DIGESTION. 71 



its way back again into the common duct, and flows into the 

 intestine, to be mixed with the food. 



The chief function of the liver is to secrete the bile, a 

 greenish-yellow, bitter fluid. The bile duct, as it enters the 

 intestine, is joined by the pancreatic duct, so that these fluids 

 reach the food at the same point, about four inches from 

 the pylorus. 



20. Kinds Of Food. Before tracing the food through 

 these several steps in the process of digestion, and showing 

 the action of the different secretions it meets with on its 

 course, it will be necessary to classify the several varieties of 

 food used by man. The almost universal habit of the human 

 race, guided by instinct and reason, shows that a mixed diet 

 is the best. The different kinds of teeth would even indicate 

 this. 



There are three kinds of food : 

 I. NITROGENOUS FOODS, OR TISSUE BUILDERS. 

 II. CARBONACEOUS FOODS, OR HEAT PRODUCERS. 



III. INORGANIC, OR MINERAL FOODS. 



To the first belong albumen, as the white of the egg, 

 casein, the principal part of cheese, the fibres of lean meat, 

 and the gluten of grain. The second class comprises the fats, 

 starch and sugar. In the third class there are water and such 

 mineral substances as salt, potash, sulphur, phosphorus and 

 iron. 



The elements of nutrition must have the power to combine 

 with oxygen. The living body, like fire in a stove, must be 

 supplied with fuel that will burn. The food, as swallowed, is 

 not fuel. It has to undergo changes, both chemical and vital, 

 before it is prepared to become blood and feed the system. 

 Albumen exists in the blood, but if the white of an egg were 

 injected into a blood-vessel, it would be worse than useless; 

 the albumen of the blood must be the product of the digestive 



