THE CONTRACTION OF MUSCLE 



547 



it is suggested that it is due to the beneficial action of the meta- 

 bolites formed in the previous contractions. When the contractions 

 are made to follow each other without any pause they finally become 

 less and less, and the relaxations become more and more drawn out, 

 a well-marked " contraction remainder " usually appearing, until 

 eventually the muscle gives no contraction at all; the muscle is 

 " fatigued " (Figs. 28.3. 284). 



; ' Fatigue " is due mainly to the accumulation of the products of 

 activity within the muscle in part, however, to the using up of food 

 material. The fatigue products are acid in nature chiefly lactic 

 acid. Fatigued mu-scles placed in oxygen recover more quickly 

 (Fig. 285). If the muscles be made to contract in an atmosphere of 

 oxygen, lactic acid does not appear, and the onset of fatigue is much 

 delayed or postponed altogether. Recent evidence tends to show that 



FIG. 285. CHANGES IN LENGTH OF A PAIR OF EXCISED GASTKOCXEMII AFTER FATIGUE. 



(W. M. Fletcher.) 



.1. Exposed to oxygen; B. exposed to air. Load 3 grammes, temperature 16 C. The 

 ordinates are m -a-ured directly from the record; levers magnified six and a half 

 times. 



lactic acid is not normally a waste product, but a stage in the metabolic 

 changes of the muscle. A muscle through which the blood is cir- 

 culating is fatigued only when either the load, or the frequency of 

 contraction is made too great. Thus the muscles of the skilled work- 

 man perform thousands of contractions without fatigue. So with the 

 respiratory muscles and the heart. To secure the maximum output 

 of athletes or workers, load and frequency must be carefully adjusted 

 to prevent overstrain. 



Summation. If a muscle be given two stimuli in quick succession, 

 the effects of the two are added together " summated " and a 

 bigger curve is obtained than either of the single curves would have 

 been if recorded separately (Fig. 286). If, however, the interval 

 between the stimuli be very short, so that the second stimulus falls 

 within the latent period of the first stimulus, then, if this be maximal, 

 no summation is obtained, the curve is unaffected by the second 



