HEAT PRODUCTION 145 



the latter. The essential point about this preparation which is 

 called the rheoscopic frog is that it actually proves the occurrence 

 of a diphasic current in muscle in consequence of its activity. 

 If the free nerve is stimulated by a tetanising current both muscles 

 go into tetanus. This secondary tetanus demonstrates that 

 although the stimuli are being applied so rapidly that the con- 

 tractions of the " battery " muscle are fused, the diphasic nature 

 of the excitatory process is still quite distinct and is indicated by 

 the contraction of the " galvanometer " muscle. 



The current of action may be considered as inclusive of the 

 current of injury. Injury is stimulation. Therefore, the current 

 produced by an injury confined to a small area should be weaker 

 than that obtained by the excitation of the whole muscle. The 

 E.M.F. of the current of action of a sartorius is about 0-085 volts, 

 while the demarcation current may be about 0-053 volts. 



An important piece of research work by A. V. Hill and W. Hartree has 

 just been published. They devised instruments and methods enabling 

 them to record photographically, not only the mechanical response of a 

 muscle to a stimulus, but the heat produced at each phase of the mechanical 

 response. Their apparatus was sensitive to 0-000001 C. One may 

 summarise their main findings as follows : 



(1) '' The time relations of the heat production in the first few seconds 

 after excitation are independent of the presence of oxygen." It has been 

 stated above that the magnitude of the heat production was independent 

 of the presence of oxygen. One may therefore consider that the energy 

 required for the initial chemical changes caused by excitation does not 

 arise from oxidation. 



(2) A complete muscle twitch may be resolved into four phases. 



HEAT PRODUCTION. 



(a) Contraction Initial rapid production diminishing 



gradually in rate as the stimulus 

 proceeds. 



(&) Maintenance of Con- Small constant value ending shortly 

 traction after stimulus ceases. 



(c) Relaxation Relatively large evolution of heat, 



occurring rather suddenly during the 

 later stages of relaxation. 



(r/) Recovery A large but slow production of heat 



occurring in the presence of oxygen 

 for some minutes after the contraction 

 is over. 



B.B. 10 



