When the different juices have all done their work, the _chyme_, which is food as it passes from the stomach into the duodenum or passage to the lower stomach or bowels, becomes a milky substance called _chyle_, which moves slowly, pushed by numberless muscles along the bowel, which squeeze much of it into little glands at the back of the bowels. These are called the mesenteric glands; and, as each one receives its portion of chyle, a wonderful thing happens. About half of it is changed into small round bodies called corpuscles, and they float with the rest of the milky fluid through delicate pipes which take it to a sort of bag just in front of the spine. To this bag is fastened another pipe or tube--the thoracic duct--which follows the line of the spine; and up this tube the small bodies travel till they come to the neck and a spot where two veins meet. A door in one opens, and the transformation is complete. The small bodies are raw food no more, but blood, traveling fast to where it may be purified, and begin its endless round in the best condition. For, as you know, venous blood is still impure and dirty blood. Before it can be really alive it must pass through the veins to the right side of the heart, flow through into the upper chamber, then through another door or valve into the lower, where it is pumped out into the lungs. If these lungs are, as they should be, full of pure air, each corpuscle is so charged with oxygen, that the last speck of impurity is burned up, and it goes dancing and bounding on its way. That is what health means: perfect food made into perfect blood, and giving that sense of strength and exhilaration that we none of us know half as much about as we should. We get it sometimes on mountain-tops in clear autumn days when the air is like wine; but God meant it to be our daily portion, and this very despised knowledge of cookery is to bring it about. If a lung is imperfect, supplied only with foul air as among the very poor, or diseased as in consumption, food does not nourish, and you now know why. We have found that the purest air and the purest water contain the largest proportion of oxygen; and it is this that vitalizes both food and, through food, the blood. To nourish this body, then, demands many elements; and to study these has been the joint work of chemists and physiologists, till at last every constituent of the body is known and classified. Many as these constituents are, they are all resolved into the simple elements, oxygen, hydrogen, nitrogen, and carbon, while a little sulphur, a little phosphorus, lime, chlorine, sodium, &c., are added. FLESH and BLOOD are composed of water, fat, fibrine, albumen, gelatine, and the compounds of lime, phosphorus, soda, potash, magnesia, iron, &c. BONE contains cartilage, gelatine, fat, and the salts of lime, magnesia, soda, &c., in combination with phosphoric and other acids.