Chapter VIII: Section V

RELATION BETWEEN THE QUANTITY OF FOOD CONSUMED BY AN ANIMAL, AND THE INCREASE IN ITS WEIGHT, OR OF THE AMOUNT OF ITS WORK.

The manifestations of that wondrous and mysterious principle, _life_, are completely dependent upon the decomposition of organised matter. Not an effort of the mind, not a motion of the body, can be accomplished without involving the destruction of a portion of the tissues. In a general sense we may regard the fat of the animal to be its store of fuel, and its lean flesh to be the source of its motive power. As the evolution of heat within the body is proportionate to the quantity of fat consumed, so also is the amount of force developed in the animal mechanism in a direct ratio to the proportion of flesh decomposed. The quantity of fat burned in the body is estimated by the amount of carbonic acid gas expired from the lungs and perspired through the skin; the proportion of flesh disorganised is ascertained by the quantity of urea eliminated in the liquid egesta. The amount of urea excreted daily by a man is influenced by the activity of his mind, as well as by that of his body. A man engaged in physical labor wears out more of his body than one who does no work; and a man occupied in a pursuit involving intense mental application, consumes a greater proportion of his tissue than the man who works only with his body.[13] In each of these cases, there is a different amount of tissue disorganised, and consequently a demand for different amounts of food, with which to repair the waste. But all the food consumed by a man is not devoted to the reparation of the tissue worn out in the operations of thinking and working. A human being whose mind is a perfect blank, and who performs no bodily work, excretes a large quantity of urea, the representative of an equivalent amount of worn-out flesh. In fact the greater part of the food consumed by a man serves merely to sustain the functions of the body--the circulation of the blood--the action of the heart--the movements of the muscles concerned in respiration--in a word, the various motions of the body which are independent of the will. According to Professor Haughton, about three-fourths of the food of a working man of 150 lbs. weight, are used in merely keeping him _alive_, the remaining fourth is expended in the production of mechanical force, constituting his daily toil.

In the nutrition of the lower animals, as in that of man, the amount of food made use of by a particular individual depends upon its age, its weight, the amount of work it performs, and probably its temper. As three-fourths of the weight of the food of a laboring man are expended in merely keeping him alive, it is obvious that the withholding of the remaining fourth would render him incapable of working. An amount of food which adequately maintains the vital and mechanical powers of three men, serves merely to keep four alive. It is the same with the horse, the ox, and every other animal useful to man: each makes use of a certain amount of food, _for its own purposes_; all that is consumed beyond that is applied for the benefit of its owner. Let us take the case of two of our most useful quadrupeds--the horse and the ox. The horse is used as an immediate source of motive power. For this purpose food is supplied to it, the greater portion of which is consumed in keeping the animal alive, and the rest for the development of its motive power. Abundance of food is as necessary to the natural mechanism, the horse, as fuel is to the artificial mechanism, the steam-engine. In each case the amount of force developed is, within certain limits, proportionate to the quantity of vegetable or altered vegetable matter consumed. The greater portion of the ox's food is also consumed in keeping its body alive, and the rest, instead of being expended in the development of motive power, accumulates as surplus stores of flesh, which in due time are applied to the purpose of repairing the organisms of men. It is evident then, that the greater sufferer from the deficient supply of food to animals is their owner. That they cannot be _taught_ to _fast_ is a fact which does not appear very patent to some minds. The man who sought by gradually reducing the daily quantum of his horse's provender to accustom it to work without eating, was justly punished for his ignorant cruelty. The day before the horse's allowance was to be reduced to pure water, and when its owner's hope appeared certain of speedy realisation, the animal died. There are men who act almost as foolishly as the parsimonious horse owner in this fable did; and who are as properly punished as he was. Such men are to be found in the farmers who overstock their sheep pastures, and whose "lean kine" are the _laughing stock_ of their more intelligent neighbours.

The weight of a working full-grown horse does not vary from day to day, as the weight of its egesta is equal to that of its food. The desideratum in the case of the working animal is that its food should be as thoroughly decomposed as possible, and the force pent up in it liberated within the animal's body: as an ox, on the contrary, increases in weight from day to day, it is desirable that as little as possible of its food should be disorganised. The wasteful expenditure of the animal's fat may be obviated by shelter, and the application of artificial heat: the retardation of the destruction of its flesh is even more under our control; for, as active muscular exertion involves the decomposition of tissue, we have merely to diminish the activity of the motions which cause this waste. This, in practice, is effected by stall-feeding. Confined within the narrow boundaries of the stall, the muscular action of the animal is reduced to a minimum, or limited to those uncontrollable actions which are conditions in the maintenance of animal life.

The proportion of the food of oxen, sheep, and pigs, which is consumed in maintaining their vital functions, has not been accurately ascertained; probably, as in the case of man, it is strictly proportionate to the animal's weight. We can determine the amount of plastic food consumed by an animal during a given period: we can ascertain the increase (if any) in the weight of its body; and finally, we can weigh and analyse its egesta. With these data it is comparatively easy to ascertain the quantity of food which produced the increase in the animal's weight; but they do not enable us to determine the amount expended in keeping it alive, because the egesta might be largely made up of unappropriated food--organised matter which had done no work in the animal body. When we come to know the precise quantity of nitrogen, in a purely, or nearly pure, mineral form[14] excreted by an animal, then we shall be in a position to estimate the proportion of its food expended in sustaining the essential vital processes which continuously go on in its body. But although we are in ignorance as to the precise quantity of flesh-formers expended in keeping the animal alive, we know pretty accurately the amount which is consumed in producing a given weight of its flesh, or rather in causing a certain increase in its weight. This knowledge is the result of numerous investigations, of which by far the most valuable are those of Lawes and Gilbert. These experimenters found that fattening pigs stored up about 7-1/2 per cent. of the plastic materials of their food, whilst sheep accumulated somewhat less than 5 per cent. That is, 92-1/2 out of every 100 lbs. weight of the nitrogenous food of the pig, and 95 out of every 100 lbs. of that of the sheep, are eliminated in the excretions of those animals.

It appears from the results of Lawes and Gilbert's experiments, that pigs store up in their _increase_ about 20 per cent., sheep 12 per cent., and oxen 8 per cent. of their (dry) food. The relative increase of the fatty, nitrogenous, and mineral constituents whilst fattening, are shown in this table.

---------------------+-------------------------------------------------
|Estimated per cent. in Increase whilst Fattening.
CASES. +--------+-----------+-----------+----------------
|Mineral |Nitrogenous| |
| matter |matter | Fat (dry).| Total dry
|(ash). |(dry). | | substance.
---------------------+--------+-----------+-----------+----------------
Average of 98 oxen | 1.47 | 7.69 | 66.2 | 75.4
Average of 348 sheep | 1.80 | 7.13 | 70.4 | 79.53
Average of 80 pigs | 0.44 | 6.44 | 71.5 | 78.40
---------------------+--------+-----------+-----------+----------------

The quantity of food consumed daily by an animal is, as might be expected, proportionate to the weight of its body. The pig consumes, for every 100 lbs. of its weight, from 26 to 30 lbs. of food, the sheep 15 lbs., and the ox 12 to 13 lbs. These figures and the statements which I have made relative to the proportions of fat and plastic elements in the animals' bodies, apply to them in their fattening state, and when the food is of a highly nutritious character. The calf and the young pig will make use--to cause their increase--of a larger portion of nitrogenous matters. The sheep, however, being early brought to maturity, will, even when very young, store up the plastic and non-plastic constituents of its food, in nearly the same relative proportions that I have mentioned.

As it is the food taken into the body that produces heat and motion, it might at first sight appear an easy matter to determine the amount of heat or of motion which a given weight of a particular kind of food is capable of producing within the animal mechanism. But this performance is not so easy a task as it appears to be. In the first place, all of the food may not be perfectly oxidised, though thoroughly disorganised within the body; secondly, as animals rarely subsist on one kind of food, it is difficult, when they are supplied with mixed aliments, to determine which of them is the most perfectly decomposed. But though the difficulties which I have mentioned, and many others, render the task of determining the nutritive values of food substances difficult, the problem is by no means insoluble, and, in fact, is in a fair way of being solved. Professor Frankland, in a paper published in the number of the _Philosophical Magazine_ for September, 1866, determines the relative alimental value of foods by ascertaining the quantity of heat evolved by each when burned in oxygen gas. From the results of these researches he has constructed a table, showing the amount of food necessary to keep a man alive for twenty-four hours. The following figures, which I select from this table, are of interest to the stock-feeder:--

Weight necessary to sustain a
man's life for twenty-four hours.

Kinds of Food. Ounces.

Potatoes 13.4
Apples 20.7
Oatmeal 3.4
Flour 3.5
Pea Meal 3.5
Bread 6.4
Milk 21.2
Carrots 25.6
Cabbage 31.8
Butter 1.8
Lump Sugar 3.9

These figures show the relative calefacient, or heat-producing powers of the different foods named _outside_ the body; but there is some doubt as to their having the same relative values when burned _within_ the body. The woody fibre of the carrots and cabbages is very combustible in the coal furnace, but it is very doubtful if more than 20 or 30 per cent. of this substance is ever burned in the _animal furnace_. However, such inquiries as those carried out by Frankland possess great value; and tables constructed upon their results cannot fail to be useful in the drawing up of dietary scales, whether for man or for the inferior animals.

I may here remark, that in my opinion the nutritive value of food admits of being very accurately determined by the adoption of the following method:--

1. The animal experimented upon to be supplied daily with a weighed quantity of food, the composition and calefacient value of which had been accurately determined. 2. The gases, vapors, and liquid and solid egesta thrown off from its body to be collected, analysed, and the calefacient[15] value of the combustible portion of them to be determined. 3. The increase (if any) of the weight of the animal to be ascertained. 4. The difference between the amount of heat evolvable by the foods before being consumed, and that actually obtained by the combustion of the egesta into which they were ultimately converted, would be the amount actually set free and rendered available within the body. The calculations would be somewhat affected by an increase in the weight of the animal's body; but it would not be difficult to keep the weight stationary, or nearly so, and there are other ways of getting over such a difficulty. An experiment such as this would be a costly one, and could not be properly conducted unless by the aid of an apparatus similar to that employed by Pettenkofer in his experiments on respiration. This apparatus, which was made at the expense of the King of Bavaria, cost nearly L600.

_Value of Manure._--It is a complication in the question of the economic feeding of the farm animals that the value of their manure must be taken into account. Of the three classes of food constituents, two--the mineral and nitrogenous--are recoverable in the animal's body and manure; the non-nitrogenous is partly recoverable in the fat. I shall take the case of a sheep, which will consume weekly per 100 lbs. of its weight, 12 lbs. of fat-formers, and 3 lbs. of flesh-formers. Twelve per cent. of the fat-formers will be retained in the _increase_, but the rest will be expended in keeping the animal warm, and the products of its combustion--carbonic acid and water--will be useless to the farmer. It is, therefore, desirable to diminish as much as possible the combustion of fatty matter in the animal's body; and this is effected, as I have already explained, by keeping it in a warm place. Of the flesh-forming substance only five per cent. is retained in the increase, the rest is partly consumed in carrying on the movements of the animal--partly expelled from its body unaltered, or but slightly altered, in composition. The solid excrement of the animal contains all the undigested food; but of this only the mineral and nitrogenous constituents are valuable as manure. The nitrogen of the plastic materials which are expended in maintaining the functions of the body is eliminated from the lungs, through the skin, and by the kidneys--perhaps also, but certainly only to a small extent, by the rectum.

The food consumed by an animal is disposed of in the following way:--A portion passes unchanged, or but slightly altered, through the body; another part is assimilated and subsequently disorganised and ejected; the rest is converted into the carcass of the animal at the time of its death. The undigested food and aliment which had undergone conversion into flesh and other tissues, and subsequent disorganisation, constitute the excrements, or manure, of the animal. The richer in nitrogen and phosphoric acid the food is, the more valuable will be the manure; so that the money value of a feeding stuff is not determinable merely by the amount of flesh which it makes, but also, and to a great extent, by the value of the manure into which it is ultimately converted.

Corn and oil-cakes are powerful fertilisers of the soil; but the three principles which constitute their manurial value--namely, nitrogen (ammonia), phosphoric acid, and potash--are purchasable at far lower prices in guano and other manures. Nevertheless, many farmers believe that the most economical way to produce good manure is to feed their stock with concentrated aliment, in order to greatly increase the value of their excreta. They consider that a pound's worth of oil-cake, or of corn, will produce at least a pound's worth of meat, and that the manure will be had for nothing, or, rather, will be the profit of the business. The richer food is in nitrogen and phosphoric acid, the more valuable will be the manure it yields. It follows, therefore, that if two kinds of feeding stuff produce equal amounts of meat, that the preference should be given to that which contains the more nitrogen and phosphoric acid. Mr. Lawes, who has thrown light upon this point, as well as upon so many others, has made careful estimates of the value of the manure produced from different foods. They are given in the following table:--

TABLE

Showing the estimated value of the manure obtained on the
consumption of one ton of different articles of food; each
supposed to be of good quality of its kind.

Estimated Money Value
Description of Food. of the Manure from
One Ton of each Food.

1. Decorticated cotton-seed cake L6 10 0
2. Rape-cake 4 18 0
3. Linseed-cake 4 12 0
4. Malt-dust 4 5 0
5. Lentils 3 17 0
6. Linseed 3 13 0
7. Tares 3 13 6
8. Beans 3 13 6
9. Peas 3 2 6
10. Locust beans 1 2 6(?)
11. Oats 1 14 6
12. Wheat 1 13 0
13. Indian corn 1 11 6
14. Malt 1 11 6
15. Barley 1 9 6
16. Clover-hay 2 5 0
17. Meadow-hay 1 10 0
18. Oat-straw 0 13 6
19. Wheat-straw 0 12 6
20. Barley-straw 0 10 6
21. Potatoes 0 7 0
22. Mangolds 0 5 0
23. Swedish turnips 0 4 3
24. Common turnips 0 4 0
25. Carrots 0 4 0

All the saline matter contained in the food is either converted into flesh, or is recoverable in the form of manure, but a portion of its nitrogen appears to be lost by respiration and perspiration. Reiset states that 100 parts of the nitrogen of food given to sheep upon which he experimented, were disposed of as follows:--

Recovered in the excreta 58.3
Recovered in the meat, tallow, and skin 13.7
Lost in respiration 28.0
------
100.00

Haughton's experiments, performed upon men, gave results which proved that no portion of the nitrogen of their food was lost by perspiration or by respiration. Barral, on the contrary, asserts that nitrogen is given off from the bodies of both man and the inferior animals. Boussingault states that horses, sheep, and pigs exhale nitrogen. A cow, giving milk, on which he had experimented, lost 15 per cent. of the nitrogen of its food by perspiration. The amount of nitrogen which Reiset states that sheep exhale is exceedingly great, and it is difficult to reconcile his results with those obtained by Voit, Bischoff, Regnault, Pettenkofer, and Haughton. Of course, men and sheep are widely different animals; but still it is unlikely that all the nitrogen of the food of man should be recoverable in his egesta, whilst nearly a third of the nitrogen of the food of the sheep should be dissipated as gas. I think further experiments are necessary before this point can be regarded as settled; and it is probable that it will yet be found that all, or nearly all, of the nitrogen of the food of animals is recoverable in their egesta.

Regarding, then, an animal as a mechanism by which meat is to be "manufactured," five economic points in relation to it demand the feeder's attention: these are--the first cost of the mechanism, the expense of maintaining the mechanism in working order, the price of the raw materials intended for conversion into meat, the value of the meat, and the value of the manure. In proportion to the attention given to these points, will be the feeder's profits; but they are, to some extent, affected by the climatic, geographic, and other conditions under which the farm is placed.

* * * * *

[Footnote 1: If the elements were only capable of combining with each other in simple ratios, the number of their combinations would be as limited as that of the letters of the alphabet; but as one, two, or more atoms of oxygen can combine with one, two, or more atoms of other elements, we can assign no limits to the number of _possible_ combinations. There are hundreds of distinct substances formed of but two elements, namely, hydrogen and carbon.]

[Footnote 2: In a paper by Professor Sullivan, of Dublin, the conversion of one of these substances into another _outside_ the animal mechanism, is almost incontrovertibly proved.]

[Footnote 3: _Experimental Inquiry into the Composition of some of the Animals Fed and Slaughtered as Human Food._ By John Bennet Lawes, F.R.S., F.C.S., and Joseph Henry Gilbert, Ph.D., F.C.S. _Philosophical Transactions of the Royal Society._ Part II., 1860.]

[Footnote 4: From the Greek _plasso_, "to form." Plastic materials are sometimes termed _formative_ elements; both terms imply the belief that they are capable of giving shape, or form, not only to themselves, but also to other kinds of matter not possessed of formative power.]

[Footnote 5: The slow conversion of phosphorus into phosphoric acid takes place in the animal organism; its gradual oxidation in the open air gives rise only to an imperfectly oxidised body--_phosphorous acid_. But the latter fact does not invalidate the general proposition, that the heat emitted by a substance undergoing the process of oxidation is proportionate to the amount of oxygen with which it combines, and is not influenced by the length of time occupied by the process, further than this, that if the oxidation be _very_ rapidly effected, a portion of the heat will be converted into an _equivalent_ amount of light.]

[Footnote 6: This statement is not absolutely correct, but the range of variation is confined within such narrow limits as to be quite insignificant.]

[Footnote 7: Doubt has recently been thrown on the truth of this belief by Frankland, Fick, and Wislicenus.]

[Footnote 8: The results of Savory's experiments on rats appear to prove that animals can live on food destitute of fat, sugar, starch, or any other fat-forming substance. I think, however, that animals could hardly thrive on purely nitrogenous food. The conclusions which certain late writers, who object to Liebig's theory of animal heat, have deduced from Savory's investigations, appear to me to be quite unfounded.]

[Footnote 9: So termed because it is the basis of the common oils; the fluid portion of fat is composed of oleine.]

[Footnote 10: The term _dry_ is applied to the _solid_ constituents of the food. Thus, a pig fed with 100 lbs. of potatoes would be said to have been supplied with 25 lbs. of dry potatoes, because water forms 75 per cent. of the weight of those tubers.]

[Footnote 11: The amounts of "mineral matter" are too high, owing to the adventitious matters (dirt) retained by the wool.]

[Footnote 12: This pig was completely analysed by Lawes and Gilbert.]

[Footnote 13: The results of recent and accurately conducted investigations prove that men engaged in occupations requiring the highest exercise of the intellectual faculties, require more nutritious food, and even a greater quantity of nutriment, than the hardest worked laborers, such as paviours, and navvies. I have been assured by an extensive manufacturer, that on promoting his workmen to situations of _greater_ responsibility but _less_ physically laborious than those previously filled by them, he found that they required more food and that, too, of a better quality. This change in their appetite was not the result of increased wages, which in most cases remained the same--the decrease in the amount of labour exacted being considered in most cases a sufficient equivalent for the increased responsibility thrown upon them.]

[Footnote 14: As ammonia, urea, uric acid, or hippuric acid; all of which are nearly or perfectly mineralised substances.]

[Footnote 15: The excrements of animals are capable of evolving, by combustion, enormous amounts of heat.]