66 PHYSIOLOGY OF M1M I.KS AND NERVES. 



determined bv inserting a chain and an electro-magnet 

 between the vices /-and //. The electric currenl then 

 passes through llie platinum poiut, the convspond- 

 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 

 minute contractions of the muscle are recognised. If 

 the weights which act as over-burden, and counter- 

 balance the tendency tn contraction in the muscle, are 

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

 of the irritation of the muscle, the current oft lie 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 maybe found. This extreme 

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

 is the same in amount as that which we theoretically 

 inferred (p. 4H) from the change in the elasticity of 

 a muscle during contract ion. Kaeh muscle has a definite 

 force dependent on the conditions of its nourishment 

 and on \\< form. On comparing the muscles of the same 

 animal, it appears thai the fore.- is dependent in noway 

 on the length of the mux-le-libivs, but on the number 

 of these libres, or, in other words, on (he diameter of 

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

 portion with the diameter of the muscle. So that a 



