66 niYSlOLOGY OF MUSCLES AND NERVES. 



determined by inserting a chain and an electro-magnet 

 between the vices k and h\ The electric cmTcnt then 

 passes through the platinum point, the correspond- 

 ing lever, the quicksilver capsule, and the coils of the 

 electro-magnet. The latter becomes magnetic, and at- 

 tracts an armature. As soon, however, as the current 

 is interrupted by the contraction of the muscle, the 

 electro-magnet sets the armature free, and the latter, 

 striking against a bell, gives a signal which shows that 

 the muscle has contracted. In this way even very 

 niinute contractions of the muscle are recognised. If 

 the weights which act as over-bm*den, and counter- 

 balance the tendency to contraction in the muscle, are 

 gradually increased, a limit is reached at which, in spite 

 of the irritation of the muscle, the current of the electro- 

 magnet is no longer interrupted. The muscle is indeed 

 irritated, and a tendency to contraction is generated 

 within it ; but this is not sufficiently great to overcome 

 the weight used ; and the muscle, therefore, remains 

 uncontracted. In this way the extent to which the 

 tendency of a muscle to contract — or its energy, as we 

 called it, can increase — may be found. This extreme 

 limit of its energy is called the force of a muscle. It 

 is the same in amount as that Avhich we theoretically 

 inferred (p. 48) from the change in the elasticity of 

 a muscle during contraction. Each muscle has a definite 

 force dependent on the conditions of its nourishment 

 and on its form. On comparing the muscles of the same 

 animal, it appears that the force is dependent in no way 

 on the length of the muscle-fibres, but on the number 

 of these fibres, or, in other words, on the diameter of 

 the muscle ; and that the force increases in exact pro- 

 port ion with the diameter of the muscle. So that a 



