Biology of Man 499 



about .05 of a second. The contraction is brought about by energy re- 

 leased in the use of foods. Part of the energy associated with muscular 

 activity is heat energy which explains the increase in temperature when 

 muscles are active. Certain metabolic products, designated as fatigue 

 substances, result in muscle fatigue. If a minimum supply of oxygen is 

 present, muscles fatigue sooner if stimulated repeatedly. Recovery from 

 fatigue results from the use or elimination of fatigue substances and the 

 replenishment of the food supply, if this is necessary. 



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Fig. 251. — Membrane theory of nerve impulse transmission shown diagram- 

 matically. A, Resting nerve, showing the polarized membrane (positive charges 

 on the outside and negative charges on the inside) ; B, nerve conducting an im- 

 pulse, showing, from left to right, the partially repolarized region behind the 

 impulse, the depolarized region where the impulse is located, and the polarized 

 region ahead of the impulse ; C, passage of the impulse along the nerve shown in 

 successive stages. (From Hunter and Hunter: College Zoology, W. B. Saunders 

 Company.) 



