170 



THE MUSCLE CELLS 



Now when 1 dyne is developed in 1 em. length of muscle, we have 



Tl 

 H 



= 5,orH 



- ere\ 

 5 ^ 



which should be accompanied by 65 X lO^^^/S = 13 X 10""^^ grams 

 of lactic acid, a figure which agrees fairly well with experimental 

 values. 



Isotonic Contraction. In the body, of course, muscle naturally 

 shortens against a load, thus keeping the tension unaltered. This 

 introduces several fresh factors for consideration. The mechanical 

 act of shortening produces alterations in the thermo-elastic 

 properties of muscle whereby heat is absorbed, while the increase 

 of internal friction resulting from the increased viscosity produces 

 heat. Further, the resistance to movement of the load against 

 which the muscle has to shorten bears a very important quantita- 

 tive relationship to the heat generated. Fenn has shown that the 

 heat generated in an isotonic contraction is greater than the 

 amount generated in an isometric contraction by an amount 

 approximately equivalent to the extra work done if the work done 

 is maximal. To take a simple example, in an isometric contraction 

 a muscle evolved 24 X 10^* calories, while when it did 2-2 X 10* 

 ergs of external work it produced 30 X 10~* calories. The extra 

 6 X 10~* calories are equivalent to 2-5 X 10* ergs, corresponding 

 to the extra external work (2-2 X 10* ergs) and the internal 

 thermo-elastic and frictional factors. 



TABLE XXV 



Heat Production uxder Isometric and Isotonic Conditions 

 (Modified from Fenn, Jour. Physiol, LVIII., p. 180) 



It will be seen from this Table (XXV.) that, under the conditions 

 of the experiment, the excess heat, measured in ergs, is about twice 

 the value of the external work done. 



Muscle causes the conversion of the potential energy of the 

 greater part of the foodstuffs into kinetic energy, i.e., heat and 

 work. The process of conversion is ultimately an oxidative one. 



