WORK THE MUSCLES 97 



such as that of the alimentary canal, whose state of " tonic " con- 

 traction is relaxed by warming. 



The effect of temperature excludes another explanation which 

 has been suggested, namely, that acid increases the amount of 

 water taken up in the swelling of colloidal structures, and that the 

 arrangement in muscle is such that the swelling causes the shorten- 

 ing of the fibrillae. This imbibition, however, has the usual positive 

 temperature coefficient ; is greater as the temperature rises. 



There is yet much to be learned about the intimate nature of 

 the process of muscular contraction, but further discussion would 

 not be profitable here (P., pp. 436-458). 



Gradation of Contraction " All-or-nothing " 



Practical experience teaches us that we can cause our muscles 

 to contract with different degrees of strength. Since any individual 

 muscle consists of a large number of fibres, the adjustment might 

 in theory be made in two ways, either by causing all the fibres to 

 contract, but with less than their maximum force, or by causing only 

 a certain varying number to contract, but each always with the same 

 maximum degree of intensity. If we call to mind the similarity of 

 a muscle to the propelling charge in a cartridge, we realise that 

 the former method is less probable than the latter. Although a 

 certain small expenditure of energy is required to move the trigger, 

 this has no relation to that set free in the explosion of the charge ; 

 and whatever the strength with which the trigger is pulled, the 

 energy set free is the same. The movement^of the trigger 

 corresponds to the stimulus applied to a muscle, and this has no 

 relation to the energy set free in a contraction. Direct experi- 

 mental proof, however, shows that the changes in degree of 

 contractile strength in a muscle are actually due to the putting 

 into action of a varying number of individual fibres, each work- 

 ing at its greatest capacity. Of course, this does not mean that a 

 fatigued muscle can exert the same degree of tension as a fresh 

 muscle. It means that, so far as any fibre is concerned, whatever 

 the strength of the stimulus, if it has any result at all, the force 

 of the contraction is the greatest that this fibre can exert in its 

 state at the time. 



We shall see later that the same statement applies to any 

 individual nerve fibre, so that it is impossible to vary the strength 

 of the stimulus to a muscle fibre. Thus, even if the latter were 

 capable of different degrees of contraction, there is no means of 

 altering the strength of the normal stimulus so as to make use of 

 the property. In the nerve, as in the muscle, it is a question 

 of "all-or-nothing." As in the muscle, adjustments are made 

 by altering the number of fibres in action. 



7 



