176 THE MUSCLE CELLS 



larger value, the total area occupied by the lactic acid liberated 

 when a fibre 1 cm, long develops a dyne would be 24 X 10"^^ times 

 the number of molecules. We have seen that under these condi- 

 tions 13 X 10^^^ grams of lactic acid are liberated (p. 170), that is 

 13 X 10-^2 



— 7 7. \ Tj approximately 10^^ molecules. The area 



wt oi a molecule ^ ^ 



coverable would, therefore, be 10^^ x 24 X lO^^^ = 24 X 10~^ 



cms^., approx. 40^00 of ^ square centimetre. The muscle fibre 



being 1 cm. long, its circumference must be 4000 cm. On this 



edge, a surface tension of 1 dyne has to be produced, i.e., the 



coefficient of surface tension required would be 4,000 dynes — a 



perfectly impossible figure. To have a surface effect much more 



lactic acid is needed. Thus muscular contraction cannot be 



purely a surface tension effect. 



(2) Osmotic Theory. The release of lactic acid and other 

 substances of low molecular weight within a membrane known to 

 be semi-permeable in the resting state would produce an endos- 

 mosis which might conceivably lead to shortening. Against this 

 theory there is the fact that the muscle does not increase in 

 volume when it thickens and shortens. 



(3) Imbibition Theory. The increased H+ concentration pro- 

 duced on stimulation might cause the cell-colloids thus removed 

 further from their isoelectric point to take up more water and 

 swell. This theory suffers from the defects of both the two former 

 theories. There is not enough lactic acid liberated to alter the /jH 

 of the muscle proteins from the alkaline side of their isoelectric 

 point to the acid side, especially in the presence of electrolytes. 

 (The pYi of muscle in situ is about 7, while the isoelectric point of 

 myoproteins lies between pYi 4-6 and 5. Myosin is isoelectric at 

 pYi 3-9.) There is a further objection, namely, it is known that 

 lactic acid is neutralised under ordinary conditions by inorganic 

 bases and to a very small extent (if at all) by proteins. The heat 

 of neutralisation of the acid by proteins is 138 calories and by 

 salts 19 calories per gram of acid. The former value would 

 disturb the energy balance sheet of muscle. 



(4) Liquid Crystal Theory. Garner and also Clark have suggested 

 that if there is a film of lipoid liquid crystals in or upon the 

 anisotropic bands, then this film might be caused to contract or 

 expand by very slight alterations in 79H. Clark supposes that the 

 substance in the doubly-refracting bands passes abruptly from a 

 liquid crystal to a solid crystal form under the influence of acid. 

 The solid crystal lattice would have a closer form than the liquid 

 one, i.e. shortening would take place. She supports her hypo- 

 thesis by the production of X-ray diffraction patterns to shovv the 



