T11JU MUSULE8. 



549 



greater pull when its effort to shorten has been resisted than when 

 it has reached the same during a contraction without resistance, 

 which is an isotonic contraction. . 



CURVE OF FATIGUE. When a muscle has become fatigued and 

 its myogram studied, at first the contractions improve for a short 



Fig. 193. Fatigue-curves of Frog's Muscle. (WALLER.) 



time. This is shown by the successive contractions being higher. 

 Afterward the latent period increases, the curve becomes less high, 

 while the contraction becomes slower and lasts longer. 



Veratrine and adrenalin greatly prolong the stage of relaxation 

 in a muscle. 



STAIRCASE CONTRACTIONS. When electrical stimuli of equal 

 strength are let into a muscle at regular intervals and the contrac- 

 tions registered, it is seen that at first each curve exceeds its pre- 

 decessor in height. It shows that the muscle is benefited within cer- 



St. 



Con 



A a c 



Fig. 194. Influence of Increasing the Strength of the Stimulus upon 

 the Contraction of Skeletal Muscle. 



St, The stimulus. Con, The resulting contraction. A, A subminimal 

 stimulus. B, The minimum adequate stimulus. C, The optimum stimulus. 



(PATON.) 



in limits by contraction, and its excitability increased for a new 

 ulus, just as we can do better muscular work when we have 

 rmed up our muscles. Bowditch first noticed the staircase con- 

 ction in cardiac muscle. 



COMPOSITION OF MUSCLE BEFORE AND AFTER CONTRACTION. 

 Experiments show that constant chemical metabolism is going on 

 in a muscle at rest. It is constantly taking in oxygen, glucose, and 



