| For respiration. | For waste of muscle, &c. | ||||
| 1¾ lbs. of bread yielding | 18 oz. starch and | 3 oz. | of gluten. | ||
| 8 oz. of beef yielding | 2 oz. | of fibrin. | |||
| Total consumed by respiration, |  | 18 oz. starch and | 5 oz |  | gluten or |
| and the ordinary waste, | fibrin. | ||||
If, again, the 7½ lbs. of potatoes be eaten, then in these are contained about 2½ ounces of gluten or albumen, so that there remain 2½ ounces to be supplied by beef, eggs, milk, or cheese.
The reader, therefore, will understand why a diet which will keep up the human strength is easiest compounded of a mixture of vegetable and animal food. It is not merely that such a mixture is more agreeable to the palate, or even that it is absolutely necessary,—for, as already observed, the strength may be fully maintained by vegetable food alone;—it is, that without animal food in one form or another, so large a bulk of vegetable food must be consumed in order to supply the requisite quantity of nitrogen in the form of gluten. Of ordinary wheaten bread alone, about 3 lbs. daily must be eaten to supply the nitrogen,[24] and there would then be a considerable waste of carbon in the form of starch, by which the stomach would be overloaded, and which, not being worked up by respiration, would pass off in the excretions. The wants of the body would be equally supplied, and with more ease, by 1¾ lbs. of bread and 4 ounces of cheese.
Of rice, again, no less than 4 lbs. daily would be required to impart to the system the required proportion of gluten; and it is a familiar observation of those who have been in India and other countries, where rice is the usual food of the people, that the degree to which the natives distend, and apparently overload their stomachs with this grain, is quite extraordinary.
The stomachs and other digestive apparatus of our domestic animals are of larger dimensions, and they are able, therefore, to contain with ease as much vegetable food, of almost any wholesome variety, as will supply them with the quantity of nitrogen they may require. Yet every feeder of stock knows that the addition of a small portion of oil-cake, a substance rich in nitrogen, will not only fatten an animal more speedily, but will also save a large bulk of other kinds of food.
3. But the blood and other fluids of the body contain much saline matter of various kinds, sulphates, muriates, phosphates, and other saline compounds of potash, soda, lime, and magnesia. All these have their special functions to perform in the animal economy, and of each of them an undetermined quantity daily escapes from the body in the perspiration, in the urine, or in the solid excretions. This quantity, therefore, must be daily restored by the food.
No precise experiments have yet been made with the view of determining how much saline matter is daily excreted from the body of a healthy man, or in what proportions the different inorganic substances are present in it; but it is satisfactorily ascertained, that without a certain sufficient supply, the animal will languish and decay, even though carbon and nitrogen in the form of starch and gluten be abundantly given to it. It is a wise and beautiful provision of nature, therefore, that plants are so organized as to refuse to grow in a soil from which they cannot readily obtain a supply of soluble inorganic food, since that saline matter which ministers first to their own wants is afterwards surrendered by them to the animals they are destined to feed.
Thus the dead earth and the living animal are but parts of the same system,—links in the same endless chain of natural existences,—the plant is the connecting bond by which they are tied together on the one hand,—the decaying animal matter which returns to the soil, connects them on the other.
4. The solid bones of the animal are supplied from the same original source,—the vegetable food on which they live. The bones of the cow contain 55 per cent. of phosphate of lime, of the sheep 70, of the horse 67, of the calf 54, and of the pig 52 lbs., in every hundred of dry bone. All this must come from the vegetable food. Of the bone-earth also, a portion,—perhaps a variable portion,—is every day rejected from the animal; the food, therefore, must contain a daily supply, or that which passes off will be taken from the substance of the bones, and the animal will become feeble.
It is kindly provided by nature, that a certain proportion of this ingredient of bones is always associated with the gluten of plants in its various forms,—with the fibrin of animal muscle and with the curd of milk. Hence, man, in using any of these latter along with his vegetable food, obtains from them, with comparative ease, the quantity of the earth of bones which is necessary to keep his system in repair; while those animals which live upon vegetables alone, extract all they require along with the gluten of the plants on which they feed.