ENERGETICS 109 



this tension. We may therefore explain the mechanism of 

 muscular contraction by supposing that a nervous impulse 

 alters in some way the rate of combustion in a muscular fibre, 

 that this alteration produces a momentary change in the 

 chemical composition of the muscular cell, and that this 

 change of chemical composition increases the surface tension 

 of the cell sufficiently to provoke its contraction into a more 

 spherical form. 



Ostwald has introduced a very useful conception for the 

 study of this question of surface energy. A liquid surface 

 contains a quantity of energy equal to its surface tension 

 multiplied by its area, hence any variation either of area or of 

 tension corresponds to a variation of its energy. This novel 

 conception constitutes a valuable addition to the experimental 

 study of the physiology of muscular action, since it gives us 

 some idea of the mechanism by which chemical energy may be 

 transformed into muscular contraction. 



Whatever the mechanism of transformation in the animal 

 machine, we have to consider the same quantities as in other 

 motor machines. These are : (1) the efficiency ; (2) the 

 potential energy; (!5) the power; (4) the energy given up 

 to the medium under the form of heat ; (5) the temperature. 



Muscles, then, are merely transformers which change 

 chemical energy into mechanical work, the diminution of 

 stored-up energy in a muscle being expressed by the sensation 

 of fatigue. A muscle may be studied in four different phases: 

 (!) in repose ; (2) in a state of tension ; (!i) when doing positive 

 \vork ; (4) when work is being done on it. 



When a muscle is in a state of tension, as when a weieht 

 is sustained by the outstretched arm, the muscle is producing 

 no external work. The entire work done is converted into 

 heat; just as it is in a dynamo or steam engine which is 

 prevented from turning by a brake. Muscular contraction 

 produces fatigue even when it does no external work. It is 

 impossible for the muscle to support even the weight of the 

 outstretched arm itself for any considerable time. 



A muscle is doing positive work when it is raising a weight 

 or moving a bodv from one point to another. 



