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first, and we shall therefore try to find out what a muscle does and how it works. Let us interrogate a muscle itself. I have prepared the muscle of a frog in the way shown in this diagram. Now I wish you to follow all I do; and you must receive all the explanations that I would think it necessary to make Fig. 7.—The gastrocnemius muscle of a frog prepared for experiment. F, femur, bone of the leg; N, sciatic nerve ending in muscle at n; J, tendon of Achilles, with a small hole in it for a hook. if you were beside me in my laboratory. How has the muscle been prepared? As you know, the frog is what is usually called a cold-blooded animal, that is to say, the temperature of the body is always not far from that of the medium in which it lives. The term "cold-blooded" is misleading, because the frog's blood may, in some circumstances, not be cold; and besides it is a term to which one would think the frog might take just exception as being a term to