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ABSTRACt'The length-tension diagram, the force-velocity relation, the characteristics of the series elasticity, and the duration of the active state have been studied on the papillary muscle preparation of the cat heart, and on other examples of cardiac muscle.Positive inotropic changes such as the staircase phenomenon and post-extrasystolic potentiation occur without lengthening, but frequently with shortening, of the duration of the active state. They are accompanied by an increased velocity of contraction, and may be caused either by an intensification of the active state or by an alteration of the force-velocity characteristics of the contractile component.The changes in the force-velocity relation point to an adaptation of the velocityefficiency relation in dependence on the frequency of contraction.

The ‘initial’ heat production of a non-medullated nerve ( Maia ) has been reinvestigated with more rapid recording equipment than was previously available. In a single impulse at 0° C a positive heat production was observed averaging about 9 x 10 -6 cal/g nerve: this is rapid and is probably associated with the active phase of the impulse. It is followed by a rather slower heat absorption averaging about 7 x 10 -6 cal/g nerve and lasting for about 300 ms. Previous methods were too slow to do more than record the difference between the two, the ‘net heat’, viz. about 2 x 10 -6 cal/g nerve: this is about one-third greater at 0°C than at 18° C. Maia nerves contain fibres from 20 to 0.3 µ in diameter, and about half the heat is probably derived from fibres less than 3.0 µ . The velocities of impulses in them at 0° C vary from 1.4 to 0.1 m/s, so impulses reach the recording thermojunctions throughout a long interval. Thus the observed course of the heat production is the resultant of positive and negative components in different fibres, and a substantial part of each is masked. The real positive and negative heats, therefore, are substantially greater than those observed: on the most likely estimate of velocity distribution, in a single impulse at 0° C they are about 14 x 10 -6 cal/g and — 12 x 10 -6 cal/g, respectively. Heat production, like ionic interchange, is probably proportional to fibre surface, which in 1 g of Maia nerve is estimated as 10 4 cm 2 . If the fibre surface is taken as 50 Å thick, the heats just calculated, if reckoned per gram of surface material, are 2.8 x 10 -3 cal and — 2.4 x 10 -3 cal, respectively. The former is about the same as the heat produced per gram in a muscle twitch. During the passage of an impulse there is known to be an interchange of Na and K ions between the axoplasm and the outside fluid. When isotonic solutions of NaCl and KCl are mixed there is a production of heat. A substantial part of the heat during an impulse may be derived from the interchange of Na and K. Another part may be associated with chemical reactions occurring in the excitable membrane during the cycle of permeability change accompanying the passage of an impulse. The negative heat production is discussed. It cannot be connected with ‘pumping back’ the Na and K ions; this is a much slower process and anyhow would probably involve a positive heat production. It may be a sign of endothermic chemical reactions, representing a first (anaerobic) stage in recovery, which occur in the surface membrane following the completion of the permeability cycle. The question is considered whether the positive and negative phases of the heat production could be due to the discharge and recharge, during the action potential, of the condenser residing in the excitable membrane. The heats so calculated are of the right order of size, but on present evidence the time relations seem to be quite wrong. The amount of K which escapes per impulse from Maia nerve during slow repetitive stimulation at 0° C was measured. It depends greatly on frequency of stimulation; at ‘zero frequency’ it was about 9 X 10 -8 mole/g x impulse.

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In a recent paper (Hill, 1949 a), it was shown that the heat produced in a single twitch is made up of two parts: (a) the heat of shortening which is simultaneous with and proportional to the shortening, and (b) the heat of activation. The latter starts at its maximum speed shortly after a shock, falling in rate from then on and finishing by the time that relaxation begins. In another paper (Hill, 1949c) the use of a quick stretch applied shortly after a shock showed that the full strength of a contraction (equal to that in a maximal tetanus) is developed very rapidly after the end of the latent period, remaining on a plateau for a time, then gradually disappearing in relaxation.These facts suggested that each of the successive shocks required to maintain a prolonged contraction merely restored the full strength of the contraction from the level to which it had relaxed in the interval between shocks, and that the heat produced in a maintained contraction was no more than the summated effect of the heats of activation of the successive responses. If this was so, we should expect the maintenance heat, like the activation heat, to be little affected by muscle length. The first object of the present investigation was to test this conclusion.If the maintenance heat is the summated accompaniment of the succession of re-activations by which the onset of relaxation is deferred, it should be greater or less according as relaxation is faster or slower. In two respects this is known already to be the case: (a) a rise of temperature increases the rate of relaxation and of maintenance heat production (Hartree & Hill, 1921), and (b) previous activity (Bronk, 1930) and the presence of C02 (Bozler, 1930) decrease both. The second object of the present experiments was to find out, in a single maintained contraction, whether the two effects run parallel.In an isometric contraction considerable shortening of the contractile material, with consequent stretching of the series elastic material, occurs in the

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When a muscle is stimulated a change occurs such that, at the end of the latent period, the contractile mechanism is already fully active. This has been demonstrated by Hill (1949cHill ( , 1950, using rapid stretches. The present experiments show that at the end of the latent period the part of the muscle at the point of stimulation begins to shorten with its maximum speed, i.e. transition from rest to full activity is abrupt.When the stimulus is applied to one end of a muscle the part of the muscle distant from the electrodes becomes active only after an extra delay representing the time taken for activity to spread along the muscle to that part. The shortening of the whole muscle therefore begins gradually; the abrupt change in each element is masked by a time dispersion between the elements.The velocity of propagation of the contraction wave, as distinct from the electrical excitation wave, along the muscle has been studied. Hill (1949b) made allowance for the time of spread of activity along the muscle in order to obtain more precisely the heat production in a twitch. This propagation time was deduced from the first derivative of the heat curve. Similar arguments apply to the effect of propagation time on the form of the shortening curve during a twitch. Comparison of the onset of shortening under a small load when a muscle is directly stimulated at one end, with that when the same muscle is simultaneously stimulated at many points along its length, has enabled the propagation velocity of activity to be calculated and related to that of the action potential. Parallel fibred muscles were required for these experiments, in which the fibres ran the length of the muscle, or, at least, in which no two independent fibres were joined end to end. Sartorius muscles from English frog (Rana temporaria) and toad (Bufo bufo) were used in the first group, and coracohyoid muscles from medium-sized small-spotted dogfish (Scyllium canicula) in a later group of experiments. METHOD An electrode assembly was constructed by Mr A. C. Downing which enabled a muscle to be stimulated either at one end or at many points along its length simultaneously. Twelve pieces of 32 s.w.g.(0.27 mm.) silver wire were embedded as electrodes 2-5 mm. apart in a perspex block, and the

The variation in volume when a muscle contracts was the subject of much discussion towards the end of the 17th century, and more recently by Fulton (1926) and by Ernst (1958). Many scholars expected an increase in volume as the vital spirits moved from the nerve into the muscle and induced activation: but Swammerdam (ca. 1660) experimented with an isolated muscle contained in a jar with a fine-bore tube attached and in fact observed a slight decrease in volume. Ernst (1925) first demonstrated the decrease with certainty and set the stage for many investigations of volume change in frog striated muscle. Meyerhof & Hartmann (1934) showed that the volume decrease during tetanic contraction parallelled the development of tension, and that a further decrease accompanied maintenance of the tension. They also made precise studies of the slow changes in volume which occur following mechanical activity. These were related quantitatively to the formation of lactic acid under anaerobic conditions and to the break-down of creatine phosphate in an iodoacetate-poisoned muscle. Fischer (1941), on the other hand, reported the occurrence of a volume increase as well as a decrease during contraction, but this was not confirmed by Ernst. All the authors found a volume decrease of about 0-002 % of the muscle volume during a short tetanus, but the response of the apparatus was too slow to record the precise time relationships of the change during contraction. A tetanus was necessary in order to obtain a change large enough to record; the muscles used were almost always large gastrocnemii and usually several muscles were necessary. Hill (1948) has criticized this choice: the fibres in the gastrocnemius muscle do not run parallel through the length of the muscle but instead are arranged at an angle to the axis of the muscle, so that during contraction a portion of the muscle tension would be directed at right angles to the axis of the muscle, thus producing an internal pressure. This was demonstrated by Hill and can also be substantiated by the observation in mammals that during