CONTRACTILE TISSUES 



443 



muscle forms lactic acid, slowly, as it dies, but the fact that chiefly concerns us 

 here is that this production is greatly accelerated by stimulating the muscle to 

 activity. These investigators have shown that muscle freshly removed, immersed 

 in ice-cold alcohol and disintegrated therein, contains only the trace of lactic acid 

 which might be expected to be formed during the slight unavoidable delay in the 

 experimental procedures. An instructive experiment, showing its production on 

 stimulation, is described on pp. 308 to 309 of the paper referred to. Hopkins' 

 delicate thiophene test for lactic acid 

 is used. , 



This, then, is the only chemical 

 change that can be demonstrated to 

 occur in the act of contraction itself. 

 It is true that carbon dioxide is 

 slowly given off by excised muscle in 

 an atmosphere of nitrogen and, as 

 we saw above (page 272), Hermann 

 represents the products of the break- 

 down of his inogen substance as 

 " myosin " (that is, the nitrogen-con- 

 taining residue, after separation of 

 lactic acid), lactic acid itself and 

 carbon dioxide. But Fletcher (1902 

 and 1913, p. 374) has conclusively 

 shown that the slow evolution of 

 carbon dioxide in an atmosphere of 

 nitrogen is to be accounted for by 

 the action of the lactic acid on bicar- 

 bonates already present in the muscle; 

 the carbon dioxide so formed grad- 

 ually escapes. And, what is more to 

 the point for our purpose is that 

 stimulation does not increase this out- 

 put. Consumption of oxygen in the 

 contractile process is excluded by the 

 continuous and prolonged activity of 

 muscle in its absence. 



As to the form of energy present 

 in muscle itself, the chief experi- 

 mental evidence is contained in the 

 work of A. V. Hill (1911, 1, 1912, 2, 

 1913, 1 and 4, 1914, 1 and 2) on the 

 formation of heat in muscular con- 

 traction. There is found to be a 

 definite proportion between the ten- 

 sion developed and the heat given off. 

 If a muscle is allowed to contract 

 isometrically at one time and allowed 



FIG. 136. SKETCH OF ONE OF THE FORMS OF 

 THERMOPILE USED FOR THE INVESTIGA- 

 TION OF HEAT PRODUCTION IN THE PiASTROC- 

 NEMIUS MUSCLE OF THE FROO. 



The muscle is inserted in the conical cavity, with its 

 tendon upwards, until it fits exactly and is in contact 

 with the junctions, b, b, b. When contracting, it 

 cannot slip away from the junctions. 



a, a, a, External junctions, imbedded in the ebonite. 

 Co., CM, Copper leads to the galvanometer. 



The muscle is tied to supports at both ends, or to a lever 

 at the lower end. 



(A. V. Hill, 1913, 4, p. 307.) 



to shorten at another time, the heat 

 formed is greater in the first case. 

 It should be remembered that the 



heat measured in these cases represents the total energy change in the con- 

 tractile process, the tension being allowed to disappear in the form of heat, 

 and the weight raised, if such is done, is allowed to fall again. If we allow 

 the muscle to shorten, by releasing it at the time that the maximal tension 

 has developed, but not earlier, the heat is unaffected. The heat produced, 

 or energy developed, is, in fact, directly proportional to the length of the fibres 

 during the time that the contraction takes place and not to their volume. In 

 other words, it is a surface phenomenon. This view was first clearly put forward 

 by Blix (1902, p. 113). The tension developed thus depends upon the area of 



