THE SENSE OF HEARING. 163 



means always thoroughly open, but usually its walls lie close to one another, 

 although so loosely that every inward curvature of the tympanum readily 

 separates them for the expulsion of a portion of the air within the cavity. It 

 opens not so readily to the air of the pharynx ; a peculiar effort is necessary to 

 effect an entrance of the air in the above mentioned experiment. In ordinary 

 speaking and singing this effort is never used; the air coming from the lungs 

 chooses the broad and unobstructed outlet of the mouth or nostril, and passes 

 by the orifice of both ear-tubes without opening them; consequently, no sound- 

 producing air- waves come ordinarily by this channel to the organs of hearing. 

 That happens only when, in speaking or singing, we make a peculiar effort, and 

 then indeed we suddenly hear our voice, which before sounded as coming from 

 without, with deafening force and as if it originated in tlie ear itself. This last 

 circumstance is of particular significance; we shall see further on that, in idea, 

 we then only judge the source, which we regard as cause of a sound, to he with- 

 out us, when the waves thereby generated reach our organ of hearing through 

 the external meatus of the car; but whenever it happens that this is impervious 

 to the air-waves, we seek the source of the sound icithin ourselves, even when 

 it is really without. There can, therefore, be no question of hearing our own 

 voice through the Eustachian tube, not to say of its destination for this purpose. 

 We could submit more of such hypotheses to criticism were it worth while. 

 Much has been idly said respecting a strengthening of the waves of sound in 

 the cavity of the tympanum through resonance, but a closer examination of these 

 suppositions would require a special investigation of the physical nature of re- 

 sonance, its conditions and laws, which would lead us too far, and, candidly to 

 speak, be difficult of explanation to novices in this kind of knowledge. 



We leave now the outworks of the organ of hearing and enter upon the 

 proper citadel, the recess in which the ramifications of the auditory nerve are 

 distributed for the reception of the waves of sound, for whose conveyance the 

 appai'atus we have been considering is destined. This recess, to which the 

 name of labyrinth has been given, is, as that name implies, a wonderfully com- 

 plicated structure, in which, though we need no thread of Ariadne to lead us 

 aright, we everywhere encounter mysterious arrangements, whose full acoustical 

 and physiological understanding is reserved for future times. The labyrinth is 

 a cavity surrounded with walls of bone and filled with a fluid, having two 

 openings, of which one {{\iQ fenestra ovalis) is closed by the stapes and its an- 

 nular ligament, the other [iho, fenestra rotunda) by a membrane; this cavity 

 bas not, however, the simple shape of a sack, but consists of many compart- 

 ments and canals in a winding form. If the collective labyrinth be carved from 

 the so-called petrous bone, out of the substance of which it is channelled, it 

 presents, when seen from without, the appearance of the accompanying figure. 

 It consists of two chief divisions, the 



vestibule V with the semicir'-.ular canals '° ' ' " 



K, and the cochlea S. We will con- 

 sider first the vestibule and the canals. 

 The proper vestibule, which forms the 

 middle part of the whole labyrinth, is 

 ovoidal in shape, and measures about 

 one-fifth of an inch from before back- 

 wards and a . little less from above 

 downwards ; its wall exhibits eight 

 openings. One of these is the already 

 mentioned y^wes^ra ovalis, which leads 

 into the cavity of the tympanum and 

 receives the foot-plate of the stapes; 

 a second leads immediately into the 

 spiral canal of the cochlea; a third 

 passes to a narrow channel bearing the 



