658 



NATURE 



\OcL 17, 1878 



only be heard, a small quantity of an odoriferous substance can 

 only be smelled, a dilute solution of a sapid substance can only 

 be tasted, but a very loud sound can be felt by producing vibra- 

 tions sensible to the nerves of the skin ; ammonia gas attacks 

 the mucous membrane of the nose, and mustard bites the tongue. 

 These are not sensations of hearing, smell, or taste, but they are 

 sensations produced by external irritations which, when feebler, are 

 perceptible by these special senses only. It is not, then, um-eason- 

 able to suppose that sudden and violent changes of rate of rota- 

 tion should be perceived by the shock communicated to all the soft 

 and movable parts of the body, although slighter changes may 

 be perceptible only by the special organ of the sense of rotation. 

 The experiment just mentioned is undoubtedly a crucial one, 

 but, in order to obtain from it a decisive answer, it would be 

 necessary to make a series of comparative trials, with varied 

 rates of rotation, upon normal rabbits, and upon rabbits whose 

 auditory nerves had been divided ; if no difference is observed, 

 even with moderate change of rate, the kinetic theory must be 

 abandoned. 



A great deal of valuable information might be obtained by care- 

 fully testing the delicacy and accuracy of the sense of rotation in 

 deaf-mutes. Many deaf-mutes have not only the cochlea, but 

 the whole internal ear, destroyed ; if, then, the inmates of deaf 

 and dumb establishments were systematically tested by means of 

 such experiments as Mach and Brown made upon themselves, 

 experiments which would, no doubt, greatly interest and amuse 

 them, and if the condition of the internal ear were, in each case 

 of post-mortetn examination of a deaf-mute, accurately noted, 

 we should soon obtain a mass of information which would do 

 more to clear up the relation between the sense of rotation and 

 the semicircular canals than any number of experiments on 

 animals unable to describe to us their sensations. 



We cannot pass from this criticism of the kinetic theory with- 

 out noticing a passage in Dr. de Cyon's thesis which seems to 

 show that he has not fully appreciated the bearing of this 

 theory : — " Quelques mots seulement pour mieux faire ressortir 

 I'invraisemblance h priori de la theorie de MM. Mach, Crum 

 Brown et d'autres. Comment admettre que les canaux semi- 

 circulaires servent a nous informer sur la rotation de la tete, 

 quand nous voyons les memes organes parfaitement biendeveloppes 

 chez les animaux qui, comme les grenouilles ou les poissons, 

 ont la tete presque immobile et qui d'ailleurs, pas plus que les 

 autres animaux, n'executent pas habituellement des mouvements 

 de rotation? 



"Pourquoi justement la presence d'un organe des sens pour 

 un mouvement peu habituel et pas pour beaucoup d'autres, pour 

 les mouvements que les animaux executent continuellement ? " — 



P. 47- 



No doubt a frog or a fish cannot move its head freely without 

 at the same time moving its body, but head and body together 

 move and perform frequent and rapid rotations. Whenever 

 a fish or other animal changes the direction of its motion, rotation 

 takes place, and a knowledge of the amount and of the axis of 

 this rotation is necessary if the animal is to retain any sense of 

 its orientation. Of all animals a fish, moving and turning with 

 great rapidity and sharpness in a dense medium often affected by 

 complicated currents, seems to have most need of such an organ 

 which serves the same purpose as a ship's compass, an instru- 

 ment surely not useless because a ship is rigid and does not 

 habitually perform movements of rotation. 



We have already alluded to some of the phenomena of optical 

 vertigo. This subject is discussed at considerable length by Dr. 

 de Cyon, and it is therefore right that we should here explain 

 somewhat fully the opinions held in reference to it by various 

 experimentalists. 



The phenomena themselves are, in the main, well known. If 

 we rotate about a vertical axis either actively (that is turning 

 ourselves) or passively (that is being turned round on a movable 

 chair or platform by an assistant) we are at first fully aware that 

 we are turning and that external objects are at rest ; gradually 

 external objects seem to move round us in a sense opposite to 

 that of our real motion. If at this stage we stop we not only 

 feel that we are being turned round in the opposite sense to that 

 of the previous real motion, but we see, or think we see, exter- 

 nal objects turning round. These two imaginary rotations, viz., 

 that of our body which we feel, and that of external objects 

 which we see, take place about the same axis, in the same sense 

 and at the same rate. The axis is parallel to the line in the 

 head which was the axis of the original real rotation, and the 

 sense is, as already explained, contrary to that of the original 



rotation. These facts are well known and were fully described 

 by Darwin in the "Zoonomia" and by Purkinje. Another 

 phenomenon closely connected with them was first noticed by 

 Purkinje, and has since been investigated by Breuer and by de 

 Cyon. When a real rotation of the body takes place the eyes 

 do not at first perfectly follow the movement of the head. 

 While the head moves uniformly the eyes move by jerks. Thus, 

 in the diagram. Fig. 3, where the abscissas indicate time and the 

 ordinates the angle described, the straight line a b represents 

 the continuous rotatory motion of the head and the dotted line 

 the discontinuous motion of the eye. 



Here it will be seen that the eye looks in a fixed direction for 

 a short time, represented by one of the horizontal portions of 

 the dotted line a b, and then very quickly follows the motion of 

 the head, remains fixed for another short time, and so on. 

 After the rotation has continued for some time the motion of 

 the eye gradually changes to that represented by the dotted line 

 c d'va. Fig. 4, The eye now never remains fixed, but moves for 

 a short time more slowly than the head, then quickly makes up 

 to it, then falls behind, and so on. At last the discontinuity of 

 the motion of the eye disappears, and the eye and head move 

 together. 



F^g.3. 



Fig. 



If now the rotation of the head be stopped (of course the 

 body stops also) the discontinuous movements of the eyeballs 

 recommence. They may now be represented by the dotted line 

 in Fig. 5. 



Fig.S 



f 



Fig 6 



X 



The intermittent motion of the eyes gradually becomes less, 

 passing through a condition such as that shown by the dotted 

 line in Fig. 6, and at last ceases. The consideration of these 

 oscillatory motions is greatly simplified if we draw diagrams 



similar to the above, but in which -/- is made proportional to 



ax 

 the apparent rate of rotation, that is, to the rate of rotation as 

 perceived by the observer, instead of to the real rate as mea- 

 sured or inferred from external observations. 



The apparent motion of the head and of the eyeballs is shown 

 in Fig. 7 when uniform real rotation is kept up until it ceases to 

 be perceived, and is then suddenly stopped.-^ The accented 

 letters «', b', &c., correspond to a, b, &c., in Figs. 3, 4, 5, 

 and 6. 



We now see what is the real nature of the oscillatory move-.: 

 ments. The eye remains for a short time in an apparently fixed ' 



' In order to represent accurately the phenomena, the oscillations should '. 

 be much more numerous and of course smaller. As five or six complete j 

 turns are required before all sense of rotation is abolished, and as Dr. i 

 Breuer finds at least ten oscillations in one complete turn, there should be at 

 least fifty between a' and e'. These could not be represented without 

 making the figure either very large or very indistinct. Such diagrams are 

 not used by any of the physiologists who have investigated the subject, and 

 must not be interpreted too rigidly, as both the duration and the extent of 

 the oscillations vary considerably. The figures, however, represent the , 

 general natu( ' of the phenomena sufficiently for cur purpose. 



