From observations of tbis kind on the human subject, we may infer that, in Man, the albumen of the adult organism under goes a loss of about 2 ounces daily ;—a quan tity which correseonds to scarcely more than T,',5-6-th of the weight of the body. While if we suppose that a new-born infant, weighing six or seven pounds, consumes daily about ten or twelve ounces of milk, containing 31 per cent. of casein, the quantity of protein thus introduced into its alimentary canal would amount to the larger proportion of about eruth of its total bodily mass.
The larger proportion of albumen thus consumed by the infant probably depends upon at least two causes. As a smaller* animal, it is subject to a more energetic waste of substance. And as a growing animal, it not only lays aside in its body a constant surplus of its income over its waste; but possibly undergoes a more active metamor phosis, that still further increases the propor tion of its effete materials.
But, apart from the influence of age or size, there is no donbt that a careful compari son of the azotized ingesta and egesta would always show a marked disproportion between the two. There are indeed obvious reasons, why the nitrogenous constituent of any suitable food should always greatly exceed that quantity which is required by the strict exigencies of the organism. A part of the casein which is contained in the milk taken by the sucking-child, is often found to pass through the alimentary canal without being absorbed into the blood. And in the case of many other varieties of food, the insoluble state of the protein-compounds actually• present affords a still greater obstacle to their absorption. In a proper mixed diet, how ever, we may detect some approximation be tween the presumable gain and loss. Thus the daily rations of the British soldier on home service include little more than 5 ounces of albuminous substance ; —a quantity which is therefore little more than double the amount of this material, which the waste of his body probably dismisses from his system within the same period of time.
2. The next group of alimentary substances is that of the fats, the composition of which has led to their receiving the generic name of hydro-carbons. They are found in both animal and vegetable food. In the milk, they are represented by its butter ; the quantity of which amounts, on an average, to about 31 per cent.
The great variety of different alimentary substances of this kind is such as to pre clude even their enumeration. The most
important are stearin, elain, and margarin. The composition of these three fats may be generally stated as almost corresponding to single equivalents of carbon and hydrogen :— or, more exactly, to ten atoms of each of these elements, minus one of hydrogen, and plus one of oxygen (C„, 112 0, ; or C,„ . 4 +09 = 100).
The uses sustained by these constitu ents of the food in the organism are easy to indicate, but difficult to specify. The protection afforded by the fat of the body to its temperature, and to the mechanical safety of its internal structures, might perhaps be }ac complished without involving any rapid waste and replacement of the material by which it is afforded. But the vast quantity of fatty matter which enters into the composition of the nervous system, and the primary import ance of this delicate and energetic organ to the maintenance of life, entitle us to infer, that its functions imply such a rapid metamorphosis of its substance, as can only be sustained by the continual supply of new materials to replace those rendered effete. And the numerical phenomena of nutrition further show, that the process of respiration is constantly dis missing from the body an amount of carbonic acid, the proportion of which to the azotized egesta proves that it must have been derived more or less directly from an oxidation of the fatty, as well as of the albuminous, tissues. The quantity of fatty matter contained in the healthy organism strongly confirms these views ; and thus helps to account for its dietetic importance. For, including all their varieties in thc tissues just alluded to, we can hardly estimate the total hydro-car bons of the human body at less than *th or ith of its weight. And since they scarcely forni -8-6-5th part of the blood, it follows, that even assuming this nutrient fluid equal to ith of the corporeal weight, its fatty con stituent amounts to little more than or ,-(1.-,,th of the fat which is deposited in the central and peripheric structures of the nervous system, and stored up in the adipose cells of other parts of the body. Such an estimate further entitles us to conjecture, not only that the quantity of fat taken up at any one tiime by the digestive organs is limited to a very small one ; but also, that it either undergoes some important metamorphosis before reach ing the general mass of the blood, or is very rapidly eliminated from this fluid.