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It was pointed out some time ago' that a first-order phase transition would be particularly suitable in the mechanism of muscular contraction at the molecular level because such a transition could provide a very large change in length from a quite small alteration in the force, or in the concentration of a molecule or ion, or in some other condition (e.g., electric field). The primary object of this paper is to call attention to the fact that the sliding-filament model2 of muscular contraction, which is now well-established experimentally, is indeed, from the statisticalmechanical point of view, a one-dimensional system that exhibits a first-order transition. Hence, it should be expected to have the required "razor-edge" sensitivity. Whether the phase transition proves to be a significant feature of the model remains to be seen. But physiologists should at least be aware of the possibility and of some of its implications (see below). Ordinary one-dimensional systems cannot exhibit a first-order transition,3 but in certain rather artificial cases,4' 5 with long-range forces, a transition is found. In contrast, the sliding-filament model is an extremely simple, real (i.e., not artificial) one-dimensional system which virtually has a phase transition built into it. In fact it is difficult to see how nature could select a more elementary thermodynamic device to achieve the sensitivity referred to above. In order to stress the essentials, we shall confine ourselves in this paper to a discussion of the equilibrium properties of a simplified version (Figs. 1 and 2) of the

PHASE TRANSITION IN THE SLIDING-FILAMENT MODEL OF MUSCULAR CONTRACTION