The most familiar experiment of this sort is sometimes known as Kircher's. Let the feet of a hen be tied together (though this is not necessary in all cases), and the hen placed on a level surface. Then if the body of the hen is gently pressed down, the head extended with the beak pointing downwards, touching the surface on which the hen stands, and a chalk mark is drawn slowly along the surface, from the tip of the beak in a line extending directly from the bird's eye, it is found that the hen will remain for a considerable time perfectly still, though left quite free to move. She is, in fact, hypnotised. We have now to inquire what parts of the process just described are effective in producing the hypnotic condition, or whether all are essential to success in the experiment. In the first place, the fastening of the feet may be dispensed with. But it has its influence, and makes the experiment easier. An explanation, or rather an illustration, of its effect is afforded by a singular and interesting experiment devised by Lewissohn of Berlin:--If a frog is placed on its back, it immediately, when the hand which had held it is removed, turns over and escapes. But if the two fore-legs are tied with a string, the frog, when placed on its back, breathes heavily but is otherwise quite motionless, and does not make the least attempt to escape, even when the experimenter tries to move it. 'It is as though,' say Czermak, describing the experiment as performed by himself, 'its small amount of reasoning power had been charmed away, or else that it slept with open eyes. Now I press upon the cutaneous nerves of the frog, while I loosen and remove the threads on the fore-legs. Still the animal remains motionless upon its back, in consequence of some remaining after-effect; at last, however, it returns to itself, turns over, and quickly escapes.'