650 MISCELLANEOUS FARM SUBJECTS 



juice of the intestine upon protein, fats, and carbohydrates. The 

 action of all the ferments is aided by the fine division of the food 

 by chewing and by the muscular contractions, the so-called peristal- 

 tic action, of the stomach and intestine. These latter motions help 

 to mix the digestive juices and their ferments with the food. 



The parts of the food which the digestive juices can not dis- 

 solve, and which therefore escape digestion, are periodically given 

 off by the intestine. Such solid excreta, or feces, include not only 

 the particles of undigested food, but also the so-called metabolic 

 products, i. e., residues of the digestive juices, bits of the lining of 

 the alimentary canal, etc. 



Absorption and Assimilation. The digested food finds its way 

 through the walls of the alimentary canal, and at this time and later 

 it undergoes remarkable chemical changes. When finally the blood, 

 supplied with the nutrients of the digested food and freighted with 

 oxygen from the lungs, is pumped from the heart all over the body 

 it is ready to furnish the organs and tissues with the materials and 

 energy which they need for their peculiar functions; at the same 

 time it carries away the waste which the exercise of these functions 

 has produced. It is a characteristic of living body tissue that it can 

 choose the necessary materials from the blood and build them into 

 its own structure. How it does this is one of the mysteries of 

 physiology. The body has also the power of consuming not only 

 the materials of the food, but also parts of its own structure for the 

 production of muscular work, or heat, or to protect more important 

 parts from consumption. How it does this is another mystery. 



Excretion. After the material has been thus assimilated and 

 utilized the resulting waste products must be removed from the 

 body. The chemical elements which this waste contains are the 

 same as those making up the structure of the body and the food 

 carbon, oxygen, hydrogen, nitrogen, calcium, phosphorus, sulphur, 

 etc. Most of the carbon and part of the oxygen are given off from 

 the lungs as carbon dioxid. Hydrogen unites with more oxygen to 

 form water, which is passed off in vapor from the lungs, in perspira- 

 tion from the skin, and in urine from the kidneys. Almost all the 

 nitrogen is excreted in the urine. Waste mineral matters are given 

 off to some extent in the perspiration, but mainly through the kid- 

 neys and intestines. 



APPARENT AND ACTUAL DIGESTIBILITY. 



The real nutritive value of a food, then, depends not simply on 

 the proportions of nutrients which it contains, but also on the 

 amount of those nutrients which can be made available to the body 

 by digestion for building material and for fuel. Part of the food 

 eaten escapes digestion and is given off from the body in the feces. 

 If we subtract the amount of this undigested residue from the total 

 food, the remainder will be the amount actually digested in the 

 stomach and intestines absorbed through their walls, and taken into 

 the circulation. This difference between the amounts eaten and 

 those undigested represents the actual digestibility of food. A part 

 of the food taken into the circulation, however, is later returned 



