ALL-OR-NONE LAW in physiology relates response to stimulus in excitable tissues. It was first established for the con traction of heart muscle by H. P. Bowditch in 1871. Describing the relation of response to stimulus, he stated, "An induction shock produces a contraction or fails to do so according to its strength ; if it does so at all, it produces the greatest contraction that can be produced by any strength of stimulus in the condition of the muscle at the time." It was believed that this law was peculiar to the heart and that the other highly specialized and rapidly responding tissues—skeletal muscle and nerve—responded in a different way, the intensity of response being graded according to the intensity of the stimulus. This impression arose because nerves and muscles are composed of hundreds or thousands of individual fibres, and since some of these do not respond to a weak stimulus the total response of the organ is apparently sub ject to such gradation. It has now been established, largely through the work of Gotch, Lucas, Pratt, Adrian, Kato and others, that the individual fibres of both skeletal muscle and nerve respond to stimulation according to the all-or-none principle. This does not mean that the size of response is immutable. Functional capacity varies with the condition of the tissue. The response to a stimulus applied during recovery from a previous response is subnormal. The energy liberated by a muscle fibre when stimulated depends, among other things, on the mechanical conditions obtaining while the response is being evoked. But the size of response is independent of the strength of stimulus, pro vided this be adequate. These facts show that the functional response is essentially alike in these specialized tissues—heart, skeletal muscle and nerve ; its precise nature is not known, but it resembles an explosive reaction in that it depletes for a time the available store of energy on which it depends. (A. Fo.)