160 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 





Fig. 10.5. Ending of a motor nerve cell fiber on a striated muscle cell, highly 

 magnified. A change sweeps over the fiber of the nerve cell to the junction with 

 the muscle fiber and there it acts upon the muscle stimulating it to contract. When 

 a nerve cell acts upon muscle the ends of the nerve fiber produce a minute amount 

 of chemical substance which stimulates the muscle to contract. This substance is 

 a neurohumor, a chemical link between stimulation and activity. (Courtesy, Gen- 

 eral Biological Supply House, Inc., Chicago, 111.) 



as they go into action and, as they work, their great demand for oxygen is 

 answered by deeper breathing. 



Stimulation. Contraction is due to a rapid succession of stimuli coming into 

 the muscle fibers from nerve cells (Fig. 10.5). If many fibers are stimulated 

 the contraction is strong; if few are stimulated it is weak. The strength of the 

 stimulation whether of one or several muscles depends originally upon stimuli 

 received through the eyes, ears, nose, and other sense organs. Making a home 

 run means that strong sensory stimuli, the sight of the opposing players, and 

 applause of the spectators, have been translated into motor stimuli and have 

 put millions of muscle cells into action. A muscle cell is stimulated, contracts, 

 relaxes, and recovers. These steps are gone through with great rapidity and 

 can be analyzed only because living muscle can be isolated and subjected to 

 experiment and observation. 



The contraction of muscle is completely dependent upon receiving messages 

 via certain nerves. Muscles also send forth messages via certain other nerves. 

 It is their ability to do this that makes it possible for us to know that our feet 

 are on the floor. 



Fatigue of Muscle. A muscle acts for a considerable period until it is 



