228 VETERINARY PHYSIOLOGY 



latent period, but lengthens the periods of contraction and 

 relaxation, and thus lengthens the whole period of con- 

 traction. 



5. Resistance to Contraction — Weight to be Lifted. — A small 

 weight attached to the muscle may actually increase the 

 extent of contraction by lengthening the fibres (p. 224 (2)), 

 but greater weights diminish it, and when a sufficient weight 

 is applied, the muscle no longer contracts, but may actually 

 slightly lengthen, because its extensibility is increased during 

 stimulation (fig. 113, 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 resembling 



JL 



1 



Fig. 113. — Influence of Load on a Muscular Contraction, (a) The effect of 

 increasing the load on the extent of contraction ; (h) the effect of load 

 on the course of contraction. 



a small after-contraction and due to the elasticity of the 

 muscle (fig. 113, h), (Practical Physiology). 



6. Electrotonus. — As already explained in dealing with 

 nerve (p. 63), the passage of a galvanic current through a 

 muscle decreases its excitability, and hence its contractility, 

 at the anode and increases them at the cathode. 



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 lasts much 

 longer than 0*07 of a second, which is about the time taken 

 by a single contraction of mammalian muscle. 



How is this continued contraction of muscles produced ? 

 To understand this it is necessary to study the influence of 

 a series of stimuli. 



(1) If, to a frog's muscle that takes O'l of a second to 

 contract and relax, stimuli at the rate of say 7 per second are 



