58 MUSCULAR CONTRACTION. 



weight is considerably less than the maximum weight 

 which the muscle is able to lift. Muscle is elastic : a 

 muscle extended by a load recovers its original length 

 when the load is removed. When a loaded muscle is 

 extended by successive additions of equal weights to its 

 load, the increase of length, resulting from each addition, 

 becomes less and less as the extension proceeds, until no 

 further increase is observable. Of two muscles, of which 

 one is in tetanus, the other at rest, the former is more 

 extended by the same weight than the latter. 



In contraction the temperature of muscle is slightly 

 raised : the greater the effort, and the less the work done, 

 the greater the rise. 



In living muscles, differences of electrical tension are 

 usually observed 'between different parts of the natural 

 surface, which differences can be shown to be intimately 

 associated with the vital properties of the tissue, and 

 cease with the cessation of its life. The greatest differ- 

 ences (often amounting to several hundredths of a Volt) 

 present themselves when a sound surface is compared 

 with an injured one, the injured part being always 

 negative to the sound. In a muscle which is at rest, 

 all parts being in the same physiological condition, the 

 surface is -(according to Hermann) isoelectric or equi- 

 potential. Such a condition is rarely met with in the 

 voluntary muscles, for the slightest exposure or injury 

 produces electrical inequality, but is easily observed in 

 the resting heart. In voluntary muscles, separated from 

 the body, it is commonly observed that the end surfaces 

 are negative to the lateral surfaces. During the state 

 of excitation, which precedes contraction (period of 

 latent excitation) the electrical state of (uninjured) mus- 

 cular tissue undergoes a change which consists in its 

 becoming negative to the unexcited parts. In tetanus 

 this change precedes each single contraction (see p. 56). 

 In nerveless (curarized) muscles, the excitatory state 



