THE CHEMISTRY OF MUSCLE. 73 



an artificial muscle from a string of catgut which, working on this principle, 

 contracts when heated and relaxes when cooled. When the heating occurs 

 suddenly this model gives curves of contraction identical with those obtained 

 from plain muscle. The apparatus is illustrated and described in Fig. 27, 

 and the curve of contraction obtained from it is shown in Fig. 28. The under- 

 lying principle of this hypothesis has met with much criticism. Fick * has 

 shown apparently that when applied quantitatively to the work done by muscle 

 it leads to an impossible conclusion. If, in a reversible process, at a temper- 

 ature TO, a certain quantity of heat, Qo, is converted to mechanical work, it 

 necessitates, according to the second law of Thermodynamics, the passage of 

 heat, Q, from a higher temperature, T! to a lower temperature, T 2 , in accordance 



with the equation, ^ = Q ( ~ - 4~ \ 



J-O \ A 2 A l / 



We may assume, in accordance with experimental results, that the efficiency 

 of muscle is equal to 25 per cent, of the total energy, or that Q = iQ, and that 

 Tj is the temperature of the body, 37 C., or expressed in the absolute scale, 

 37 + 273 = 310 C. T is the same as T t . If, now, in the equation we sub- 

 stitute iQ for Qo and 310 C. for T 2 and solve the equation for T! it gives a 

 value of 387 C., or, expressed in centigrade units, 387 273 = 114 C. 

 That is to say, to perform the work indicated the muscle must show a fall 

 in temperature from 114 to 37 C., and it seems clearly impossible to sup- 

 pose that the muscle in contracting attains any such temperature as 114 C. 



This criticism has been accepted by most authors as demonstrating that the 

 muscle cannot work as a heat engine by transforming a part of the heat of the 

 chemical reaction to work. A difference in temperature is necessary that is 

 not possible in the case of muscle. Other theories have been proposed, accord- 

 ing to which the chemical energy is supposed to be converted into work either 

 directly (Fick) or through a change in surface tension. The muscle is supposed 

 to act in such theories as a chemical or chemodynamic engine. The various 

 forms which the theories employing the conception of surface tension have 

 taken make it difficult to describe them in general terms. According to one 

 presentation (Macallumf) the sarcous elements (dim bands) may be considered 

 as having interfaces with the sarcoplasm along the lateral planes and with the 

 isotropic substance (light bands) at their ends, at which surface tension exists. 

 If the results of the chemical changes within the elements are such as to cause 

 a diminution in surface tension along the lateral walls, or an increase in this 

 energy at the end surfaces, the elements would tend to change from a cylinder 

 with straight to one with curved lateral walls, and this change, when multi- 

 plied by the total number of sarcous elements in the muscle, would account 

 for the shortening. The theory is deficient in not explaining how the surface 

 energy is changed, and also in failing to give an approximate quantitative 

 determination of the total amount of mechanical energy that might be obtained 

 in this way from the muscle. According to calculations made by Bernstein, j 

 the work energy exhibited by a contracting muscle is greater than can be 

 accounted for by probable changes in surface tension. 



Another theory has been adopted in recent years by an increasing number 

 of workers, and has been supported by many suggestive experiments. This 

 theory holds that the shortening of muscle in contraction is essentially a 

 phenomenon of imbibition. The instrument by which the shortening is 

 effected is the fibril, which is regarded as a coherent gel structure embedded in 

 a more liquid material, the sarcoplasm. When the muscle is stimulated, lactic 

 acid is formed in the fibril or in the sarcoplasm around it, and the effect of this 

 acid is to increase the power of imbibition of the fibril or of certain structures in 

 it, for example, the anisotropous discs. The swelling thus produced causes the 

 shortening of the muscle, and the subsequent relaxation is explained as due to 

 the removal of the acid by diffusion or by oxidation. The heat developed in 



* Fick-Pfluger's "Archiv," 1893, 53, 606. 



t Macallum, " Surface Tension and Vital Phenomena," University of 

 Toronto Studies, Physiological Series, No. 8, 1912. 



I Bernstein-Pfluger's "Archiv," 85, 271, 1901. See also Berg, "Biochem- 

 ical Bulletin," 2, 101, 1912. 



