182 



THE POPULAR EDUCATOR. 



partly derived from bone destruction, but principally from the 

 wear and tear of nerve substance, as they are always increased 

 after any undne mental exertion, or any other circumstance 

 producing nervous exhaustion. The chlorides are probably 

 derived directly from the food taken. All these elements vary 

 much in amount, but are all of subordinate importance to the 

 excretion of urea. 



Having now traced the blood from its origin, in the products 

 of digestion, through the various tissues of the body, and having 

 described one of the methods by which the worn-out materials 

 are removed from it, we shall in the next article consider the 

 subject of respiration, which relieves the blood of the remaining 

 impurities, and refits it to continue its unceasing work of re- 

 building and maintaining the different tissues of the body. 



LESSONS IN FRENCH. LXXXVIII. 

 146. PEENCH HOMONYMS AND PARONYMS (continued). 



