RECOVERY FROM FATIGUE OF MUSCLES 439 
nerve that enters the muscle. This important fact enables 
us to understand how the nervous system is able to control 
the actions of our muscles. 
349. Food of Muscles. — The blood flowing through the 
muscles carries food to the muscle cells. The most important 
of the food substances used in the contraction of a muscle is 
grape sugar, or glucose. This sugar is formed in the body in 
digestion from starch and cane sugar. Glycogen, the stored- 
up sugar in the liver, is also used to furnish energy in con¬ 
traction, but not until it has been transformed into sugar. 
A small amount of fat is present in muscles and may be 
used up during their contrac¬ 
tion. Under ordinary circum¬ 
stances the protein foods do not 
furnish energy for contraction, 
but are used to repair the actual 
wastes that take place in the Figure 391. — Heart Muscle 
muscle cells as they do work. Cells. 
350. Fatigue of Muscles. — After you have played hard for 
a time, you become tired and want to rest. You are tired or, 
to express it more scientifically, your muscles are fatigued. 
When you are taking your physical exercises, some of the 
movements make your arms ache, and you do not do them in 
good form. You are using muscles that do not get much 
exercise and they become quickly fatigued. You may con¬ 
tinue to move your arm until it is impossible to move it any 
longer. In such cases of extreme fatigue not only are the 
muscles tired but also the nerve cells. Two things are in¬ 
volved in the fatigue process of a muscle: first, there is the 
using up of the food energy necessary for muscular contrac¬ 
tion ; and secondly, there is the accumulation of waste sub¬ 
stances produced by the activity of the muscles. 
351. Recovery from Fatigue of Muscles. — It is difficult 
to separate entirely a consideration of the muscles from the 
nervous system, as already explained, but some facts indicate 
