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IN a recent paper Hill [1938b] (a g.cm. being the " extra heat" for 1 cm. shortening, b g.cm./sec. the increase of the rate of energy production for 1 g. tension loss).This formula relates two variables, the speed of shortening v and the external force P, which can be determined without any heat measurements. According to this formula, which can also be written as (v+b) (P+a)=(Po+a) b=const., the relation between force and speed is a rectangular hyperbola, with asymptotes at P=-a, and v=-b. As has been verified by Hill [1938b], the value of a derived from a mechanical P-v relation is equal to the extra heat per cm. shortening (frog's sartorius, 00 C.). It was of particular interest to find whether this relation remains valid if the applied force becomes greater than the isometric tension. Obviously, for P >PO, v becomes negative: the muscle lengthens as was found already by Fick

. There appears to be general agreement that the resting potential has a low temperature coefficient, but the evidence concerning the effect of temperature on spike amplitude is conflicting. The experiments of Gasser (1931) suggest that the temperature coefficient of the spike is large and positive, while those of Schoepfle & Erlanger (1941) indicate that it is small and negative. Recent work on the giant axon of the squid suggests that the effect of temperature should be examined with this preparation. In particular, it is important to know whether the characteristic relation between action potential and resting potential holds over a wide range of temperature. The use of an internal recording electrode facilitates this study, since membrane potentials can be measured directly and are independent of the amount of external short circuiting. Another important advantage of this technique is that the temperature of the preparation can be altered rapidly by changing the sea water in which the axon is immersed. METHODThe experiments were made on the largest axon (diameter 500-700,) of the stellar nerve of Loligo forbesi. The mounting of the fibre, insertion of the microelectrode and the method of recording action potentials and rates of potential change have been described by Hodgkin & Huxley (1945) and Hodgkin & Katz (1949).The temperature was varied by replacing the sea water-bath around the axon (about 100 c.c.) and was read on a thermometer inserted close to the nerve. The effects of temperature change on the spike and resting potential were established without noticeable lag, provided that the temperature was not raised much above 350 C., at which point the axons tended to fail progressively. It was, therefore, possible to obtain a continuous range of measurement by allowing the bath to

Crustacean muscle has been of special interest to physiologists because its myoneural junctions show the properties of central nervous synapses. The muscles are supplied by excitatory and inhibitory nerve fibres which can be stimulated separately, and whose interaction can be studied at the level of the nerve-muscle junction. As was pointed out in recent reviews (Wiersma, 1941;Katz, 1949), much valuable information has been obtained about the organization of the crustacean nerve-muscle system, but little progress has been made in elucidating the mechanisms by which antagonistic nerve impulses exert their effects on the muscle fibres.As a first step, it seemed necessary to study the properties of crustacean muscle fibres, quite apart from their nerve junctions. Hitherto, most of the elementary properties of the crustacean muscle membrane have remained unexplored, largely because of technical difficulties in obtaining suitable fibre preparations. With the help of intracellular electrodes, this difficulty can be overcome, and in the present work extensive use has been made of this method, for stimulating and recording across the surface membrane of individual muscle fibres.The object of this paper is to present electrical measurements on the crustacean muscle membrane, in particular its resistance and capacity, its resting and action potential, and its electrical reactions in various ionic environments. As was indicated in previous notes (Fatt & Katz, 1951b, 1952, the electric response of crustacean muscle fibres, and especially its persistence in sodium-free media, differs from that of many other excitable tissues, and the evidence for this will be discussed in detail. METHODSApplication of intracelular electrodes. The usual procedure was to introduce two microelectrodes into the same fibre, one to paas current through the membrane, the other to record the membrane potential. The circuits were completed by separate external electrodes whose connexions are shown in Fig. 1. The micro-electrodes were operated by separate manipulators and