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



molecular tension along longitudinal surfaces, or whether by the process of 

 imbibition, causing a resultant increase of tension in the fibre. A catgut 

 fibre in water will contract if its temperature be raised, as Engelmann 

 showed here in his Croonian Lecture of 1895, and it will contract if acid be 

 brought to it, relaxing again on its removal. We know that artificial 

 application of lactic acid to muscle causes contraction, reversible by 

 removal, as one of us showed long ago(l), and we have noticed to-day the 

 shortening produced in muscle as lactic acid accumulates after fatigue and 

 the lengthening which follows it's removal by oxygen (Fig. 4). Engelmann 

 thought of muscle as a heat engine, but we know now that, apart from 

 other theoretical objections to that view, the heat production of contraction 

 may take place after the mechanical contraction is over, and, in physiological 

 conditions with oxygen present, the greatest heat production is always 

 subsequent in time to the contraction. But, in spite of this dislocation in 

 time, there is, as Hill has shown (7), a constant ratio between the new tension 

 set up in the contractile elements and the heat of the contraction. The 

 heat of the contraction, however, will be proportionate to the chemical 

 reaction yielding it, so that we may say that there is a constant ratio 

 between the increased fibrillar tension and the new chemical condition 

 causing it, and regard the muscle as a chemical instead of as a heat engine ; 

 this, indeed, very many other considerations, into which time now forbids us 

 to enter, force us to believe. Such a chemical event, proportionate to heat 

 production, and also to the new tensile stress, would be the appearance, 

 close to the muscle-fibrils, of the H-ions of lactic acid as this arises from some 

 forerunner. The new condition of elastic state of the fibrils will give 

 contraction if the mechanical conditions allow it, and work will be done 

 according to the opposition given during the contraction. The work 

 actually done, however, will bear variable and quite accidental relations to 

 the* heat production, as we have long known that it does, and the 

 mechanical efficiency of the machine will vary accordingly with the 

 conditions. The removal of lactic acid under the influence of oxygen will 

 give relaxation, as the original state of tension in the fibril is restored. 



To complete the image before us, we have still to consider the nature and 

 the results of this oxidative removal of the lactic acid, hoping to reconcile in 

 hypothesis the ascertained facts with regard to energy exchanges and 

 chemical events. 



On the simplest view, we might picture the muscle fibre as endowed 

 initially with a supply of a substance (probably derived from carbohydrate) 

 capable of yielding lactic acid by a non-oxidative molecular rupture. This 

 rupture is exothermic, and heat is yielded proportionately to the acid 



