45: 



NA TURE 



\_Sept. 5, 1889 



of rotation about an axis the position of which we can foretell from 

 the relative positions of the planes of thc^e three canals. And 

 clinical observations on persons suffering from this disease show 

 that the vertigo actually is about this axis. Deaf-mutes are persons 

 who from a veryearlyage have had no sense of hearing at all. This 

 condition may arise from the imperfect development of the organ 

 of hearing, or from its early destruction by disease. In either case 

 it often happens that the organs we have been discussing, from 

 their nearness to the organ of hearing, are involved in the mis- 

 chief, and are also imperfectly developed, or destroyed by disease. 

 Deaf-mutes have therefore not unfrequently the semicircular 

 canals in a state unfit for use. Experiments have been made on 

 deaf-mutes with the object of testing the accuracy of their sense 

 of rotation. Those who have made these experiments report that 

 many deaf-mutes are insensitive to rotation. If these o'lserva- 

 tions are confirmed, the theory I have just been explaining will 

 receive a very great support. 



In order to illustrate this theory, and to show that the principle 

 on which it is based is a sound one, I have devised a sort of 

 working model which I shall now show you. I may say that 

 when I accepted your invitation to lecture here, and had selected 

 the subject of our sensations of motion, the idea occurred to me of 

 making a sort of working model of the semicircular canals. The 

 difficulty was to find an instrument-maker who could help me 

 over the obstacles which always lie in the way of a designer who 

 is not himself an engineer. I take this opportunity of thanking 

 Mr. Alex. Frazer for his help in this matter. He at once under- 



stood what I wanted, and so gave me the use of his skill and 

 experience that the instrument here is exactly what I intended it 

 to be, and a great deal better than my most sanguine hopes. 



The model, as perhaps you will better see from this somewhat 

 diagrammatic drawing than from the machine itself, consists 

 essentially of two heavy wheels, placed side by side, with their 

 axes parallel, in a frame which itself can be turned round about 

 an axis parallel to that of the wheels. These heavy wheels 

 correspond to two parallel canals, say the two horizontal canals. 

 As it is the inertia of the fluid in the canals which enables them 

 to work, so here it is the inertia of the heavy wheels. Each 

 wheel has a stop, which altogether prevents its turning, in one 

 way round, beyond a certain point. The one wheel is thus 

 checked in turning the one way, the other in turning the other 

 way. Each wheel is just held against its stop by a spring which 

 is stretched when the wheel turns away from the stop. Each 

 wheel with its stop and spring is as nearly the mirror image of 

 the other as it could be made. When I turn round the frame, 

 both wheels tend to lag behind the rotation of the frame, on 

 account of their inertia. One of them cannot lag behind at all 

 because of its stop, and the other cannot lag much behind because 

 of its spring ; the stronger we make the spring the less can it 

 lag behind. This lagging behind is, of course, a turning of the 

 wheel on its axis, relatively to the frame, in the opposite sense 

 to that in which the frame is turned. As we continue to turn 

 the frame with uniform speed, the spring brings the wheel back 

 to its original place against the stop, and further rotation at the 



same rate makes no change in the relative position of the parts 

 of the machine. But if we now quickly bring the frame to rest, 

 both wheels in virtue of their inertia tend to continue their rota- 

 tion : one, that one which made the relative movement before, 

 cannot continue its rotation because of its stop ; the other can 

 rotate a little, not much, because of its spring — it turns a little, 

 but is soon brought back to its original position against its stop 

 by the spring. You will easily see that just as in the model the 

 inertia of the wheel corresponds to the inertia of the fluid, so 

 here the stretching of the spring corresponds to the stretching of 

 the ampulla. All that we want to make the model complete is 

 to find some way of making the stretching of the spring visible, 

 something which shall correspond to the message sent to the brain. 

 You cannot easily see the stretching of the spring while the frame is 

 turning round, and it was necessary to devise some way of making 

 it visible. We must here leave the an.alogy of the living organ. 

 The brain turns with the labyrinth, but we are the brain of this 

 machine, and we do not turn with it. After a good deal of con- 

 sideration, and after thinking of and rejecting a good many plans, 

 I fell upon the one I shall now show you. In the lower end of 

 the axle of each wheel there is fixed a stop-cock, through which 

 gas can pass from one pipe to another. When the wheel is 

 against the stop only a very little gas passes — just enough to 

 prevent the jet going out. When the wheel turns away from its 

 stop, the stop-cock is opened, and the stop-cock is so adjusted 

 that the quantity of gas passing shall be roughly in proportion 

 to the stretching of the spring. By a contrivance indicated in 



the diagram, the two gas pipes, one from each stop-cock, are 

 brought through the axle of the frame and led each to a gas jet. 

 Now, when I begin to turn the frame one jet flares up, but as 

 I continue the turning, as nearly uniformly as I can, you see the 

 jets remain at their minimum, which I shall call zero of sensation. 

 Of course I could have made this zero the zero of gas too, but 

 then we should have needed a subsidiary flame to light the gas 

 when the stopcock opened. I now stop the frame, and yon see 

 the other jet flare up for a little. That corresponds to the second- 

 ary imaginary rotation which we feel when a real rotation is 

 stopped. I ought to apologize for so often calling this a 

 secondary or imaginary rotation. I hope you all see now that 

 it is as really an acceleration in the strict meaning of the word as 

 the original start from rest. 



I have taken this question — What is the organ by means of 

 which we perceive acceleration of r^.'^/t'/^ motion ? —first, be- 

 cause it has been most fully worked out. We now come to the 

 question. How do we perceive acceleration of translatory motion ? 

 This, as we have seen, is the same as the question. How do we 

 by our senses recognize the direction and estimate the intensity 

 of what is to us at the moment the force of gravity ? A very 

 natural suggestion as to the way in which we perceive the in- 

 tensity of this force is that it is a skin sensation ; that it is by 

 the greater or less pressure which we feel on that part of our 

 body which rests on our support. Prof. Mach, to whose experi- 

 I ments I have had often to refer, and to whom we owe, more 

 than to any other investigator, our knowledge of the whole sub- 



