MAXIMUM CONTRACTION. 463 



anything that leads to an accumulation of the chemical products 

 which arise from muscle activity. Hence fatigue or over-stimu- 

 lation cause a slow contraction (Fig. 186). 



Moderate increase of temperature greatly shortens the time oc- 

 cupied by the single contraction of any given muscle. Excessive 

 heat causes a state of continued contraction. 



The reduction of temperature causes a muscle to contract more 

 slowly, and, when extreme, the muscle remains contracted long 

 after the stimulus is removed. 



Wave of Contraction. If one extremity of a long muscle be 

 stimulated without the aid of the nerve (it is best to employ a 



FIG. 189. 



Curves drawn by the same muscle while being cooled. Showing that the 

 latency and the contraction become longer as the temperature is reduced. 



muscle from a curarised animal), the contraction passes along the 

 muscle from the point of stimulation in a wave which travels at 

 a definite rate of 3-4 metres per sec. in a frog, and 4-5 metres 

 per sec. in a mammal. Reduction of temperature and fading of 

 vital activity cause the velocity of the wave to be lessened, until 

 finally the tissue ceases to conduct ; then only a local contraction 

 occurs, severe stimulus causing simply an elevation at the point 

 of contact. This seems analogous to the idio-rnuscular contrac- 

 tion, which marks the seat of severe mechanical stimulation after 

 the general contraction has ended. 



MAXIMUM CONTRACTION. 



The extent to which a muscle will contract depends upon the 

 conditions in which it is placed, and varies, as we have seen, with 

 the load, its irritability, the temperature, and the force of the 



