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161 muscles at 0°C, the events are still too fast to be followed by chemical methods ; there are in fact no chemical data whatever on what happens during a single twitch. Physical methods, of which the majority have been devised by Hill himself, can follow easily and accurately the heat-production during and after the twitch, the tension development, and electrical and optical changes. Though very hard to interpret, these physical data are clear and reliable. More than anything else they provide a framework into which chemical theories must fit. Some theories can thus be discarded, but more than one can be mote or less squared with the physicist’s findings. The fundamental event in contraction is the shortening of a particular protein molecule in the myofibril. Prof. H. H. Weber, of Tubingen, observed at the meeting that muscle contains several proteins, of which apparently only two-actin and myosin-are concerned in the actual contractile process. A possible model of the process was given in a communication by Prof. W. T. Astbury F.R.S. Imagine two strips of paper stretching parallel to each other from north to south, and turned edge-on to the observer. Fold each of them like a concertina, so that each is crinkled, running first south-east then south-west, then south-east, and so on. These represent the myosin molecules in the relaxed state. Between these two scatter some tennis -balls-the globular form of actin. When the muscle contracts, said Astbury, the actin tennis-balls arrange themselves in chains, one sticking to another in a straight north-south line ; this is now the fibrous form of actin. At the same time the myosin molecule shortens with a concertina action ; the crinkles instead of running south-east and south- west run more nearly east and west, and the edges of the crinkles now touch the actin chain. Astbury was led to this picture by X-ray studies, but not all workers in this field agree that it is true. What is needed, he holds, is an X-ray study of muscle actually while it twitches; but here, as in chemical studies, technical difficulties supervene. What is it, then, that causes the change in form of the actin and myosin molecules and their association ’This is the central question, to which there is still no answer. When a nerve impulse reaches the neuro- muscular junction a wave of activity spreads down the surface membrane of the fibre, very much as an impulse travels down a nerve. Somehow this surface diminution of electric charge sets off the contractile mechanism in the interior of the fibre ; and probably the movement of ions is one of the links in the chain. But is this literally a trigger action Is the muscle like a stretched spring waiting for a chance to contract-that is, to lower its potential energy Is contraction or relaxation the more probable state, in the technical sense of the phrase ’* This is another question on which there is no agreement. Professor Hill holds strongly to the view that contraction is active and relaxation passive, and the relaxed state the lower in potential energy. From physical measure- ments he cited considerable evidence to support this view ; but the evidence is not decisive, and many of the chemists had theoretical reasons for holding the opposite opinion. Since it is believed that the energy source of a twitch is the breakdown of adenosine tri- phosphate the crucial experiment would be to determine whether this substance breaks down during the contrac- tion phase of the twitch or only when the twitch is over. This is asking too much of the chemists ; Mrs. Dorothy -Needham, F.R.s., indicated that new spectrophometric methods might be able to detect chemical changes after a single twitch, but she held out no hope of establishing the time relations of the chemical events. Bit by bit an intricate cat’s-cradle of fact is being woven, but the shape of the cat still eludes us ; and one can scarcely see the string for the holes. Not that there is anything mysterious or unique about muscular activity ; as Professor Astbury emphasised, the con- tractile muscular protein molecules are only a special adaptation in a general group which includes keratin, fibrinogen, and the epidermal protein of mammals. In human hair there persists the skeleton of muscle-fibres -the machine without the fire to drive-so the per- manent wave has some relation to the athlete’s strength. EFFECTS OF HEAT AND HUMIDITY IN DEEP MINES AT the first post-war Empire Mining and Metallurgical Congress, the technical sessions of which were held at Oxford from July 12 to 16, interest in man’s reactions to underground environments was intensified because of the various factors now increasing the difficulties of the mining industry. Not only is it becoming more .. difficult in different parts of the world to attract labour into the mines, but it is apparent that in South Africa, India, and even Britain heat and humidity in the deeper mines diminish working efficiency to an unknown extent. By contrast with the outstanding success of personnel research during the war on similar subjects, the mining industries have lagged behind in organising heat physio- logy units, not only in this country but also in South Africa. The problems discussed must have left little doubt of the urgent need for further physiological information. A survey of the environmental conditions in British coalmines by Mr. A. E. Crook, Mr. F. Edmond, Mr. J. Ivon Graham, and Mr. B. R. Lawton revealed that in Britain a wet-bulb temperature of 85°F had been reached at 3000 feet below the surface. It was agreed on theoretical grounds by Sir David Brunt, F.R.S., and from Dr. A. Caplan’s experience in India, that efficiency for strenuous work falls off at wet-bulb temperatures of 83-85°F. Caplan and J. K. Lindsay assessed the fall in muscular efficiency as 20% at a wet-bulb tempera- ture of 87°F, and at similar temperatures Mackworth has demonstrated a reduction in psychological perform- ance. It might be inferred from the agreement between Caplan and Brunt that Indian mineworkers work at a level of energy output defined as strenuous-i.e., about 300 Kcal./m2/hr. This would be surprising in the light of Dr. J. S. Weiner’s observations on the working ability of Bantu mine-labourers in hot humid conditions. Weiner demonstrated that " raw " and " experienced " Bantu are less well adapted to hot humid environments than young healthy Europeans, acclimatised in experi- mental chambers. It is suggested that Bantu labourers normally work at a lower energy output than that of the acclimatisation routine (110 Kcal.jm2/hr.). There is no information on the energy output at the various tasks in mining. This extraordinary gap in our knowledge delays the use by ventilation engineers and physiologists of Brunt’s integration into a vastly improved thermal balance equation of the factors concerned in heat- production and its transmission to and dissipation from the body surface to the environment. Thermal balance may be expressed simply as : Heat-production . heat-loss by evaporation z- heat-storage - z- convection z- radiation. But energy exchanges by convection and radiation cannot be measured directly. C. E. A. Winslow, in 1941, determined these factors experimentally, in terms of mean temperature of the skin and the walls and the dry- bulb thermometer reading. Brunt has integrated Winslow’s with his earlier work and expressed convection, evaporation, and radiation in terms of readily measurable quantities, thus : Heat-production = 3,6 (T - Tr) 66-7 Vv (1. - I ) - heat-storage -- ’ s Tr) ’ v B a - w where Ts is mean skin temperature ; Tr is mean temperature of walls ; Ig and Iw are the total heat content of unit mass of dry air, plus the water required to saturate it, at skin temperature of the wet-bulb thermometer of the ambient air.