19 HUMAN PHYSIOLOGY 



period are the shorter the higher the temperature. The 

 height of the contraction also changes with the temperature, 

 but it is not increased merely with the raising of temperature, 

 for a .cold muscle may give greater contraction than a warm 

 muscle. 



2. The load. In general, the height of the contraction is 

 less the greater the load of the muscle. But it must be 

 observed that the height of contraction of a muscle without 

 any load is slightly less than that of a moderately loaded 

 muscle. 



3. Fatigue. If a muscle has made many successive con- 

 tractions, the duration of the contraction and latent time 

 increases; the height of the contractions at first slightly 

 increases, but later on gradually decreases. 



Concerning the influence of the strength of the stimulus 

 upon contraction see page 197. 



Wave of contraction. Though but a limited portion of 

 a muscle is stimulated, the whole muscle contracts. The 

 contraction is propagated in the form of a wave in both 

 directions from the spot stimulated throughout the muscle 

 fibres. If the motor nerve of a muscle is stimulated, the 

 wave of contraction spreads from the place of entrance of the 

 nerve through the fibres. 



The rapidity of the contraction wave is measured by 

 stimulating a certain part of the muscle and placing two> 

 recording levers at unequal distances from the stimulated 

 spot. The increase in diameter of the muscle due to con- 

 traction will not meet the two levers at the same time, and 

 this difference in time will represent the length of time taken 

 by the contraction wave to travel through the distance which 

 separates the two levers. 



The rate of the contraction wave in the skeletal muscle of 

 a frog at room temperature is three metres per second, for 

 the muscles of a rabbit four to five metres, and for a human 

 muscle ten to fifteen metres. In smooth muscles it is ten 

 to fifteen mm per second. The rate is decreased by cooling 

 the muscle and by fatigue. The duration of the contraction 



