124 MUSCULAR CONTRACTION. [CH. X. 



is reached which the muscle is unable to lift. The latent period is some- 

 what longer with a heavy load than with a light one. 



3. Influence of fatigue. This can be very well illustrated by letting the 

 muscle write a curve with every revolution of the cylinder until it ceases 

 to contract altogether. The next diagram shows the early stages of fatigue. 



Fig. 146. fatigue. 



At first the contractions improve, each being a little higher than the pre- 

 ceding ; this is known as the beneficial effect of contraction, and the graphic 

 record is called a staircase. Then the contractions get less and less. But 

 what is most noticeable is that the contraction is much more prolonged ; 

 the latent period gets longer ; the period of contraction gets longer ; and 

 the period of relaxation gets very much longer ; there is a condition known 

 as contracture, so that the original base line is not reached by the time 

 the next stimulus arrives. In the last stages of fatigue, contracture passes 

 off. 



Fig. 147. Effect of temperature on a single muscular contraction ; N, normal ; //, warm ; 

 Ci, cooling; Cz, very cold; P, point of stimulation. The above tracing is a con- 

 siderably reduced fac-simile of a tracing taken with the pendulum myograph. 



4. Effect of temperature. Cold at first increases the height of contrac- 

 tion, then diminishes it ; otherwise the effect is very like that of fatigue 

 increasing the duration of all stages of the curve. 



Moderate warmth increases the height and diminishes the duration 

 of all stages of the curve, latent period included. This may be 



Fig. 148. Veratrine curve, taken on a very slowly-travelling cylinder; the time tracing 

 indicates seconds. 



