54 Life and Health 



Muscles are almost invariably stimulated to contraction 

 by a nervous impulse. The delicate nerve fibrils which end 

 in the muscle fibers communicate with the spinal cord or 

 the brain, the center of the will power. Hence, when the 

 brain commands, a nervous impulse, sent along the nerve 

 fibers, becomes the exciting stimulus which acts upon the 

 muscles and makes them contract. 1 If the nerves of a 

 part, as the face or the leg, be severed the muscles will 

 fail to act, because while they are intact and ready 

 to work, there are no means of sending commands 

 to them and they remain idle. In short, the parts are 

 paralyzed. 



Contraction is not, however, the natural state of a 

 muscle. In time it is tired and begins to relax. Even 

 the heart, the hardest-working muscle, rests between its 

 beats. 



479. The Object of Contraction. The object of contrac- 

 tion is obvious. If one end of a muscle be fixed and the 

 other attached to some object which is free to move, the 

 contraction of the muscle will bring the movable body 

 nearer to the fixed point. Thus by their contraction muscles 

 are able to do work. They even contract more vigorously 

 when resistance is opposed to them than when it is not. 

 If, however, the contractions are too rapid or too long con- 

 tinued the muscles become exhausted. When the feeling 

 of exhaustion passes away with rest, the muscle recovers 

 its power. 



1 This property of contraction is inherent and belongs to the muscle 

 itself. It is often independent of the brain. Thus, on pricking the heart 

 of a fish an hour after removal from its body, obvious contraction will 

 occur. In this case it is not the nerve force from the brain that supplies 

 the energy for contraction. The power of contraction is inherent in the 

 muscle substance, and the stimulus by irritating the nerve ganglia of the 

 heart simply affords the opportunity for its exercise. 



