200 ANIMAL PHYSIOLOGY. 
The effectiveness of a given muscle must, of course, depend 
on the degree to which it shortens, which is from one half to 
three fifths of its length; and the number of fibers it contains 
—i. e., upon its length and the area of its cross-section, taking 
into account in connection with the first factor the arrange- 
ment of the fibers; those muscles in which the fibers run 
longitudinally being capable of the greatest total shortening. 
There is, as shown by actual experimental trial, a relation 
between the work done and the load to be lifted. With double 
the weight the contraction may be as great as at first, or even 
greater; but a limit is soon reached beyond which contraction 
is impossible. This principle may be stated thus: The contrac- 
tion is a function of the stimulus, and is illustrated by the 
diagram below (Fig. 193). 
“t EE 45 OO use a ae TE 
Fig, 193.—Diagram of muscular contractions with same stimulus and increasing weights. The 
numbers represent grammes (McKendrick). 
It has been shown experimentally that the chemical inter- 
changes in a muscle, acting against a considerable resistance, 
are increased—i. e., the metabolism and the working tension are 
related. 
These experimental facts harmonize with our experience 
of a sense of satisfaction and effectiveness in the use of the 
muscles when weights are held in the hands; and it must bea 
matter of practical importance that each person should, in 
taking systematic exercise, keep to that kind which does not 
either overweight or underweight the muscles. 
CIRCUMSTANCES INFLUENCING THE CHARACTER OF MUSCULAR 
AND NERVOUS ACTIVITY. 
The Influence of Blood-Supply. Fatigue.—Fig. 194 shows at a 
glance differences in the curves made by a contracting muscle 
suffering from increasing fatigue. 
Suppose that in such a case the blood had been withheld 
from the muscle, and that it is now admitted, an almost im- 
mediate effect is seen in the nature of the contractions; but 
even if only saline solution had been sent through the vessels 
of the muscle, a similar change would have been noticeable. 
We may fairly conclude that the blood and saline removed 
something which had been exercising a depressing effect on the 
