METABOLISM 



79 



colactic acid, which can be shown to be a product 

 of the breaking down of substances in the muscle 

 fiber. 



Heat. During contraction the muscle also de- 

 velops heat, and it has been estimated that the 

 amount of energy liberated as heat is five times that 

 utilized as mechanical energy, i.e. " work " in the 

 ordinary sense. This is shown by the " warming 

 up " that physical exercise 

 brings. The contracting mus- 

 cle liberates still a third form 

 of energy, namely, electricity, 

 which may be measured by a 

 capillary electrometer. 



Electricity. Both heat and 

 electricity as accompaniments 

 of muscular activity are, in a 

 sense, waste products and repre- 

 sent a certain necessary loss. 

 On the other hand, heat is a 

 transformation of energy very 

 necessary to the organism, not only in the higher 

 animals, where a definite bodily temperature must 

 be kept up, but in all animals and plants as 

 well, since it is only in the presence of a cer- 

 tain amount of heat that the necessary oxidations 

 and reductions involved in metabolism can take 

 place. We are all familiar with the rise of tempera- 

 ture produced by fermenting yeast. In both animals 

 and plants the evolution of heat is usually brought 

 about by the cleavage of a carbohydrate. 



FIG. 28. Noctiluca, 

 a phosphorescent marine 

 Protozoan; magnified 30 

 diameters. (From 

 Weysse after Cienkowski.) 



