598 FOOD REQUISITE FOR THE NATURAL WASTE. 



moved inwardly and rejected in the excretions — the place of that which 

 18 removed being supplied by new portions of matter derived from the 

 food. 



This removal, tliough imfelt by us, goes on so rapidly that in a space 

 of time, which varies from one to five years, the whole body of the ani- 

 mal is renewed. There does not remain, it is said, in any of our bodies, 

 a single particle of the same matter which formed their substance three 

 or five years ago. It is just as if we were to take a single old brick every 

 day out ofthe corneuof a house, and put in a new one — the form and 

 dimensions ofthe house would remain unaltered, and yet in the course 

 of a few years its walls would be entirely renewed. 



In full grown animals, some parts of the body are renewed more ra- 

 pidly than others — the muscles, for example, more frequently and rapidly 

 than the bones and the brain. In young animals, again, the whole body 

 is oftener renewed than in such as are advanced in years, but all the 

 parts of all animals are believed to be more or less quickly removed and 

 replaced. ^ 



The new materials which are conveyed to the different parts of the 

 body are derived directly from the food. The fibrin of the muscles is 

 replaced from the gluten which the food contains — the fat from its oil — 

 and the earthy matter of the bones and the salts of the l)lood, fiora the 

 phosphates and saline substances which are naturally present in it. On 

 the other hand, those parts which are extracted from the muscles and 

 bones, and carried off in the excretions, are decomposed during their re- 

 moval. New chemical compounds are produced from them, which are 

 found in the urine and dung of the animal, and which give to these ex- 

 cretions their richness and value in the manuring ofthe soil. 



§ 6, Ofthe kind and quantity of food necessary to make ujjfor the natural 

 ■waste in the body of a full grown animal. 



The substances which constantly disappear from the body in conse- 

 quence of the natural waste above describf^d, are of three kinds — the fbrin 

 and other analogous organic compounds, whicli form the muscles and the 

 cartilage of the bones — the earthy phosphates (of lime and magnesia), 

 which form so large a proportion of the bones, and exist in small quan- 

 tity in the muscles also — and the soluble saline substances, which abound 

 in the blood and in the other fluids of the livmg animal. In the solid and 

 liquid excretions, a larger quantity of each of these three classes of com- 

 pounds is carried out ofthe body. How much of each must be contained 

 in the daily food of a full-grown animal in order that it may be kept in 

 its actual condition? 



1°. Quantity of fibrin or other analogoiis compounds {albumen or 

 casein) which the daily food must contain. — The most accurate experi- 

 ments that have yet been made upon this subject (Lecanu) appear to 

 show that a full grown man rejects in his urine alone about half an ounce 

 of nitrogen (230 grs.) every 24 hours. This quantity of nitrogen is con- 

 tained in about three ounces of dry muscular fibre, which must, therefore, 

 every day be decomposed or removed in order to yield it. 



But if the body is kept in condition, this quantity of fibrin must be 

 daily restored again by the food. Now, to supply three ounces of dry 

 fibrin, there must be eaten about — 



