THE MUSCLES 133 



action, such as mountain climbing, hardly increases the 

 amount of albuminous material (urea) excreted as waste. 

 Such activity, however, does largely increase the amount 

 of carbon dioxid given off. It is believed, therefore, that 

 our energy is largely derived from starches and sugars 

 (carbohydrates) and fats. The fact that beasts of burden 

 depend chiefly upon carbohydrate foods strengthens this 

 view. That muscles use sugar in their action is indicated 

 by the fact that the arteries take more sugar to the muscles 

 than is carried away from them by the veins. 



225. The nerve impulse reaching the muscle causes it to 

 contract. This takes about ^V f a second. It would relax 

 in gV f a second also, and it can be kept in a state of con- 

 traction only by successive impulses, usually about 10 per 

 second. Thus it remains in a tremulous state of contraction. 

 The vibrations may become apparent during excitement, 

 or when a great effort is being made. If a muscle con- 

 tracts continuously for a long time, a time comes when 

 it can no longer be made to contract, not even by an 

 electric stimulus, but it becomes soft and relaxed. What 

 is the cause of this ? Are the oxygen and sugar and fat all 

 combined and changed into carbon dioxid ? Or, as we 

 say, is the strength all used up ? No, it is found that if 

 the carbon dioxid is allowed to escape but no fresh blood 

 allowed to come, the muscle can be made to contract again. 

 This shows that the accumulation of the carbon dioxid 

 gas and perhaps some acids besides, prevent the nerve 

 stimulus from having effect. The paralysis and aching of 

 extreme fatigue are thus caused. It is a very beneficent 

 thing that a healthy person undisturbed by poisonous drugs 

 or stimulants can never use up all his strength. A man 

 may have been ill for weeks, yet when delirious, and stimu- 

 lated by the poisons formed in the course of the disease, he 

 proves that he still has strength enough to overpower 

 several strong men. 



