This rule of course only applies within the boundaries of the intensity between the threshold of stimulation and maximal stimulus. The interval, however, between these intensities varies considerably in different living substances. In this connection there are several forms of living substance which call for our special attention. In these the surprising condition seems to exist, that the interval between the threshold and the maximal stimulus is zero; that is, every stimulus which acts at all always produces a maximal response. _Bowditch_[26] first observed this behavior in the frog’s heart and this has also been confirmed by _Kronecker_.[27] The induction current produces, as _Bowditch_ says, either a contraction or nothing. If the former, it is the strongest contraction which can be produced by an induction shock at the given time. Here for the first time a constancy of response was discovered which has been termed the _all or none law_. _McWilliams_[28] has later verified the same fact for the mammalian heart. _Gotch_[29] has also arrived at the same conclusion in connection with the nerve. He states that “the comparison of submaximal with maximal responses shows that although there is an obvious difference in the amount of E. M. F., there is little or no difference between such time relations as the moment of commencement, the moment of culmination of E. M. F. and the rate at which E. M. F. disappears.” Further: “the rate of propagation of the excitatory wave is the same whether this is maximal or submaximal.” He likewise assumes that the “all or none law” is applicable to the constituent fibers, and that the variations in the strength of response with weak and strong stimulation are brought about in the first instance by stimulation of a few, in the latter by a greater number of fibers in the nerve trunk. The same conclusion has been reached by _Keith Lucas_[30] for the single cross-striated fiber of the skeletal muscle, founded on the fact that by direct stimulation of a bundle of curarized muscle fibers, the contraction only increases inconstantly and not regularly with the increasing intensity of the stimulus. This is only comprehensible if one takes into consideration that, with the increasing intensity of the stimulus, a greater and greater number of fibers are stimulated. _Keith Lucas_[31] came to the same conclusion in the case of the muscle stimulated indirectly through the nerve. He, therefore, sees, because of the nature of the response of the single muscle cell, no difference between heart muscle and skeletal muscle. The “_all or none law_” applies to the individual muscle cells of both kinds. The difference between the heart and skeletal muscle, according to him, lies in the fact that in the heart the individual muscle cells in their totality stand together as conductors of excitation, whereas in the skeletal muscle the individual muscle fibers are separated, as far as conduction of excitation is concerned, by the sarcolemma.