Section V.—of the Feeding of Animals, and the Purposes Served by the

FOOD THEY CONSUME.

The food of plants we have seen to consist essentially of two kinds, the organic and the inorganic, both of which we have insisted upon as equally necessary to the living vegetable—equally indispensable to its healthy growth. A brief glance at the purposes served by plants in the feeding of animals, will not only confirm this view, but will also throw some additional light upon the kind of inorganic food which the plants must be able to procure, in order that they may be fitted to fulfil their assigned purpose in the economy of nature.

Man, and all domestic animals, may be supported, may even be fattened, upon vegetable food alone: vegetables, therefore, must contain all the substances which are necessary to build up the several parts of animal bodies, and to supply the waste attendant upon the performance of the necessary functions of animal life. Let us consider what these substances are, and in what quantities they must be supplied to the human body.

1. The food must supply carbon for respiration.

A man of sedentary habits, or whose occupation requires little bodily exertion, may respire about 5 ounces of carbon in twenty-four hours—one who takes moderate exercise, about 8 ounces—and one who has to undergo violent bodily exertion, from 12 to 15 ounces.

If we take the mean quantity of 8 ounces, then to supply this alone, a man must eat 18 ounces of starch or sugar every day. If he take it in the form of wheaten bread, he will require 1¾ lbs. of bread, if in the form of potatoes, about 7½ lbs. of raw potatoes, to supply the waste caused by his respiratory organs alone.

When the habits are sedentary, 5 lbs. of potatoes may be sufficient, when violent and continued exercise is taken, 12 to 15 lbs. may be too little. At the same time, it must be observed, that where the supply is less, the quantity of carbonic acid given off will either be less also, or the deficiency will be supplied at the expense of the body itself. In either case the strength will be impaired, and fresh food will be required to recruit the exhausted frame.

2. The food must repair the daily waste of the muscular parts of the body.

When the body is full grown, a portion from every part of it is daily abstracted by natural processes, and rejected either in the perspiration or in the solid and fluid excrements. This portion must be supplied by the food, or the strength will diminish—the frame will gradually waste away.

The muscles of animals, of which lean beef and mutton are examples, are generally coloured by blood, but when well washed with water, they become quite white, and, with the exception of a little fat, are found to consist of a white fibrous substance, to which the name of fibrin has been given by chemists. The clot of the blood consists of the same substance; while skin, hair, horn, and the organic part of the bones, are composed of varieties of gelatine. This latter substance is familiarly known in the form of glue, and though it differs in its sensible properties, it is remarkably analogous to fibrin in its elementary constitution, as both of these substances are to the white of the egg (albumen), to the curd of milk (casein), and to the gluten of flour. They all contain nitrogen, and all consist of the four elementary bodies (organic elements), very nearly in the following proportions:—

Carbon, 55 Hydrogen, 7 Nitrogen, 18 Oxygen, 20 ---- 100

They all contain, likewise, a small proportion of sulphur and of phosphorus.

The quantity of one or other of these removed from the body in 24 hours, either in the perspiration or in the excretions, amounts to about five ounces, containing 350 grains of nitrogen, and this waste at least must be made up by the gluten or fibrin of the food.

In the 1¾ lb. of wheaten bread we have supposed to be eaten to supply carbon for respiration, there will be contained also about 3 ounces of gluten. Let the other 2 ounces be made up in beef, of which half a pound contains 2 ounces of dry fibrin, and we have

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,} } gluten or and the ordinary waste, } 18 oz. starch and 5 oz.} 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, 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.

The flour being supposed to contain 15 per cent. of dry gluten, on which supposition all the above calculations are made.

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.

The provision is very beautiful by which the young animal,—the muscle and bones of which are rapidly growing,—is supplied with a larger portion of nitrogenous food and of bone-earth, than are necessary to maintain the healthy condition of the full grown animal. The milk of the mother is the natural food from which its supplies are drawn. The sugar of the milk supplies the comparatively small quantity of carbon necessary for the respiration of the young animal; as it gets older, the calf or young lamb crops green food for itself to supply an additional portion. The curd of the milk (casein) yields the materials of the growing muscles, and of the animal part of the bones,—while dissolved along with the curd in the liquid milk is the phosphate of lime, of which the earthy part of the bones is to be built up. A glance at the constitution of milk will shew us how copious the supply of all these substances is,—how beautifully the constitution of the mother’s milk is adapted to the wants of her infant offspring. Cow’s milk consists in 1000 parts by weight of—

Butter, 27 to 35 Cheesy matter (casein), 45 to 90 Milk sugar, 36 to 50

Chloride of potassium, and a little chloride of sodium, 1½} Phosphates, chiefly of lime, 2¼} to 10

Other salts, 6 Water, 882¼ to 815 ----- ---- 1000 1000

The quality of the milk, and, consequently, the proportions of the several constituents above mentioned, vary with the breed of the cow,—with the food on which it is supported,—with the time that has elapsed since the period of calving,—with its age, its state of health, and with the warmth of the weather; but in all cases this fluid contains the same substances, though in different quantities.

In warm weather the milk contains more butter, in cold weather more cheese and sugar.

Milk of the quality above analyzed contains, in every ten gallons, 4½ lbs. of casein, equal to the formation of 18 lbs. of ordinary muscle, and 3½ ounces of phosphate of lime (bone-earth), equal to the production of 7 ounces of dry bone. But from the casein have to be formed the skin, the hair, the horn, the hoof, &c. as well as the muscle, and in all these is contained also a minute portion of the bone-earth. A portion of all the ingredients of the milk likewise passes off in the ordinary excretions, and yet every one knows how rapidly young animals thrive, when allowed to consume the whole of the milk which nature has provided as their most suitable nourishment.

And whence does the mother derive all this gluten and bone-earth, by which she can not only repair the natural waste of her own full grown body, but from which she can spare enough also to yield so large a supply of nourishing milk? She must extract them from the vegetables on which she lives, and they again from the soil.

The quantity of solid matter thus yielded by the cow in her milk is really very large, if we look at the produce of an entire year. If the average yield of milk be 3000 quarts, or 750 gallons in a year—every 10 gallons of which contain bone-earth enough to form about 7 ounces of dry bone—then the milking of the cow alone exhausts her of the earthy ingredients of 33 lbs. of dry bone. And this she draws necessarily from the soil!

If this milk be consumed on the spot, then all returns again to the soil in the annual manuring of the land. Let it be carried for sale to a distance, or let it be converted into cheese and butter, and in this form exported, there will then be a yearly drain upon the land of the materials of bones, from this cause alone, equal to 30 lbs. of bone-dust. After the lapse of centuries, it is conceivable that old pasture lands in cheese and dairy countries should become poor in the materials of bones—and that in such districts, as now in Cheshire, the application of bone-dust should entirely alter the character of the grasses, and renovate the old pastures.

Thus, as was stated at the commencement of the present section the study of the nature, and functions of the food of animals throws additional light upon the nature also and final uses of the food of plants. It even teaches us what to look for in the soil—what a fertile soil must contain that it may grow nourishing food—what we must add to the soil when chemical analysis fails to detect its actual presence, or when the food it produces is unable to supply all that the animal requires.

The principles above explained, therefore, shew that the value of any vegetable production, considered as the sole food of an animal, is not to be judged of—cannot, in short, be accurately determined—by the amount it may contain of any one of those substances, all of which together are necessary to build up the growing body of the young animal, and to repair the natural waste of such as have attained to their fullest size.

Hence the failure of the attempts that have been made to support the lives of animals by feeding them upon pure starch or sugar alone. These substances would supply carbon for respiration, but all the natural waste of nitrogen, of saline matter, and of earthy phosphates, must have been drawn from the existing solids and fluids of their living bodies. The animals in consequence pined away, and sooner or later died.

Some have expressed surprise that animals have refused to thrive, and have ultimately died, when fed upon animal jelly or gelatine (from bones) alone, nourishing though that substance as part of the food undoubtedly is. When given in sufficient quantity, gelatine might indeed supply carbon enough for respiration, with a great waste of nitrogen, but it is deficient in the saline ingredients which a naturally nourishing food contains.

Even on the natural mixture of starch and gluten in fine white bread, dogs have been unable to live beyond 50 days, though others fed on household bread, containing a portion of the bran—in which earthy matter more largely resides—continued to thrive long after. It is immaterial whether the general quantity of the whole food be reduced too low, or whether one of its necessary ingredients only be too much diminished or entirely withdrawn. In either case, the effect will be the same—the animal will pine away, and sooner or later die.