I. I. In his illuminating book on The Nature of Thermodynamics, Bridgeman (1941) points out an intrinsic contradiction between the concepts of physical and biological evolution. In his words: ‘The view that the universe is running down into a condition where its entropy and the amount of disorder are as great as possible has had a profound effect on the views of many biologists on the nature of biological phenomena. It springs to the eye, however, that the tendency of living organisms is to organize their surroundings—that is to “produce” order where formerly there was disorder. Life then appears in some way to oppose the otherwise universal drive to disorder. Does it mean that living organisms do, or may violate the second law of thermodynamics?…’

An intact living muscle has such a regular structure that it diffracts light or X-rays, thereby providing patterns that contain uniquely valuable information. Interpretation of these patterns is not straightforward, but is helped by light microscopy and electron microscopy, which can often provide similar though less reliable information. At all levels of complexity, from that of the fibrils to that of the molecules, structure in a muscle is orderly. No other natural cell assembly is so suited to study by the diffraction method, and the results obtained in recent years are an outstanding example of how this method can elucidate a biological problem. In contrast to protein crystallography, where the system studied is artificial, muscle can be examined in its natural state, during normal activity. The levels of structure explored as yet in muscle are above that of the atoms in the molecules. Such structure is more commonly investigated by electron microscopy, and the application of the diffraction method to living muscle has provided a valuable check on the preparative artifacts that worry the microscopist. The great complexity of a muscle, as compared with a protein crystal, and the fact that the system is only semi-crystalline, giving a much less detailed diffraction pattern, make the problems of interpretation especially difficult. But a great deal of useful information is available about other properties of muscle and its constituents, and the flourishing state of muscle biology at present is a major factor contributing to the successful application of the diffraction method.