104 THE PHILOSOPHY OF BIOLOGY 



muscle : carbon dioxide is given off to the lymph 

 and blood stream, and the amount of this is increased 

 when an increased amount of work is done by the 

 muscle ; heat is produced and this too increases with 

 the work performed ; glycogen is used up, and lactic 

 acid is produced ; finally oxygen is required, and more 

 oxygen is required by an actively contracting muscle 

 than by a quiescent one. Now the obvious hypothesis 

 correlating all these facts is that the muscle substance 

 is oxidised, and that the heat so produced is trans- 

 formed into mechanical energy. " We must assume," 

 says a recent book on physiology, " that there is some 

 mechanism in the muscle by means of which the energy 

 liberated during the mechanical change is utilised in 

 causing movement, somewhat in the same way as the 

 heat energy developed in a gas-engine is converted 

 by a mechanism into mechanical movement." 



Now, must we assume anything of the kind ? To 

 begin wdth, life goes on, and mechanical energy is pro- 

 duced in many organisms living in a medium which 

 contains no oxygen. Anaerobic organisms are fairly 

 well known, and we cannot suppose that in them 

 energy is generated by the combustion of tissue sub- 

 stance in the inspired oxygen. A muscle removed 

 from a cold-blooded anim.al will continue to contract 

 in an atmosphere containing no oxygen, and it will 

 continue to produce carbon dioxide. It is true that 

 the contractions soon cease, even after continued stimu- 

 lation under conditions excluding the fatigue of the 

 muscle, but do the contractions cease because the 

 oxygen supply is cut off, or because the muscle dies 

 in these conditions ? We know that some complex 

 chemical substance is disintegrated during contraction 

 and that mechanical energy and heat are produced and 

 that carbon dioxide is also produced. We know that 



