Sept. 5, 1889] 



NATURE 



451 



depending on this principle, which I have seen played. The sub- 

 ject of the joke, who ought, of course, to be a ])erson not con- 

 versant with the laws of the sensation of motion, is asked to hold 

 a poker upright on the flo">r, and, placing his forehead on the top 

 of it, to walk three times round it, rise up, and walk across the 

 room. Ilis march round the poker is a rotation about the 

 fore-and-aft axis of his head ; when he rises up he feels the con- 

 trary imaginary rotation, about the same axis, now, of ciurse, 

 horizontal, so that, if he went round with the sun, he falls to the 

 right, and to the left if he went round the other way. In a 

 very interesting experiment with the turn-table we have a com- 

 bination of real and apparent rotation. Lie down on the table, 

 say on the left side, so that the left ear is vertically under the 

 right ear, so making the right-and-left axis of the "head the axis 

 of rotation ; let the table be turned round at a uniform rale, wait 

 until all senation of rotation has ceased, and then, while the 

 uniform turning is still going on, roll yourself over on y lur back. 

 You will then experience a very startling sensation. The new 

 axis of rotation of the head is the fore-and-aft one ; there was 

 no rotation aV)out it l^fore, therefore you feel that real rotati m, the 

 real rotation about the old axis— right-and-left — has just ceased, 

 therefore you feel the imaginary opposite rotation. The sudden 

 occurrence of the combination of these motions, felt as real, 

 resulting from a cau-:e so seemingly inadequate, rolling from your 

 side to your back, gives rise to an almost dreadful sense of 

 insecurity. 



Thus then in rotational as in tran'^lational motion it is change 

 of motion, whit is technically called acceleration, which we per- 

 ceive. There are two questions which naturally arise in this 

 connection : (i) What is the use of this sense? and (2) What is 

 the organ of this sense, and how does it work ? What is the use 

 of it? Everyone will, I am sure, admit that it must be of great 

 use to us to have a constant knowledge of the direction of the 

 vertical ; to have, as it were, a private level of our own, which 

 we cannot mislay. As to our sense of rotation, it is to it chiefly 

 that we owe what we call our knowledge of the airts ; it enables 

 us, as we walk about on winding roads, or through narrow 

 crooked streets, to retain some idea of the real directions. But 

 the chief use, no doubt, of the sense of rotation is to enable us 

 to control and regulate the rotatory movements of our head — 

 movements we are constantly making as we look about us. It 

 may be asked, in these short quick movements of the head, 

 where is the secondary sensation of turning the other way which 

 I described ? We never experience it at all. Mach has very clearly, 

 and with great penetration, explained how this comes about. 

 These quick movements, our habitual movements of rotation, are 

 so short in their duration, that daring theai we do not come to 

 feel that our head is at rest. The sense of the original real ro- 

 tation is still vivid when the rotation is stopped, so that the 

 secondary sensation of an imaginary motion the other way round 

 merely annuls the primary sensation, puts an end to it when the 

 real rotation stops. Without such a stopper of sensation we 

 should go on feeling the rotation for a short time after it was 

 done. 



What is the organ of this ?ense ? There is in our head a very 

 remarkable organ which has been for long a puzzle to physio- 

 logists, an organ which is found not only in our head?, but also in 

 the heads of all mammals, of all birds, and of all but the very 

 lowest fishes (and even in the very loweU fishes it occurs in a 

 less developed form). This organ is so closely related in posi- 

 tion to the organ of hearing, that it was long supposed to be a 

 part of it, and we shall see what attempts were made to explain 

 It as an organ of hearing. I shall give as short a description of 

 it as is compatible with making it plain how it can act as the 

 organ of the sense we have been considering. I must at the same 

 time confess that in some points our knowledge of the matter is 

 still imperfect, and that much has ctill to be done before we can 

 explain its action as fully as we can that of the eye, for instance. 



The organ in question is lodged in a bony cavity continuous 

 with that which contains the organ of hearing, and for this 

 reason was long, and perhaps by some is still, regarded as itself 

 having something to do with the perception of sound. This 

 cavity in the hardest bone in our head consists of four parts — the 

 vestibule, and the three semicircular canals. The vestibule is 

 an irregular chamber, in man about \ of an inch long and I of 

 an inch broad and deep. In its walls are five openings leading to 

 the semicircular canals. These are tunnels in the bone having an 

 elliptical or circular section, and opening at each end into the 

 vestibule. The central line of each canal lies nearly in one plane, 

 which we may call the plane of the canal. At one end of each 



canal there is an enlargement called the ampulla. The planes of 

 the three canals are approximately at right angles to one another. 

 The canals are named from their position — the horizontal, the 

 superior, and the posterior ; the two latter unite at their non- 

 ampullary ends before joining the vestibule, so that there are 

 five and not six openings into the vestibule from the canals — 

 three ampullary, one for each canal, and twonon-ampullary, one 

 for the horizontal, and one common to the superior and the pos- 

 terior canals. The plane of the horizontal canal is nearly hori- 

 zontal in the ordinary position of the head in all animals, and is 

 therefore at right angles to the mesial plane : the planes of the 

 two other canals make nearly equal angles with the mesial plane. 

 In the bony labyrinth just described there is inclosed a mem- 

 branous labyrinth of a generally similar form. It consists of the 

 utricle, lodged in the vestibule, and of three membranous canals, 

 each furnished with a membranous ampulla. The membranous 

 labyrinth does not fit tight into its bony case. The utricle is much 

 sma'ler than the vestibule, which contains, besides, the saccule, 

 an organ connected with the cochlea ; and the diaoneter of the 

 membranous canals, except at the ampullar, is much less than 

 that of the bony canals. The membranous ampullae, on the 

 other hand, nearly fill the bony ampullar. The entire cavity is 

 thus divided into two spaces, one within, the other around, the 

 membranous labyrinth ; each is filled with a liquid, the endo- 

 lymph and the perilymph. The nerves are distributed to one 

 spot in the utricle, and to a crescent-shaped ridge near the 

 middle of each ampulla. The nerves end in hair-cells, the hairs 

 of which project into the endolymph. The macula acuslica, the 

 spot in the utricle to which nerves are distributed, is covered 

 with a gelatinous layer in which are embedded small crystals of 

 carbonate of lime. 



Everyone must see that an apparatus so purpose-like in its 

 arrangement must have a use, and this use must be one applic- 

 able to all the higher animals. 



It was long .supposed that it had to do with our perception of 

 the direction 'from which sounds come to us. The idea is not 

 unnatural, and is obviously derived from the nearness of the ap- 

 paratus to the organ of hearing, and from the relation of its form 

 to the three dimensions of space. No explanation has ever been 

 given how it could serve this inirpose ; and we can easily show 

 that it does not do so by experimentally showing that we have 

 no means of ascertaining the direction from which a sound comes 

 except by two or more simultaneous or successive observations. 

 If a sound is heard louder in the right ear than in the left we 

 conclude that it comes from the right, and by turning round the 

 head we soon get a sufficient number of observations to enable 

 us to judge of the exact direction. If a short sharp noise is 

 made at a point equidistant from the two ears, we do not know 

 the direction from which it comes unless we see what causes it. 



But the apparatus is admirably fitted to act as the organ of the 

 sense of rotation, or rather of the sense of acceleration of 

 rotatory motion. Let us first consider the action of one canal. 

 If the head is rotated about a line at right angles to the 

 plane of the canal, with the ampulla leading, you will see from 

 looking at the diagram that there will be a tendency of the 

 endolymph to go from the utricle into the ampulla, and of 

 perilymph to go from the space between the bony and the mem- 

 branous canals into the utricle. These will conspire to stretch 

 the membranous ampulla where the nerve-endings are, and we 

 can easily see how this will stimulate the nerves, and send a 

 message to the brain. But this stretching will not take place if 

 the head be rotated the other way al.oui. In that case the tendency 

 of the fluids will be in the opposite directions, and will rather make 

 the ampulla less tightly stretched, and we can readily suppose 

 that this may not stimulate the nerves, and no message will be 

 sent to the brain. One canal will thus be able to give indications 

 of rotation about one axis, in one of the two ways round. But 

 for each axis we have two canals, one turned the one way and the 

 other the other. And as by means of three rectangular axes we 

 can represent any rotation, so any rotation will be perfectly re- 

 cognized by means of the six canals. That this is actually the 

 function of this organ is further proved by the effects of injury or 

 di.-ease. One ear is sometimes attacked by inflammation while 

 the other ear is unaffected. In such cases the patient suffers from 

 persistent vertigo — that is, sense of rotation where no real rotation 

 occurs. This is, at least, one form of what is known as Meniere's 

 disease, so-called from the name of the physician who first drew 

 the attention of medical men to it. Now it is obvious that, if the 

 six canals act in the way I have indicated, the pathological 

 irritation of the three canals of one ear will produce a sensation 



