*OOD, WORK, AND GROWTH OF AN1MAL8 187 



apparently cease to grow, but in reality they go on 

 growing, though their size remains about constant. 

 They are continual lv repairing and rebuilding the worn 

 places in their bodies; twice each year a new crop of 

 hair is grown, the various muscles are constantly re- 

 newed, and in this way the body is continually grow- 

 ing. These repairs and changes cease only when the 

 animal dies. Life means change and constant repair- 

 ing; an addition of new growth and a replacing of old 

 parts by new. In plants these changes usually mean 

 the addition of new growth, most plants continuing to 

 increase in size so long as they live. Most animals, on 

 the other hand, reach a fixed point where they cease 

 to grow in size, and such changes in the bodies as take 

 place consist in renewing worn parts. 



180. Food Necessary for Growth.— To effect the 

 changes that are constantly taking place in the animal 

 body a constant supply of the proper kinds of food is 

 necessary. Not only is food necessary for the repairs 

 that are constantly going on, but it is necessary to keep 

 up the tempe ratur e of the body. If the temperature 

 of an animal's body is much reduced, a chill is the result 

 and sickness usually follows. Food and a proper supply 

 of fresh air are necessary to prevent the cooling off of 

 the animal body. 



181. Food Necessary for Work. — Food and air are 

 also necessary to enable the animal to use its muscles 

 in moving about. The animal body has often been 

 compared to a steam engine. Feed an engine fuel, 

 water,. and air, and it does work; take away its supply 

 of any one of these three things, it ceases to move, and 



