570 



HEARING. 



three, which are so situated that they correspond 

 to the three dimensions of a cube, its length, 

 breadth, and depth, and that every sound ar- 

 riving in one of these three directions will 

 always strike one canal at right angles to its 

 axis, and another in its length. The position 

 of these canals is likewise such, that the cor- 

 responding canals of opposite sides cannot be 

 parallel, and that therefore any sound which 

 strikes the head in any given direction affects 

 the semicircular canal of one side much more 

 than the corresponding one of the opposite 

 side, whereby it may be determined whether 

 the sound coming in a straight line (from west 

 to east for example,) has really moved from 

 west to east, or from east to west."* They 

 state that in animals in whom the semicircular 

 canals are highly developed, the power of dis- 

 tinguishing the direction of sounds is marked 

 to a proportionate degree. Thus in the mole, 

 the development of these canals is very con- 

 siderable, and from a simple experiment it 

 appears that this animal readily distinguishes 

 the direction of sounds. A mole was intro- 

 duced into a wide but flat vessel filled with 

 earth, in which he was allowed to burrow, 

 and it was found that the mole could be made 

 to move about by sounding an instrument out- 

 side the vessel ; if the instrument were sounded 

 on one side, the animal would always imme- 

 diately turn to the other.f The fox seemed to 

 distinguish the direction of sounds better than 

 cats : if at the same time, and at opposite sides, 

 the high tones of a little bell and the deep 

 tones of a bass viol were sounded, the fox 

 always turned to the side whence the high 

 notes came. Cats seem to be sensible of the 

 direction of high notes only. When upon a 

 violin, or a flageolet, or upon a glass goblet 

 containing water, high notes were sounded, 

 the cats always turned towards the place 

 whence the sound came, even although the in- 

 strument was concealed from them : on the 

 other hand, when a person seated on the ground 

 sounded the low notes of a bass viol before several 

 cats in a garden, they seemed to seek in all direc- 

 tions for the place of the sounding body, without 

 nitting upon the right one. The cow, the 

 horse, the pig, and the rabbit seemed to mani- 

 fest particularly little sensibility to the direction 

 of sounds. The dog appears to have less power 

 to distinguish the direction of sound than man; 

 his smell seems to assist him, and it is well 

 known that when a dog is called by his master, 

 he commonly runs backwards and forwards 

 for some time before he finds out the right 

 direction. The human semicircular canals 

 greatly exceed in width all others examined by 

 Autenrieth and Kerner, but this excess is more 

 as regards the canals properly so called, but 

 does not apply to the ampullae. Scarpa had 



* Op. cit. p. 363. 



t This experiment, however, was repeated by 

 Esser, who assures us that the direction of the 

 movements of the mole was not influenced by the 

 direction of the tones of the instrument. Kastner's 

 Archiv. fur die gesammte Naturlchre, B. 12, s. 56. 

 quoted in Treviranus, Erschcin. und Geseue des 

 Organisclien Lcbens. 



already remarked, that although the canals of 

 oxen and horses were narrower than those of 

 man, the ampullae were scarcely at all smaller 

 than in the human subject. These observers 

 further remarked, in many animals they ex- 

 amined, an inverse ratio between the width 

 of the ampulla and that of the canals ; that 

 the former were wider in proportion as the 

 latter were narrow. In fine, they conclude 

 that the wider the semicircular canals, the size 

 of the animal being taken into account, the 

 greater is the power of distinguishing the direc- 

 tion of sound. Of the lower animals, the 

 first in order as regards this power is the 

 hedge-hog, which, after the human subject, 

 has, relatively to its size, the widest semicircu- 

 lar canals ; we may form some idea of the 

 width of these canals from the fact that in 

 their centre they are nearly as wide as the 

 semicircular canals of the pig, which is so 

 very much larger an animal. Next to the 

 hedge-hog stands the mole, whose canals are, 

 proportionally to the size of the animal, both 

 remarkably wide and long ; they are peculiar 

 also as projecting free (visible without any prepa- 

 tion) into the cavity of the cranium. The 

 mouse and the bat come next, then the fox and 

 the dog,* the rabbit, the cat, the pig, the cow, 

 the horse, and lastly the sheep. 



Professor Wheatstone advocates the theory 

 that our notions of audible direction depend 

 upon the excitation of those portions of the 

 auditory nerve which belong to the semi- 

 circular canals. He conceives that we dis- 

 tinguish best the direction of those sounds 

 which are sufficiently intense to affect the bones 

 of the head, and that it is from the portion 

 which is transmitted through those bones that 

 our perception of the direction is obtained. 

 Thus, we always find it difficult to tell by the 

 ear the position whence the feeble tones of the 

 CEolian harp proceed. The three semicircular 

 canals, then, being situated in planes at right 

 angles with each other, are affected by the 

 sound transmitted through the bones of the 

 head with different degrees of intensity accord- 

 ing to the direction in which the sound is trans- 

 mitted ; for instance, if the sound be trans- 

 mitted in the plane of any one canal, the ner- 

 vous matter in that canal will be more strongly 

 acted on than that in either of the other two ; 

 or if it be transmitted in the plane intermediate 

 between the planes of this canal and the ad- 

 jacent one, the relative intensity with which 

 those two canals will be affected will depend 

 upon the direction of the intermediate plane. 

 The direction suggested to the mind will cor- 

 respond with the position of the canal upon 

 which the strongest impression has been made.f 



* The width of the canals in dogs was found to 

 vary in the different races. Autenrieth and Kerner, 

 loc. cit. 



f Dr. Young thought that the semicircular canals 

 seemed very capable of assisting in the estimation 

 of the acuteness or pitch of a sound by receiving 

 its impression at their opposite ends, and occa- 

 sioning a recurrence of similar effects at different 

 points of their length, according to the different 

 character of the sound ; while the greater or less 



