44 



HUMAN BIOLOGY 



As the boy wills to throw the stone, nerve impulses are 

 sent to all the organs that can assist, and they are excited 

 to just the amount of action needed. 



The Nerve Impulse and the Contraction. Each nerve 

 that goes to a muscle is composed of many fibers ; the 

 fibers soon separate and go to all parts of the muscle, 

 and each muscle fiber receives its nerve fiber (see Fig. 45). 



In the brain each fiber is 

 stimulated at once, and all 

 the fibers shorten and thicken 

 together. This change is 

 spoken of as contraction ; but 

 since the muscle does not be- 

 come smaller, the word may 

 be misleading. When the 

 muscle shortens, it thickens 

 in proportion and occupies as 

 much space as it did when 

 relaxed. 



Where does Muscular En- 

 ergy come from? The nerve 

 does not furnish the energy 

 which the muscle uses when 

 contracting. The muscle cells 



FIG. 45. MOTOR NERVE FIBERS, 

 ending among fibrils of voluntary 

 muscle. Compare with Fig. 48. 



have already stored up energy from the food and oxygen 

 brought to them by the blood, and the process called oxida- 

 tion sets free the energy. Activity of muscle may increase 

 the carbon dioxid output fivefold. Mental work has prac- 

 tically no effect upon it. 



How a Muscle stays Contracted. The muscle relaxes at 

 once after contraction; and in order to keep it contracted, 

 nerve impulses must be sent in quick succession, causing 

 in fact many contractions ; the effect of this is sometimes 



