Muscle transforms the energy supplied to it by the blood into mechanical work. It is doubtful whether any hypothesis as to structure will help us to an understanding of the way in which this transformation is effected. Explanations are seductive, but all attempts at explaining the connection between molecular change and change in shape must be viewed with suspicion. It is quite clear that muscle as a motor is not to be compared with any form of motor with which we are acquainted. It is also clear that the theory of muscle must be applicable to all its varieties—striped, cardiac, and plain. It must cover the alterations in form of an amœba and the streaming movements of protoplasm within a vegetable cell. Probably it must extend farther, and cover the discharge of electricity by an electric organ and the emission of light by the lamp of a firefly. We are on ground so treacherous that we are not sure whether, in crossing it, we may lean with confidence on the laws of thermodynamics; and doubt as to the applicability of these laws to living tissue almost upsets one’s mental balance. Until we have evidence to the contrary, we are bound to exclude such a misgiving from our minds. If we allow it to influence us at all, it is merely to the extent of causing us to hesitate to assume that the explanation of muscular contraction can be based upon an analogy between muscle and any known mechanical contrivances for generating power, not even excluding apparatus designed for the purpose of measuring osmotic force. If living muscle is frozen, pounded with snow containing 0·6 per cent. of sodic chloride, and placed upon a filter, a fluid plasma passes through the filter as the mixture thaws. Like blood-plasma, it clots spontaneously—without, however, so far as is known the intervention of a ferment.