THE MUSCULAR SYSTEM 



397 



is reduced to about -^ second. The entire contraction from 

 the instant of stimulation to the end of relaxation occupies about 

 Y$ second. 



The latent period, contraction, and relaxation of a muscle are 

 affected by certain conditions. It is found that with an increase 

 in the weight a muscle may be called upon to lift, the less the con- 

 traction while the latent period is prolonged. By continuing to 

 weight the muscle a point is reached when it is unable to lift the 

 load. With an increase in the strength of the stimulus there is, up 

 to a certain point, increased contraction, and also a shorter latent 

 period ; beyond this point, known as a ' maximal stimulus,' an 

 increased stimulus produces no effect. It will be remembered 

 that this is very different from the effect of stimulation on heart 

 muscle (p. 50), the weakest or minimal stimulus applied to 

 cardiac muscle producing a max- 

 imal contraction. If a muscle 

 is kept contracting and record- 

 ing its movements, it is easy to 

 observe that as it settles down 

 to its work the contractions 

 improve, each being higher than 

 the preceding. This is known as 

 the beneficial effects of contraction, 

 and the character of the tracing 

 recorded is described as a ' stair- 

 case ' (see Fig. 115). If the 

 contractions be maintained suffi- 

 ciently long the muscle becomes 

 fatigued. Under the influence 

 of fatigue the period of contrac- 

 tion, and especially of relaxation, 

 becomes longer (see Fig. 116) in 



which the muscle has been made to contract at the same point 

 at each revolution of the drum. If the muscle be left to itself it 

 will recover, though on rest imulat ion it is more easily fatigued 

 than before. 



The effect of warming a muscle is to shorten the latent period 

 and quicken the contraction. The results obtained are not, 

 however, so simple as would appear from this statement ; there are 

 two maximal temperatures at which the frog's muscle does its best 

 work— viz., at about y° C. and about 28 C. (45 F. and 82 F.). 

 There are two temperatures at which the muscle loses its irrita- 

 bility—viz., o° C. and 37 C. (32 F. and 98 F.). Cold prolongs 

 the latent period and also the period of contraction, but heightens 

 the excitability. When the temperature of muscle is raised 

 beyond a certain point heal rigor occurs (see p. 418). 



Fig. 115. — 'Staircase,' Skeletal 

 Muscle : Frog (Stewart). 



