456 Dr. W. M. Fletcher and Prof. F. G. Hopkins. 



is to be emphasised again, is associated with any fresh yield of carbon dioxide, 

 except such as is directly due to expulsion, by the lactic acid, of previously 

 formed carbon dioxide loosely combined in the muscle. 



In the oxygen atmosphere, however, we have a removal of acid, a simul- 

 taneous yield of newly-formed carbon dioxide, with a restoration of the status 

 in quo ante and of the previous " potential " of the muscle. Carbon dioxide 

 production in an atmosphere of oxygen is a sign and a measure of an 

 immediate contemporary combustion. 



The Heat Production of Muscle. 



These results, it must be pointed out, are the results of the chemical study 

 of an integrated series of contractions in a muscle. The present limitations 

 of chemical method do not allow us to measure and follow the time relations 

 of the relatively minute changes which accompany and succeed each single 

 act of contraction. 



During recent years, however, Mr. A. V. Hill has conducted, at Cambridge, 

 a long series of investigations into the heat production of muscle by means of 

 the most refined thermo-electric methods. Of this work we must not pretend 

 here to give any adequate account, but we must notice in general that, 

 following \ip the results of our own experimental and chemical work, and 

 using the same general experimental methods of analysis of the conditions of 

 fatigue, of rigor (whether inflicted by heat or by chloroform), of recovery in 

 oxygen, and so on, Mr. Hill has obtained a valuable series of parallel 

 observations of heat production which have fundamental importance for the 

 theory of muscular metabolism. By ingenious modifications of the thermopile 

 and with a highly sensitive galvanometer Mr. Hill has been able to record the 

 temperature changes associated with a single act of contraction, and so by 

 exposing the muscle either to nitrogen or to oxygen the anaerobic heat 

 production can be distinguished from the aerobic. 



He finds that if the muscle contracts after being an hour or more in 

 nitrogen, the heat production observed as the accompaniment of contraction 

 does not continue beyond it (12). In oxygen, however, the heat production of 

 contraction is continued for long periods after the mechanical event is over. 

 The amount of heat liberated during the recovery process in oxygen he found 

 to be at least as great as that due to the anaerobic act of contraction itself. 



We must not now stay to speak of the many other general respects in 

 which Mr. Hill's work has confirmed and further illuminated our own 

 observations made on the chemical side. 



Before passing, however, to some general considerations, we would draw 

 attention to the fact that for important reasons of technique (which we must 



