378 PROTOPLASMIC ACTION AND NERVOUS ACTION 



those yielding the energy for contraction, may be under 

 similar kind of control. In these cells the chief reactions 

 following stimulation probably occur at the surface of 

 the contractile fibrils, and apparently certain reaction- 

 products, e.g., lactic acid, are directly concerned in 

 the resulting contraction.^ We may assume that the 

 temporary breakdown of the interfacial film (between 

 fibril and sacroplasm) will have a double effect: (i) 

 permit access of diffusible substances (possibly of the 

 lactic acid) to the interior of the fibril; and (2) form the 

 condition of a change of surface-tension, in the same 

 general manner as in the Hg-HaOa system. Under these 

 conditions contractile effects (essentially of an electro- 

 capillary kind) would result. Hence the consideration 

 of the relation of film-structure to the chemical reactions 

 of protoplasm has an obvious bearing on the problem of 

 the conditions of contractility in muscle and other con- 

 tractile tissues. 



^ For a discussion of the part played by acids in the contractile 

 mechanism cf. the recent review of A. V. Hill, Physiological Reviews, II 

 (1922), 310. 



