda— both of them closed by membranes, against the 
former of which the stapes abuts, whilst the latter is free. 
519. The whole internal ear is lined by a delicate mem¬ 
brane, on which the auditory nerve (o, fig. 204) is very 
minutely distributed, especially on the membranous portion 
of the partition between the two passages of the cochlea. 
The cavities are completely filled with fluid, which is set in 
vibration by the movements of the stapes, communicated 
through the membrane of the fenestra ovalis; and these vibra¬ 
tions are probably rendered more free by the existence of the 
second aperture—the fenestra rotunda . It is by the influence 
of these undulations upon the expanded fibrils of the auditory 
nerve, that the sensation of sound is produced; but in what 
way the different parts of the labyrinth (as this complex 
series of cavities is not unaptly called) contribute to the per¬ 
formance of this function, is not yet known. In all Fishes 
but the lowest, the three semicircular canals exist; they have, 
however, no vestige of a cochlea. In the true Eeptiles, a 
rudiment of the cochlea may be generally discovered. In 
Birds, this cavity is more completely formed, though the 
passage is not spiral, but is nearly straight; of its real 
1 There is a double spiral staircase constructed exactly on this plan 
in Tamworth church. 
