268 OF SENSATION, AND THE ORGANS OF THE. SENSES. 



from side to side, and at last will fall obliquely on the membrane, thus perhaps 

 contributing to the notion of direction. The power of the lining of the meatus 

 to conduct sound from the external ear, is made evident by the fact, that, when 

 both ears are closely stopped, the sound of a pipe having its lower extremity 

 covered by a membrane, is heard more distinctly, when it is applied to the 

 cartilage of the external ear itself, than when it is placed in contact with the 

 surface of the head. The resonant action of the air in the tube is easily 

 demonstrated, by lengthening the passage by the introduction of another tube ; 

 the intensity of external sounds, and also that of the individual's own voice, as 

 heard by himself, are then much increased. 



361. 'Many facts prove, however, that the fluid of the labyrinth may be 

 thrown into vibration in other ways than by the tympanic apparatus. Thus 

 in Osseous Fishes, it is only by the vibrations transmitted through the bones 

 of the head that hearing can take place. There are many persons, again, 

 who can distinctly hear sounds which are thus transmitted to them ; although, 

 through some imperfection of the tympanic apparatus, they are almost insensible 

 to those which they receive in the ordinary way. It is evident, where this is 

 the case, that the nerve must be in a state fully capable of functional activity ; 

 and, on the other hand, where sounds cannot thus be perceived, there will be 

 good reason to believe that the nerve is diseased. 



362. A single impulse communicated to the Auditory nerve, in any of the 

 foregoing modes, seems to be sufficient to excite the momentary sensation of 

 sound; but most frequently a series of such impulses is concerned, there 

 being but few sounds which do not partake, in a greater or less degree, of the 

 character of a tone. Any continuous sound or tone is dependent upon a suc- 

 cession of such impulses ; and its acuteness or depth is governed by the rapidity 

 with which they succeed one another. It is not difficult to ascertain by 

 experiment, what number of such impulses or undulations are required to 

 give every tone which the ear can appreciate. Thus, if a circular plate, with 

 a number of apertures at regular intervals, be made to revolve over the top of 

 a pipe through which air is propelled, a succession of short puffs will be 

 allowed to issue from this ; and, if the revolution is sufficiently rapid, these 

 impulses will unite into a definite tone. In the same manner, if a spring be 

 fixed near the edge of a revolving toothed wheel, in such a manner as to be 

 caught by every tooth as it passes, a succession of clicks will be heard ; and 

 these too, if the revolution of the wheel be sufficiently rapid, will produce a 

 tone. The number of apertures in the plate which pass the orifice of the pipe 

 in a given time, or the number of teeth which pass the spring being known, 

 it is easy to see that this must be the number of impulses required to produce 

 the given tone. Each impulse produces a double vibration, forwards and 

 backwards ;* hence the number of impulses is always half that of the single 

 vibrations. The maximum and minimum of the intervals of successive pulses, 

 still appreciable by the ear as determinate sounds, have also been determined 

 by M. Savart more satisfactorily and more accurately than had previously 

 been done. If their intensity is great, sounds are still audible which result 

 from the succession of 24,000 impulses in a second ; and this, probably, is not 

 the extreme limit in acuteness of sounds perceptible by the ear. From some 

 observations of Dr. Wollaston's, it seems probable that the ears 'of different 

 individuals are differently constituted in this respect, some not being able to 

 hear very acute tones produced by Insects, or. even Birds, which are distinctly 

 audible to others. Again, the sound resulting frn 16 impulses per. second, 

 is not, as has been usually supposed, the lowest appreciable note ; on the con- 

 trary, M. Savart has succeeded in rendering tones distinguishable, which were 



*This is seen when a string is put in vibration, by pulling it out of the straight line. 



