408 TYMPANIC APPARATUS !—INTERNAL EAR. 
may be tightened, so as to vibrate in accordance with sharper 
or higher tones ; but it will then be less able to receive the 
impressions of deeper sounds. This state we may artificially 
produce either by holding the breath and forcing air into the 
Eustachian tube, so as to make the membrane bulge-out by 
pressure from within ; or by exhausting the cavity by an effort 
at inspiration with the mouth and nostrils closed, which will — 
cause the membrane to be pressed inwards by the external : 
air. In either case the hearing is immediately found to be — 
imperfect ; but it will be observed that while the experimenter — 
thus renders himself deaf to grave sounds, acute sounds are 
heard even more distinctly than before. There is a different : 
limit to the acuteness of the sounds of which the ear can 
naturally take cognizance, in different persons. If the sound 
be so acute (or high in pitch) that the membrana tympani will © 
not vibrate in unison with it, the individual will not hear it, 
although it may be loud ; and it has been noticed that some 
persons cannot hear the very shrill tones produced by par- 
ticular Insects, or even by Birds, which are distinctly audible 
to others. There is good reason to think that the two 
muscles which have been mentioned (§ 516) as tightening 
and relaxing the tympanum, exert a regulative influence upon 
its tension analogous to that which the contractile fibres of 
the iris possess in regard to the diameter of the pupil (§ 534) ;_ 
preparing it to be acted-on by faint sonorous undulations 
when we are listening, and moderating the concussion of very 
loud sounds which are anticipated. 
518. The internal ear is composed of various cavities that 
communicate with each other ; of these the vestibule (7, fig. 204) 
may be regarded as the centre, whilst from it there pass-off 
on one side the three semicircular canals, m, and on the other 
the cochlea, n. The vestibule is the part which corresponds 
with the simple cavity that constitutes the whole organ of 
hearing in the lower animals (§ 512), and the others may be 
regarded as extensions of it for particular purposes. It com- 
municates with the cavity of the tympanum by a small orifice 
in the bony wall that separates them, termed the fenestra ovdlis 
(oval window) ; but this orifice is closed by a membrane, to 
which the lower end of the stapes is attached. The three 
semicircular canals are passages excavated in the solid bone, 
and lined by a continuation of the same membrane as that 
