82 EXCREMENTS AND URINE. 



pie, if a pound of starch when burnt away in an oven 

 evolved 1,000 degrees of heat during the length of 

 fifteen minutes, the same amount of heat would be 

 emitted during twelve hours, if the starch underwent 

 the same physical alteration within twelve hours, as 

 by its combustion in the oven ; but it would, when 

 evolved in an interval forty-eight times as long, be 

 also forty-eight times feebler ; or, what indeed is tan- 

 tamount to this, it would be sufficient to maintain 

 the body for the length of twelve hours at a temper- 

 ature of nearly 21 degrees. 



For this reason, those proximate constituents of 

 provender which consist simply of the three sub- 

 stances carbon, oxygen, and hydrogen, are usually 

 denominated elements of breathing or respiration. 

 In moderate feeding none of these substances re- 

 main in the animal body ; in excessive feeding, on the 

 other hand, such as occurs in fattening stock, a part 

 of the surplusage is converted into fat or suet^ and 

 laid up in the body. Those proximate constituents 

 of plants which contain nitrogen in addition to the 

 elements above mentioned must be regarded, on the 

 contrary, as the strictly nutritive ingredients of food, 

 because they alone can be transformed into flesh, 

 skin, muscles, nerves, and other animal tissues. For 

 this reason they are called plastic or flesh-forming 

 elements of nutrition. As long as an animal lives, 

 a constant renovation of its bodily organs takes 

 place ; inasmuch as these are made fluid, and re- 

 moved by the excrements, and more especially by 



