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province to discuss these remarkable physical Fig. 5.—Muscles in human arm. b, biceps muscle. phenomena; but I ask you to remember this simple and familiar experiment, because it not only is an example of what we mean by molecular movement, but it explains the construction of many physiological appliances we shall use in future lectures. Now we come to movements associated with life. These are best studied in a muscle. Flex your arm at the elbow joint, and you will feel the flesh above the joint and in front of the arm become firm and hard. Extend the arm and it again becomes soft. These movements are made by the action of special organs we call muscles; and the particular muscle I ask you to notice in the arm is called the biceps. Every boy knows where his biceps muscle is, and