1 84 



SCIENCE. 



[Vol. XIV. No. 345 



head alone or by the head and body together. The argument by 

 which this has been established is interesting, if only as furnishing 

 another instance of the ins and outs of that great scientific method 

 by which truth is being constantly tracked to its lair. 



It was first noticed that the semicircular canals could be destroyed 

 without any injury to the sense of hearing, but that their destruc- 

 tion was followed, in various animals, by convulsive motions of the 

 eyes, the head, and the body, and that these motions were in dif- 

 ferent directions as different ones of the three canals were destroyed. 

 It was next pointed out that certain diseases in human beings 

 which were accompanied by feelings of dizziness and loss of equi- 

 librium were connected with injury to those organs. Finally, dis- 

 tinct sensations were said to be felt upon the performance of cer- 

 tain motions, not to be confounded with either a muscle-sense or a 

 sense of touch, and not to be explained except by the hypothesis 

 of some peripheral organ. But at this stage of the discussion, 

 physiologists went too fast and too far ; many of them held that 

 the semicircular canals are spatial sense-organs upon whose ac- 

 tivity depends every perception of the position, or of the direction 

 of motion, of the body. The limitation of their functions to the 

 perception of rotations only has been performed by Delage, and in 

 the following way. He pointed out the aid to be got, in such 

 cases, from the study of illusions. 



The mind is able to interpret the data of the several senses in 

 accordance with external fact only when the sense-organ in ques- 

 tion is working under normal conditions. Unusual conditions are 

 a frequent cause of illusions, and the source of a given illusory 

 sensation can easily be made out, when it is certain that one organ 

 and only one is subjected to circumstances which it is not accus- 

 tomed to. Even when this simple situation cannot be arrived at, 

 the same thing can sometimes be accomplished more indirectly. 

 Thus when the head is turned far to either side, the eyes being 

 shut, objects which can be correctly pointed at with the head in a 

 normal attitude seem to have shifted their position about fifteen 

 degrees in the opposite direction. This indicates that the organ 

 which gives us the sense of direction when we are at rest is in the 

 head. But it is in the eye and not in the ear, for the illusion per- 

 sists when we turn the eyes without the head, and it vanishes 

 when, on moving the head, we force the eyes to remain at rest 

 relatively to it. When we have occasion to look far around, we 

 usually accomplish it by moving the head part of the way and the 

 eyes in the head the rest of the way. We have thus acquired the 

 habit, when we move the head to the right, of moving the eyes 

 still farther to the right, and it is this wrong position of the eyes in 

 the sockets which gives rise to the above illusion. It is the 

 muscles of the eye, therefore, which gives us our static sensations 

 of direction. In a similar way it is shown that our knowledge of 

 the position of the body at any instant is derived, when the eyes 

 are shut, from muscular and cutaneous sensations and from a gen- 

 eral sensitiveness to the direction of gravitation of the fluids and 

 internal organs of the body. 



The feelings which inform us that we are undergoing a pro- 

 gressive motion in any direction, have a similar general origin, 

 but the case is very different with rotations. In the first place, we 

 have a far more delicate sense of rotation than of progressive mo- 

 tion, — a velocity only one third as great can be detected. The 

 illusions that are produced by turning the head to one side while 

 the body is being rotated about any axis are opposite in direction 

 from those produced by the eye, and much greater in amount ; 

 they are, in fact, partly counteracted by the eye-illusions. To 

 produce rotation about a vertical axis, the person is seated in a 

 dark rotating box. He feels himself to be rotating about a vertical 

 axis, as he is, but he has only to turn his head over towards his 

 right shoulder to make himself think that the axis of rotation of 

 the box is inclined towards the left, or that the space about him 

 had been shifted to his right. In other words, he cannot help 

 feeling as he would if his whole body were in a continuous line 

 with his head. A sudden change in the position of the head dur- 

 ing a swift rotation is enough to cause dizziness, nausea, and a 

 general feeling of' extreme unpleasantness, — so much so that 

 Delage says that it requires a very considerable amount of courage 

 to perform the same operation again. There is no other unoc- 

 cupied organ in the head which might be taken to be the source of 



this illusory sensation, except the semicircular canals, and hence 

 we are under the necessity of attributing it to them. 



A few years ago Prof. William James made the interesting dis- 

 covery that deaf people were in very many instances not subject to 

 dizziness nor to sea-sickness, and that they had, for the most part, 

 given up diving, because they found it impossible to tell one direc- 

 tion from another when under water. A disease of the organ of 

 hearing would be very likely to attack the closely adjacent semi- 

 circular canals, and hence these curious observations add great 

 weight to the theory that they are the seat of sensation for certain 

 motions of the body and the head. It may be mentioned that 

 Professor James suggests trying blisters behind the ears, or even a 

 gentle rubbing, as a cure for sea-sickness. 



The argument at this point is not absolutely conclusive, though 

 it is exceedingly strong, but it is put beyond any shadow of doubt 

 by the recent experiments of Brener upon doves, already referred to. 

 He cuts down to the bony semicircular canals, and, without having 

 injured them in the least, he' succeeds in sending an electric cur- 

 rent through them. The head moves in absolute obedience to the 

 current in either one of the three planes according to the canal 

 which is stimulated, and in each plane it moves in one direction- or 

 the other, according to the direction of the electric current. With 

 the interrupted current, no motion at all is produced. But how is 

 it certain that it is only the canals that are stimulated, and that the 

 motion is not due to direct stimulation of the brain ? This objec- 

 tion, which has long been considered a very weighty one, has been 

 absolutely set at rest by Brener. He inserts the needle, which con- 

 veys the current, into the matter of the brain, and motions of the 

 head are, indeed, produced. But he next diminishes the intensity 

 of the current until it is no longer strong enough to produce any 

 effect in that place, and then transfers the needle to the semi- 

 circular canals. The motions are immediately set up again. It 

 is, of course, perfectly natural that the effect of the current upon 

 either the brain- centres or the fibres communicating with them 

 should be the same as upon the nerve-ends, but the fortunate 

 circumstance that the nerve-ends are stimulated by a current 

 too weak to affect the adjacent parts of the brain proves con- 

 clusively — and by a very pretty piece of logic — that the specific- 

 function of those nerve- ends is, in fact, the regulation of the con- 

 vulsive motions of the head. That they regulate the motions- 

 through reflex responses to sensations, — in other words, that the 

 motions are by way of compensation for a subjective feeling of fall- 

 ing in the opposite direction, — is proved by the experience of those 

 individuals who execute the same movements under the influence 

 of disease. The chain of evidence is, therefore, now absolutely 

 complete that the nerves which are distributed upon the enlarged 

 ends of the semicircular canals are sensory nerves whose function 

 (or, at least, one of whose functions) is to give us knowledge of the 

 character and extent of all rotations executed by the head. 



Mach, in his "Bewegungsempfindungen," pubhshed in 1875, de- 

 scribed many very ingenious experiments which went to show that 

 we are conscious of a specific sensation when the activity of the 

 semicircular canals is excited. These have not been considered 

 conclusive by other writers, and in a later work of his {Analyse der 

 EmpfindinigeJi, 1886) he lays less stress upon the excitation of 

 specific sensations, and is content to assume that they set free 

 purely reflectory innervations (p. 73). The semicircular canals 

 may still be called a sense-organ, even though we are not immedi- 

 ately conscious of the sensations which they give rise to. The use 

 of the phrase " unconscious sensation " implies that in the opinion 

 of physiologists there is something which may be properly termed 

 a sensation, but which is woXfelt by us in the ordinary meaning of 

 that word. Any message which is sent in to the brain by an affer- 

 ent nerve, and which gives rise to actions suitable to the circum- 

 stances, is called a sensation, even though our conscious self knows 

 nothing about what is going on until after the action is accom- 

 plished, if even then. Thus in the eye-illusion first mentioned, the 

 full explanation of what takes place is this : — the angle through 

 which the head has moved is measured by the semicircular canals, 

 and this information is transmitted to the centres of the eye-mus- 

 cles, whereupon the eyes make the amount of motion appropriate 

 to that position of the head ; their unusual position in their sockets 

 is then telegraphed in by other nerves of sensation, and this infor- 



