466 MANUAL OP PHYSIOLOGY. 



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 

 muscle from a curarized 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-muscular 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 

 stimulus. A fresh muscle, then, at the ordinary temperature with 

 a medium load, will contract more and more as the intensity of 

 the current employed increases. There is a limit to this increase, 

 and with comparatively weak stimulation an effect is produced 

 which cannot be surpassed by the same muscle, no matter what 

 stimulus be applied. This greatest contraction is the same for 

 all medium stimuli while the muscle is fresh, and is called the 

 maximum contraction, being the greatest shortening which can be 

 produced by a single instantaneous stimulus. 



Summation. Each time a muscle receives an induction shock 

 of medium strength it contracts to its maximum. If a second 

 shock be given while the muscle is in the contracted state, a new 

 maximum contraction is added to the extent of the contraction 

 the muscle was in at the moment of the second stimulation, and 

 if stimulated when the lever is at the apex of the curve, the sum 

 of the effect produced will be equal to two maximum contractions. 



If applied in the middle of the period of the ascent or descent 



