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 (I, 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 ovalis 

 (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 



