THE ACTIVITIES OF THE ORGANISM 105 



the carbon contained in the latter gas corresponds 

 roughly with the carbon contained in the muscle sub- 

 stance which undergoes disintegration, but does all 

 this justify us in saying that this substance is oxidised 

 in order that its potential chemical energy may be 

 transformed into mechanical energy ? Obviously 

 not, since we might equally well suppose that the 

 complex metabolic substance of the muscle splits 

 down into simpler substances and that in this trans- 

 formation energy is generated. Suppose that these 

 simpler substances are poisonous and that they must 

 be removed as rapidly as formed. The role of the 

 oxygen may be to oxidise them, thus transforming 

 them into carbon dioxide, an innocuous substance 

 which can be carried away quickly in the blood stream. 

 This line of thought, according to which the role of 

 oxygen is an anti-poisonous one, is held at the present 

 day by some physiologists, and many considerations 

 appear to support it ; the existence of ' oxidases," 

 for instance, enzymes which produce oxidations which 

 would not otherwise occur in their absence. Such 

 enzymes exist in very many tissues, and they may, 

 apparently, be present in an inactive form, requiring 

 the agency of a " kinase " before they are able to act. 

 The usual view among physiologists is that the 

 muscle fibre is a thermodynamic apparatus transform- 

 ing the heat generated during metabolism into 

 mechanical energy. How is this transformation 

 effected ? It cannot be said that we have any one 

 hypothesis more convincing than another. It has 

 been suggested that alterations of surface tension play 

 a part, or that the heat produced by oxidation causes 

 the fibre to imbibe water and shorten. Engelmann 

 has devised an artificial muscle consisting of a catgut 

 string and an electrical current passing through a coil 



