148 ON THE SENSES. '^ 



depends on two circumstances : first, on the width of those excursions ; and 

 secondly, on the distance of the particle from the string. 



The wider the vibrations of the string or the departure of c' and c" from c, so 

 much greater, of course, is the displacement of the nearest particles of air, and 

 consequently of those which follow in succession. As regards the diminution 

 of the displacement in proportion to the distance from the string, an example 

 drawn from a different quarter will afford a simple and intelligible illustration. 

 If a stone be thrown into the water, there arises a circular wave, which con- 

 tinues to recede on all sides from the centre of disturbance, andin this wave the 

 elevation and depression follow one another in regular succession, like the con- 

 densation and rarefaction in the sound-producing waves of the air ; but whereas, 

 in the immediate neighborhood of the place where the stone fell, the elevations 

 are high, the depressions deep, the former, though the width of the wave remains 

 unchanged, lose their height, the latter their depth as they recede, until both 

 are at length lost in the level expanse. Such is the case, too, with the air- waves ; 

 the magnitude of the displacement of the several particles diminishes in definite 

 proportion with the increasing distance from the source of the sound, and at a 

 certain distance, which is greater as the movement nearest the source, or, in the 

 present case, the excursion of the string is more considerable, the displacement 

 wholly ceases. If our oar be beyond this limit, it is not reached by the air- 

 wave, and can therefore not perceive the vibrations of the string as sound ; 

 probably, indeed, the perceptibility of the vibrations ceases within that limit, 

 for it is little likely that, when the movement of the particles of air has become 

 very slight, it should any longer have sufiicient intensity to excite the terminal 

 apparatus of the organ of hearing. At the distance of a few feet supposed, in 

 our example, to intervene between the ear and the string, even weak vibrations 

 reach the ear with sufficient intensity, the particles of air next the organ driven 

 at the assumed rate of 400 forward and 400 backward oscillations in a second, 

 forming, with the reqxiisite distinctness, the alternating waves of condensation 

 and rarefaction. Let lis now inquire the operation and destiny of these waves 

 in the organ itself. 



The part of the ear which is first struck by the air-wave is the auricle or 

 pinna, constituting the external organ, in regard to whose structure and arrange- 

 ment every one may easily satisfy himself. This external portion presents a 

 sort of hollow shell with several eminences and channel-like depressions, the: 

 base of the shell containing a funnel-shaped cavity which opens immediately 

 into the inner passages of the organ. In the lower animals the whole ear has, 

 for the most part, more of the funnel, or, at least, spoon-shape, and is in general 

 much more movable than in man. The human ear is, however, by no means 

 immovable; the reader will scarcely fail to have met, in the course of his life, 

 ■ with some wonder-worker, who, with more or less skill, pretends to the power 

 of 'wagging his ears. This egregious accomplishment, be it known, is cheap ; 

 the ear of every one is provided Avith several muscles, which proceed in different 

 directions from its base to the surface of the skull, and which, by their contrac- 

 tion, may change the position of the auricle ; to these muscles are distributed 

 nerves, whose relations to the brain and the will are like those of the innume- 

 rable other nerves, and which are capable, therefore, of communicating motion 

 to the muscles. But just as the deaf-mute possesses the same muscles and the 

 same nerves of volition with him who articulates, and has only not learned to 

 use them because for him the natural instructor, the sense of hearing, is deficient, 

 so it is no absence of the original power but a neglect to use the muscles of the 

 ear in early life, which, in most men, leaves that member incapable of voluntary 

 motion. It may sound strangely, but still is true, that for tliis neglect the blame 

 lies in the tasteless as well as injudicious custom of muffling the heads of infants 

 in caps, by which the ears are. so long kept immovably boimd to the head, and 

 precisely at the time when the soul is getting acquainted with its bodily machine, 



