THE SEMICIRCULAR CANALS. 1195 



crista in the utricle ; but while we admit this ingenious suggestion, it 

 does not appear to account for the presence of such bristles in the cristae 

 of the ampullae themselves, nor does it account in any way for the 

 remarkable form of the canals. 



Let us consider the base of the stapes moving inwards and outwards 

 with the varying pressures caused by waves of sound. We have already 

 seen how impulses must thus be communicated to the cochlea and 

 the saccule. They must also be communicated to the utricle, into which 

 we find the five openings of the ampullae of the canals. It is evident 

 that the pressures must act equally on all the ampullae, and we can 

 conceive the pressure tending to compress or push back the fluid at the 

 mouth of each ampulla. If the pressure were less at the mouth of one 

 ampulla than at the mouth of the other, there would be a flow of fluid 

 in the canal. But as the pressure at the mouth of each canal must be 

 the same, and as the fluid is incompressible, no movement of the fluid 

 can take place, but there will be an increase of pressure on the walls of 

 the ampullae, and the effect of this will be to cause the ampulla and 

 the other end of one canal to separate slightly, and to press the 

 wall bearing the nerve-endings against the osseous wall. If the tube 

 were slightly elastic, such a movement would undoubtedly take place. 

 Thus, with a positive pressure the ampulla and the other end of the 

 canal would tend to move away from each other, thus, -^ . . ^, and, 

 with a negative pressure towards each other, thus, ^ ^ . In this 

 way one can conceive of sound-waves acting on the ampullae. 1 



Again, as the three canals lie in the three directions of space, it has 

 sometimes been held that they may have to do with the appreciation of 

 the direction of sound. An obvious objection to this view is, that as a 

 matter of fact our perception of the direction of sound, or, in other 

 words, of the position in space of the sounding body, is by no means 

 accurate, and that we look with the eyes for the source of the sound, and 

 instinctively direct the ears or the head, or both, in the direction from 

 which the sound appears to proceed. We usually judge of the direction 

 of a sound by making two or more simultaneous or successive observa- 

 tions. If the sound be heard more loudly in the right ear than in the 

 left, we turn the head to the right, and by repeated observations judge 

 of the direction ; if the sound be produced at any point equidistant from 

 both ears, we cannot tell its position unless we see what produces it. 

 It is conceivable that we might have been furnished with an organ by 

 which the source of sound might have been at once located, without 

 moving the head, but we do not possess this faculty. 2 



The relation of the canals in the two ears is remarkable, and must 

 have some physiological significance. This has been carefully investi- 

 gated in the human subject by Crum Brown, 3 who devised a strict 

 method by which the angles formed by the planes of corresponding 

 canals were measured. The results were as follows : (1) The horizontal 

 (external) canal has its plane sensibly at right angles to the mesial plane, 

 and therefore the two (right and left) horizontal canals are sensibly in 

 the same plane. (2) The planes of the superior (vertical) and posterior 

 (vertical) canals of the same side make nearly the same angle with the 



1 Suggestion by Dr. M'Kendrick. Not previously published. 



2 See also Preyer, Arch. f. d. ges. PhysioL, Bonn, 1887, Bd. xl. S. 586. 



3 Nature, London, 1878, vol. xviii. p. 663; also M'Kendrick's "Physiology," vol. ii. 

 pp. 694-702. 



