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36 work by lifting a weight, but that it becomes hotter in doing so. Energy is thus set free from the muscle as mechanical energy and heat.

The first time one sees this experiment it

is not easy to be satisfied that the electricity is used merely as a means of irritating the nerve going to the muscle, and that it is not the agent that causes the muscle to contract. Suppose a bit of nerve like a thread is stretched over two wires connected with our electric apparatus, as in this diagram on the blackboard, the current of electricity enters the nerve, from a to b. nerve to the muscle, but, in passing along the bit of nerve, the nerve is irritated, a molecular change of some kind is generated in it, and this change travels down the nerve to the muscle in the direction of the arrow. The change that runs along the nerve, as the