THE TISSUES 57 



becomes less and less. This may be represented diagram- 

 matically in the accompanying figure, where the continuous 

 lines represent the strength of the stimuli, and the dotted 

 lines the extent of the contractions (Fig. 24). 



After a certain strength of stimulus has been reached, 

 further increase of the stimulus does not_cause any increase 

 in the muscular contraction. Tljis smallest stimulus which 

 causes the maximum muscular contraction is called the 

 optimum stimulus. 



Increasing the str^ngth^nf t.Vm pt.imnbia shortens the 

 latent periocl/but lengthens the periods of contraction and 

 relaxation. 



6. Resistance to Contraction Weight to be Lifted. 

 Starting from the extent of muscular contraction without 



IG. 25. Influence of Load on a Muscular Contraction, (a) The effect of increas- 

 ing the load on the extent of contraction ; (b) the effect of load on the course 

 of contraction. 



any load, it is found that small weights attached to the 

 muscle actually increase the extent of contraction, but 

 greater weights diminish it, until finally, when a sufficient 

 weight is^tpplied, the muscle no longer contracts at all, but 

 may actually slightly lengthen, because its extensibilities 

 increased during contraction (Fig. 25, a). 



The application of weights to a muscle causes the latent 

 period and period of contraction to be delayed, while it 

 renders the period of relaxation more rapid, and an over- 

 extension may be produced followed by a recovery resem- 

 bling a small after-contraction (Fig. 25, b). (Practical 

 Physiology, Chap. VI.) 



7. Successive Stimuli. So far we have considered the 

 influence of a single stimulus on the shape of muscle. But 

 in nearly every muscular action the contraction of the 

 muscles must last much longer than y^-th of a second. 



