Every part of the body contains nitrogen, also carbon and the elements of water, but the latter in no case in proportion to form water. The blood contains 17 per cent. of nitrogen, and no organ in the body less than that amount. Experiments show conclusively that the body is utterly incapable of producing an elementary body, such as carbon or nitrogen, out of substances which do not contain them. No nitrogen is absorbed from the air, and as it enters into the composition of every part of the body, it is clear that it must exist in all kinds of food fit for the production of any portion of the body.

There is another substance, identical in composition with albumen and fibrin, and that is caseine. It is identical also with vegetable caseine, so that certain plants are capable of yielding the same substance that is formed from the blood of the mother. The young animal therefore receives caseine, which is extremely soluble, the chief constituents of its mother's blood.

We have said that all the organs in the body contain a certain amount of nitrogen, and that this nitrogen must be taken into the body by means of food. These azotized forms of nutriment, may, in the vegetable world be divided into three parts. When the juices of vegetables are allowed to stand, we find in a few minutes a gelatinous precipitate, which we readily recognize as one of the azotized substances serving as nutriment to animals, viz. vegetable fibrin. Heat the remaining fluid to a boiling point, and it coagulates, resembling the white of an egg, or the serum of blood, when diluted with water and heated to the boiling point. This is vegetable albumen, or the second nitrogenized substance. From these three azotized substances, vegetable fibrin, albumen, and caseine, are the principal azotized constituents of our food. These three substances, as has already been stated, contain the same organic elements, and are identical in composition with the chief constituents of blood. We have seen that nitrogen exists in every organ of the body, and that those substances necessary for nourishment are generally rich in nitrogen, and yet there are substances, such as sugar, starch, gum, pectine, etc, containing no nitrogen, a certain proportion of which is absolutely essential to our existence, and without which we could not survive.

Milk contains one nitrogenized substance, caseine;.

besides this, its chief ingredients are butter, or fat, and sugar of milk which contain no nitrogen. From the caseine the young animal of course obtains its blood, muscle, cellular tissue, bone and nerve. "Why is it then that the other ingredients, the butter and sugar of milk are absolutely essential to life?

By means of these compounds there is added to the nitrogenized constituents of blood an excess of carbon and hydrogen, which are expended in the production of animal heat. The food taken into the body goes in the first place to restore the waste of matter in the various organs, which, in the exercise of their functions are constantly undergoing a change, old particles being consumed and their places supplied by new matter constantly conveyed to all parts of the system by those never-idle channels, the arteries and their ramifications. What then becomes of the new compounds produced by the transformation of the different parts of the body, whose places have been taken by the new material? The never-tiring, never-failing current of the blood constantly performs its office, and when not interfered with performs it well. The arterial blood flowing outward, carrying the principles of life and reproduction, as the transformation in tissues takes place crowds before it the new compounds into the channels prepared for them, the veins, by which in the form of venous blood, it is transmitted in part, to the heart, and from thence passing into the lungs, there subjected to the action of the atmosphere, throwing off its impurities, its character of dark venous blood is changed on the reception of oxygen to arterial, and is again distributed, divested of its impurities, and freighted with life passes to every part of the system. The venous blood, before reaching the heart, is made to pass through the liver, where there is taken from it those substances that are incapable of nutrition. From the arterial blood, the kidneys take up those new compounds 22 containing the nitrogen of the transformed tissues, which, being no longer of any use, are expelled in the form of urine. Those on the contrary, which contain the carbon of the transformed tissues, are taken up by the liver, and pass into the gall-bladder in the form of a compound of soda, the bile. This is transmitted through the' gall-duct into the duodenum, and there' mingling with the chyme is taken up in part by the absorbents and returned into the system. By means of the blood a current of oxygen passes into every part of the body. This oxygen in passing through the capillary vessels, meets with the compound produced by the transformation of tissues, combining with the carbon to form carbonic acid, which passes off through the lungs, and with the hydrogen to form water, which passes off in the form of perspiration. What escapes this process of oxydation is sent back into the circulation in the form of bile, which by degrees completely disappears.

In the vegetable world, the carbon which we find deposited in seeds and fruits, in the form of oil and fat, was previously a constituent of the atmosphere and was absorbed by the plant as carbonic acid. By the influence of light and the action of the vital force of the vegetable it was converted into fat.

If we turn to the animal world, and examine their food, we are forced to the conclusion, that the fat found in the body, is formed in its organism. In the food of the cow, we find no butter, no suet in the fodder of cattle, and no lard in the food of swine. But we find on separating a certain amount of oxygen, which they contain, from the starch, gum, and sugar, that enter the composition of one class of food, fat is produced. On separating a certain amount of oxygen from fibrin, albumen, and caseine, contained in another class of food, we also obtain fat. In fatty bodies we have on an average for 120 equivalents carbon, 10 of oxygen. In vegetable fibrin albumen, and caseine for 120 equivalents of carbon there are 36 of oxygen. Therefore we have only to separate 26 equivalents of oxygen from these elements to produce fat. In gum, sugar, and starch, we find the amount of oxygen varies from 100 to 140 equivalents to 120 of carbon, so that to form fat from these substances we have only to separate from 90 to 110 equivalents of oxygen. This oxygen is not given out in a free state, but in combination with other substances, in the form of perspiration, urine and carbonic acid, the result of which combination is of course animal heat. The formation of fat then is occasioned by a deficient supply of oxygen taken into the system to equalize the amount of carbon. For the body to acquire no increase in size it is necessary that the same amount of carbon shall be consumed or given out that is taken into the system. Where a large quantity of carbonized food is taken into the body, and the amount of oxygen is not increased by exercise or Tabor, of course an accumulation of fat is the result. We have an illustration of this in the Bedouin whose active life would prevent the accumulation of much fat, and in the Turkish and Chinese Ladies, where obesity is considered a requisite for beauty, and their indolence and gluttony are favorable to that condition. Whoever heard of a fat Bedouin? The carbon contained in the fat thus deposited is capable of being consumed, and entering other combinations for the purpose of supplying animal heat.