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 bone, 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 plaa 

 in Tamworth church. 



