THE ANIMAL ACTION-SYSTEMS 75 



impulse, however, seldom or never passes into a muscle : what 

 the latter receives is a rapid succession of such and therefore the 

 muscle passes into more or less sustained twitches, which blend 

 into a contraction. 



A nervous impulse is '' all or nothing " and presumably the 

 associated contraction of a muscle-fibre is also all or nothing. 

 But the muscle contraction can be graduated according to the 

 action that is to be performed : Therefore some or all of the 

 fibres do not contract, the " strength " of the muscle-pull being 

 proportional to the number of fibres that contract. Why this is 

 so is obscure, but there are probably " quanta " of nervous 

 impulse. 



25^. The Energy-Transformations in a Muscular Con- 

 traction. We consider only one fibre : when it is stimulated 

 by a single nervous impulse its tension increases. If one or both 

 of the parts to which the ends of the fibre are attached are free 

 to move, then the tense fibre applies force to one or other of these 

 parts, which thereupon move. 



The muscle-fibre consists of ultimate parts, in the physical 

 sense, and these parts are the fibrils, or sarcostyles, or they may 

 be a series of fibrils continuous with the nerve endings in the 

 motor-plate (the precise structure is not yet known without doubt). 

 When the state of tension is initiated by the nervous impulse it 

 may be that the sarcostyles tend to thicken transversely and so 

 become tense, or the ultimate fibrils may simply writhe. 



The muscle-fibre is " protoplasmic," but it contains some 

 carbohydrate substance in a highly reactive condition and this 

 substance has potential (chemical) energy. When the nervous 

 impulse enters the fibre the carbohydrate undergoes sudden dis- 

 integration with the result that lactic acid is formed. Lactic acid 

 has less available energy than the original carbohydrate and the 

 balance of energy thus set free goes to raise the tension of the 

 fibre (or the ultimate parts of the latter) that is, potential chemical 

 energy transforms into the potential energy of the increased tension 

 of the fibre. The transformation is complete, that is, the ratio 



energy of tension 



= I 



potential chemical energy set free 



If now the ends of the fibre are free to move they apply forces. 



The fibre shortens, thus doing work. It then returns to its 



original state until another nervous impulse enters it and the series 



