Organic foods that contain nitrogen are called nitrogenous foods; they contain carbon, hydrogen, oxygen, nitrogen, and very often sulphur or phosphorus. Now these nitrogenous food substances are also divided into two large classes, and one smaller and less important class. The first class, which is the most important of all, includes substances like albumen and fibrine, substances that contain a considerable proportion of nitrogen, and that are called albuminous bodies or protein compounds; and the second class includes one important substance - viz., gelatine, and a few less important ones. There is also a smaller class containing nitrogen, which includes the essential principles of tea, coffee, cocoa, etc.

Among substances that do not contain nitrogen there are, then, two important classes, the fata and oils, and the starches, sugars, and gums. We shall see what becomes of these when they are taken in our foods.

The fats that are taken in our foods are not altered in the mouth, they are not altered in the stomach, but in the small intestine; when mixed with the pancreatic juice and bile, they form what we call an emulsion. I can best illustrate what is meant by an emulsion by mentioning milk, which contains very various sub-Btances: one is fat, which is in the state of an emulsion, being divided into very small particles.

Fat, then, is reduced by the action of the pancreatic juice into the state of an emulsion; the fine particles of fat then pass through the walls of the villi of the small intestine into the lacteal vessels, which begin in the villi, and so get into the blood unchanged; then part of the fat combines with the oxygen in the blood, and because there is a large quantity of hydrogen and only a small quantity of oxygen in the fat a very large amount of combination can take place, for almost all the carbon and hydrogen can combine with free oxygen in the blood, so that a large amount of oxidisation occurs, and a considerable quantity of heat is produced. Fats are among the most important foods that we have for producing heat; they partly do this, and partly are deposited in the various tissues.

Another important function that fats perform is that of aiding the digestion of other substances. An animal cannot live upon a diet that contains no fat, because he cannot properly digest or absorb the other substances contained in his food.

"What becomes of the sugars, starches, and gums? They are converted in the mouth by the saliva into a form of sugar which goes by the name of grape sugar. When the food gets down into the stomach no further change occurs, but in the small intestine, if all these substances have not been changed in the mouth, the pancreatic juice changes the remainder into sugar, which is absorbed into the blood-vessels of the villi, and passes through the portal vein into the liver. It has been shown that the blood in the portal vein contains sugar, but the blood in the vein that leaves the liver does not contain sugar, and the blood generally in the body contains no sugar, or only a very small quantity: when, either from disease or other cause, the blood does contain sugar, especially if in large quantity, it is got rid of by the kidneys as sugar. What, then, becomes of the sugar that goes into the liver? It has been found by a large series of experiments that the liver is capable of preparing, out of the substances that come to it in the blood, a substance that goes by the name of liver starch, some of which it stores up, and the liver in consequence always increases in weight after the consumption of a diet containing much starch. This starch is a substance capable of very readily being turned back again into sugar, and that is why it goes also by the name of glycogen, which means sweet producer. But it is found that ordinarily there is very little or no sugar in the blood that leaves the liver; what, then, becomes of the liver starch stored up in it? The liver has at least one other property - that of manufacturing fat; but the precise way in which this is done is not understood: it is probably from the liver starch, though other substances may have something to do with the process; so that the liver may be regarded as an important apparatus for manufacturing fat and turning it into the blood, and so we see why people who take large quantities of food containing starch frequently become stout, and how it is that animals, especially poultry, are fattened upon foods containing a large percentage of starch and very little fat. In this way these food substances, starches and sugars, are turned first into grape sugar, and then by the liver they are changed into fat, which is disposed of in the ways already described. We can perceive, then, that non-nitrogenous foods have especially the important function of combining with oxygen in the blood and producing animal heat, and for this reason they have been called calorific foods.

No mixture of non-nitrogenous substances is capable of keeping up the life of an animal, because loss of nitrogenous matter is occurring continually, and so it is necessary that foods containing nitrogen must be taken. When any tissue is used it wastes, and that waste must be replaced.

Then with regard to nitrogenous foods: the first class of these includes albumen, which we find pure in the white of an egg, also in meat; fibrin and syntonin, which we also find in flesh: it also includes casein, found in milk; legumen, found in beans and peas; and several others. These substances are known by the name of protein compounds. They are not attacked by the saliva in the mouth, but they are attacked by the gastric juice in the stomach, and are converted into substances that go by the name of peptones, and are absorbed from the chyle into the blood through the walls of the capillaries of the villi, and also into the lacteal vessels of the villi.