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46 by touching it, but instead of the frog we will electrify my assistant standing on an insulated stool. So long as I abstain from touching him he feels nothing, but you see, if I touch his head, or his neck, or the tip of his nose, how the sparks fly out, and he then feels a smart and disagreeable sensation. During electrification we feel nothing, but it is only when we pass from one stage of electrification to another that we have a sensation, and the more rapidly this change takes place, the more irritating the sensation is.

We have now seen how nerves and muscles may be irritated in a definite and precise way, and we have found that the irritation of its nerve causes a contraction of a muscle. In the case of a single twitch, however, the movement is too rapid to be appreciated, and still less analysed, by the unaided eye. We cannot tell, for example, whether the contraction occurs in a shorter time than the relaxation, and still less whether the contraction is at a uniform rate in time, or whether it contracts faster at the beginning and more slowly towards the end, or the reverse. We must not, therefore, trust only to our senses