APPLICATIONS OF THE GRAPHIC METHOD. 901 
vitality of the muscle. In a working muscle, like all living 
tissues, there are products of vital action (metabolism) that are 
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180 DV. 
Fig. 194.—Curves of a muscle contraction in different stages of fatigue (after Yeo). A, curve 
when muscle was fresh; B, C, D, E, each just after muscle had already contracted two 
eee times. The alteration in length of latent period is not well brought out in these 
racings. 
poisonous. We have already learned that a working muscle 
generates an excess of carbonic anhydride, and something which 
gives it an acid reaction; and that it uses up oxygen as well as 
other matters derivable from blood. 
Fatigue will occur, it is well known, if the muscles are used 
for an indefinitely long period, no matter how favorable the 
blood-supply—another evidence that there is, in all probability, 
some chemical product, the result of their own activity, depress- 
ing them; and this is rendered all the more likely when it is 
learned that the injection of lactic acid, to take one example, 
produces effects like ordinary fatigue. 
It is also a matter of common experience that exercise, while 
beneficial to the whole body, the muscles included, as shown by 
their enlargement under it, becomes injurious when carried to 
the point of fatigue. 
Why the use of the muscles is conducive to their welfare is 
but a part of a larger question, Why does the use of any tissue 
improve it ? 
When the nerve which supplies a muscle is stimulated its 
blood-vessels dilate, and it has been assumed that the same 
happens when a muscle contracts normally in the body; and 
when muscular action is increased there is a corresponding 
augmentation in the quantity of blood driven through the 
muscles in a given period, even if there be no actual increase 
in the caliber of the blood-vessels, for the heart-beat is greatly 
accelerated. . 
But repose is as necessary as exercise for the greatest effect- 
iveness of the muscles, as the experience of all, and especially 
athletes, proves. 
That the nervous system plays a great part in the nutrition 
of muscles is evident from the fact, among countless others, 
that it is not possible to use the brain to its greatest capacity 
and the muscles to their fullest at the same time; the individual 
