CONVERSION OF CHEMICAL ENERGY 87 



of the fibrillae and the hypothesis based on this assumption 

 explains all the facts of muscular contraction.* 



The discussion of this subject is confusing because the term 

 permeability is frequently used as being equivalent to pene- 

 trability. The former implies a membrane surrounding the 

 cells whilst the latter merely states that the substance in 

 question does or does not enter the cell. 



FIG. 18. Diagram to scale showing the 

 effect in two segments of four fibrils 

 of frog's sartorius muscle when liquid 

 passes by osmosis from t the isotropic to 

 the anisotropic portion of the muscle. 



Shaded portion anisotropic substance ; unshaded por- 

 tion isotropic substance. A , relaxed, B, contracted 

 condition. (From Proc. Roy. Soc., H. E. Roaf.) 



* H. E. Roaf, Proc. Roy. Sac., 1914, B vol. 88, p. 139. 



It has been stated that muscle contraction cannot be due to changes 

 in osmotic pressure because preliminary extension of a muscle increases 

 the force liberated by muscle and that surface tension is the only force 

 which, like muscle contraction, has a negative temperature coefficient. 

 The first reason is invalid because an extension of the model described 

 elsewhere will cause a squeezing of liquid from the anisotropic material 

 so that the concentration in it will be greater. Therefore a greater ten- 

 sion will be exerted by the stretched muscle when it contracts. The 

 second reason is also invalid because, as described on p. 59, the com- 

 bination of hydrogen and oxygen to form water has a negative tem- 

 perature coefficient, i.e. it is reversed at high temperatures. From 

 the last statement it follows that the hydrolysis of salts of weak acids 

 and bases is increased at higher temperatures and if muscle contrac- 

 tion be due to such salts the contraction may be weaker at higher 

 temperatures. 



