CHAPTER XL 

 EXCRETION AND FATIGUE 



241. Getting tired. When you "chin" yourself on a bar 

 four, five, six times, until you can do no more, this does 

 not mean that you will never be able to chin yourself again. 

 After resting awhile, perhaps a day or an hour, or perhaps 

 only ten or fifteen minutes, you can chin yourself again as 

 well as at first. What happens in the first place to make you 

 stop, or what happens during the rest to enable you to do 

 the work again ? 



A modern explanation is that the waste substances begin 

 to accumulate in the cells as soon as the work commences ; 

 the wastes are formed faster than they can be carried away, 

 and the result is a poisoning of the protoplasm of the working 

 cells. Experiments have enabled us to discover the importance 

 of these wastes. 



242. Fatigue poisons. If a muscle taken from the leg of a 

 frog is made to work (by being stimulated with an electric 

 current) until it is too tired to do any more, it may be restored 

 to working power by the simple process of washing it in salt 

 water. The salt water certainly does not supply fuel to the 

 muscle ; on the contrary, it would seem rather to take some- 

 thing away. Moreover, if the salt water that has been used 

 to wash the tired muscle is now injected into a fresh muscle, 

 one that has not been working, the latter immediately becomes 

 too tired to work. This would show that some unknown sub- 

 stance has been taken away from the tired muscle to make 

 it fresh, and that this same substance has been added to 

 the fresh muscle to make it tired. This substance has been 

 called fatigue poison. 



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