ABSTRACT
If the limbs of an animal are to propel the body along a straight horizontal line the propulsive forces exerted by the right and left pairs of limbs must be equal and the vertical thrusts exerted by each of the four limbs must be such that their combined resultant is equal to the weight of the body and act through its centre of gravity (Gray, 1944). During the normal ambulation of a toad these conditions are not strictly fulfilled, for the body exhibits a slight but definite tendency to rotate about its principal axes (Barclay, 1946). Since all such movements are liable to induce compensatory reflexes of labyrinthine origin, it is of interest to know how far the normal level of co-ordination between the limbs is affected by labyrinthectomy. In all cases the operation has been performed from the dorsal side.
The ‘circling’ response to a strong unilateral stimulus is remarkably persistent in the sense that it continues for relatively long periods after the cessation of the stimulus. It is also remarkable in another respect: a stimulus applied to one side causes persistent circling which eventually ceases; if shortly afterwards a second stimulus is applied to the opposite side the direction of circling is not reversed but is resumed in the original direction. Only after a prolonged period of rest or after a period of ‘hypnosis’ does the direction of circling invariably occur towards the unstimulated side of the animal. The significance of these phenomena is obscure. Circling does not occur in a unilaterally labyrinthectomized animal. If the animal is gently pressed between two glass plates it can be turned with its dorsal side uppermost, without (in the case of the labyrinthectomized animal) exhibiting any tendency to struggle. The upper plate can now be very slowly raised until out of contact with the animal which then lies motionless on its back. In this position the toad may remain motionless for as much as 5 or 10 min., after which it rights itself spontaneously; if, however, during the ‘hypnotic ‘state the toad is very gently stimulated it immediately rights itself. A similar phenomenon can be observed in intact animals after steady compression, in the inverted condition, between the palms of the hand.
