THE SEMICIRCULAR CANALS. ti 97 



remains unaltered. It was also noted that the sensation of movement 

 was experienced even when the eyes were closed, and when no tactile 

 sensations were possible ; nor could Purkinje avoid observing the dis- 

 turbance of equilibrium all have experienced, and the involuntary 

 tendency to steady one's-self by seizing hold of external objects. Purkinje 

 explained the phenomena by supposing that when rotation takes place, 

 the brain, of soft consistence, and surrounded by a thin layer of fluid, 

 has a tendency to drag behind the movements of the walls of the 

 cranium, and that the dragging might, without tearing the brain sub- 

 stance, exercise tension on the cerebellum and cerebral peduncles. He 

 did not associate the phenomena with the semicircular canals ; but he 

 supposed that, as during rotation the images of external objects flash 

 across the retinae, the objects appear to be in movement, and the 

 ocular muscles contract spasmodically or intermittently to secure fixa- 

 tion. A struggle takes place between the sensations excited by retinal 

 stimulation and the nervous impulses connected with the muscles. 

 This struggle, in the language of Purkinje, along with the tension on 

 the brain itself, and especially on its peduncles, gives rise to vertigo and 

 the peculiar sensations of movement. 



As bearing on this subject, the following interesting experiments, first 

 studied by Purkinje, may be readily performed. Stand erect and rotate round 

 a vertical axis for some minutes, and then stop. Surrounding objects appear 

 still to move round, and there may be vertigo. The direction of the apparent 

 motion depends on the direction of the preceding real motion, and is always 

 opposite to it, and the axis about which the apparent motion takes place is 

 always that line in the head which was the axis of the preceding real rotation. 

 Bend the head forwards, rotate, and then stop. In this case the apparent 

 motion is round a vertical axis. Again, bend the head forwards, rotate, stop, 

 and then raise the head so as to look forwards, and the apparent motion will 

 take place round a horizontal axis, the horizontal axis having been vertical 

 when the real rotation took place, and the floor will seem to rise on one side 

 and to fall on the other. Again, certain relations of these movements to 

 visual phenomena are of interest. During real rotation round a vertical axis, 

 at first the eyes move by a series of jerks ; that is to say, they remain fixed for 

 an instant, then follow the head, again they remain fixed, again move with a 

 jerk, and so on. After a little time, however, if the rotation is continued, the 

 eye does not continue fixed for even an instant, but moves more slowly than 

 the head, then quickly makes up to it in speed, then falls behind. At last 

 both head and eye move together. If, then, the rotation stops, the movements 

 of the eyeballs are renewed, and gradually become less and less until they 

 cease. The oscillatory movements of the eyeballs are the effect of the visual 

 vertigo. When the real rotation ceases, the person has still a sensation that 

 his head is turning round, but he feels also that his body is at rest relatively 

 to his head ; he sees that objects are at rest relatively to his head, and he con- 

 cludes that as his head is turning round, his body, and external objects, must 

 be rotating also, and his eyes oscillate exactly as they would do if these move- 

 ments were real. That these oscillations of the eyeball may be related to 

 irritations of the semicircular canals, is rendered probable by the observation of 

 Cyon, that irritation of the canals excites oscillatory spasms of the ocular 

 muscles at the rate of from 20 to 150 per minute. The eyeballs oscillate 

 about an axis perpendicular to the plane of the irritated canal. Oscillatory 

 movements of the eyeballs of a similar kind may be observed in rapid railway 

 travelling when the person looks at near objects, and probably they form do 

 inconsiderable part of the cause of the fatigue in such circumstances. 1 

 1 M'Kendriuk's "Physiology," vol. ii. p. 701. 



