Page:Life in Motion.djvu/52

32 The obvious phenomenon of a muscle, then, is that it contracts or changes its shape when it is irritated. We say that a muscle, like other living matter, is irritable. By this we mean that it responds to a stimulus, and the response is a change of form, a contraction. We shall see, however, that the contraction or change of shape is associated with many other changes of a molecular character not obvious to the senses but to be looked for by special methods. We must also, in next lecture, study more carefully the contraction itself.