The theories as to the nature of muscular contraction have undergone profound changes since the opening of the twentieth century. Only a few years ago the working mechanism was still freely compared with a steam engine in which the direct combustion of fuel leads to the liberation of energy in the form of heat and motion. This analogy between the muscle and a heat engine received its greatest setback in the demonstration that muscular contraction is still possible in the absence of oxygen, without which true combustion promptly fails. As a sequence came the views that somehow very unstable energy-yielding compounds are built up in the contractile tissue, and that these disintegrate somewhat like the reaction of an explosive so as to do work. The chemical processes involved were vaguely conceived to consist essentially in "the building up of elaborate, unstable and oxygen-charged molecules, by the processes of so-called 'anabolism,'