86 AGRICULTURE. [CH. 



The first duty draws our attention to the waste of the 

 body, which is constantly going on. Every movement 

 of the body causes a waste of the part exercised. If 

 the strain or effort be violent, the waste of tissue is 

 greater than if the movement be gentle. The waste 

 is, however, quickly repaired, and, as if to prepare 

 that part of the body for future demands, it is actually 

 strengthened. Hence, although exercise causes a 

 waste of the tissues, it tends to increase the strength 

 of the part exercised. All are familiar with the well 

 known strength of the blacksmith's arm, resulting, as 

 it does, from severe exercise ; and in contrast with 

 this we have the weak and feeble arm, which has been 

 kept from exercise. The waste of the body makes its 

 demand upon the several substances of this nitro- 

 genous group, and this is the first duty they have to 

 perform. Any surplus which remains is available for 

 muscular growth. This group therefore represents the 

 flesh-forming matter of food. 



1 80. There is another class of bodies present in 

 food, and these are known as the mineral matter of 

 food. Their chief duty is to supply materials for the 

 growth of the skeleton. 



1 8 1. The various articles of food which we employ 

 for feeding farm stock generally contain a mixture of 

 different bodies, belonging to two, and sometimes to 

 three of these groups. Our judgment upon the action 

 of food, is therefore influenced by what we know of its 

 composition. If, for instance, there should be none 

 of the flesh-forming bodies the nitrogenous group 

 present, then it would be impossible for flesh to be 

 formed. We are not justified, however, in saying that 

 if none of the heat-producing matters the non-nitro- 

 genous group are present, that it would be impos- 

 sible to maintain the warmth of the body. There is 

 reason to believe that the nitrogenous group, which 

 contains (in addition to nitrogen) carbon, hydrogen, 

 and oxygen, the elements of which the heat-producing 



