PHYSICAL METHODS IN PHYSIOLOGY Z7 



of carbon-dioxide and lactic acid. The twitch of a muscle fibre 

 cannot be varied in strength by varying the stimulus — its action 

 is " all or none." The heat is liberated, partly in the phase of 

 contraction, partly in the phase of relaxation, and in large 

 measure — in the presence of oxygen — in the " recovery " phase. 

 It appears that the energy exchanges of a muscle are like those 

 of an accumulator employed in driving an electric motor : on 

 applying the stimulus, stored chemical energy is discharged, 

 heat and work are produced, and in the phase of recovery the 

 system is recharged at the expense of some oxidative process. 

 The elastic changes are due to chemical reactions, being largely 

 affected by temperature or by chemical substances in contact 

 with the muscle. It would seem that the stimulus causes a 

 momentary alteration in the permeability of some surface in 

 the fibre, an alteration accompanied by the electric change. 

 This momentary change of permeability allows lactic acid (pre- 

 pared from sugar or glucose under the influence of a katalyst) 

 to pass out, and by some physico-chemical action to increase 

 the tension of certain longitudinal surfaces in the fibre. In 

 relaxation the acid is removed from these sensitive surfaces 

 by some reaction unknown ; in recovery part of this lactic 

 acid is oxidised, and the remainder is restored to its previous 

 position as the sugar or glycogen from which it came. The 

 ordinary voluntary contraction is the summed effect of a series 

 of such individual " twitches," graded and co-ordinated by the 

 nervous system. 



The application of physical methods to the heart is another 

 field which has proved very fertile of recent years. Our 

 present knowledge of this organ is due largely to the use of 

 the string galvanometer, an instrument designed specifically 

 for recording the very rapidly changing electric currents 

 produced by the beating heart, and since employed for such 

 diverse purposes as locating enemy guns or recording the 

 movements of the hand. By connecting the galvanometer 

 either directly on to an exposed and beating heart, or indirectly 

 to the same tissue through the intact conducting body of a 

 human subject, the passage of the wave of contraction over 

 the heart has been recorded, and its characteristics in health 

 and disease described. The spontaneous activity also of the 

 isolated heart has been the chief means by which a preliminary 

 account has been given of the effect of various salts upon the 

 living cell, and the same organ has contributed considerably 

 to our knowledge of the oxidative and chemical processes 

 occurring in activity and of their relation to the mechanical 

 changes produced. 



In dealing by physical methods with the intact animal, as 

 distinguished from the isolated tissue, perhaps the most striking 



