172 



ON THE SENSES. 



interlink witli one another, in order to make one or tlie other intelligible ; this 

 important help does the sense of touch, in community with the "muscular sense," 

 afford to the sense of hearing, inasmuch as they inform us of the direction in 

 which lies the external source of sound. In the first place, the sensibility of 

 the tympanum, just alluded to, furnishes us the ready means of deciding on which 

 side of us the source of sound is situated, since we naturally seek it on the side 

 of that ear whose tympanum, being more strongly agitated by the wave of sound, 

 communicates to us a more intense sensation of touch. The muscles which 

 move our head convey to us, through the sensations continually connected with 

 their action, accurate ideas of the position of that part of our frame, and conse- 

 quently of both our organs of hearing, so that we are conscious of the position 

 in space of the more strongly vibrating tympanum. If the difference of these 

 sensations of the tympanum is not sufficiently distinguishable to afford a certain 

 judgment, we turn the head on the cervical axis hither and thither until the 

 sensations of touch and hearing acquire greatest intensity in one of the ears, and 

 then present this ear, whose position the muscular feeling teaches us, in a right 

 line to the source of sound, for we know by experience that in this way a given 

 sound is most distinctly heard. If the sensations be equally strong in both ears, 

 we have no alternative but to seek the intonating body directly before or behind 

 us. Most probably, in discerning whether it is before or behind, above or below 

 us, we are guided by another class of sensations of touch, that, namely, which 

 results from the action of the air- waves on the cartilaginous outer ear or auricle. 

 There are simple experiments which show that this member materially assists 

 in the discernment of the direction from which sounds proceed. If we press 

 both auricles closely to the head, we lose the power of discerning with certainty 

 whether the source is before or behind, deceive ourselves easily, and, if the ob- 

 ject is before, seek it behind us. If we firmly compress the auricles, and, more- 

 over, place our hands flat in front of both organs so that they shall stand like 

 the auricles, only on the wrong side, we regularly confound before and behind, 

 on an impartial examination of the impressions received. But for those who 

 would repeat these experiments we must add that prepossession in such trials ig 

 not easily avoided ; we must eliminate with the utmost care all possible tokens, 

 from which, by the help of the other senses or from experience, we might form 

 a judgment of the position of the object. It is not easy to explain with certainty 

 in what manner the auricle, through its perceptions of touch, leads to a deter- 

 mination of the direction of sounds ; but it cannot be doubted that, as regards 

 this function of the organ, we gradually learn to associate the different qualities 

 of the sensations, which is the necessary result of the impact of differently 

 directed waves on different parts of the auricle, with determinate ideas of place, 

 just as we gradually learn by experience to form a correct judgment in respect 

 to all the impressions received through our widely organized sense of touch. 



The faculty of estimating the remoteness of a source of sound is also one 

 which we acquire through the indirect process of experience. In many cases it 

 is the sense of sight which enables us to form a judgment of the distance of the 

 object which generates the sound. If we do not see that object, the judgment 

 is only then in some degree exact when it relates to definite sounds or noises of 

 which we have had sufficient experience to be able to estimate the diminution 

 of intensity with the increasing distance. Thus is faithfully inculcated upon 

 our memory the intensity of the impression of a human voice in ordinary speech 

 or loud calling, in immediate proximity and at all distances ; hence it is that we 

 are capable of estimating with considerable correctness the remoteness of an in- 

 dividual from the intensity with which his voice reaches us. Various circum- 

 stances, such, for instance, as a strong wind opposite in direction to the sound, 

 may, indeed, render our judgment uncertain ; yet in this case also we possess to a 

 certain degree experiences respecting the influence of such circumstances, which 

 serve at least for an approximately correct estimate of intervening distances. 



