ON THE GROWTH AND COMPOSITION OF ANIMALS.
SECTION I.
ANIMAL AND VEGETABLE LIFE.
Functions of Plants.—It is the primary function of plants to convert the inorganic matter of the soil and air into organised structures of a highly complex nature. The food of plants is purely mineral, and consists chiefly of water, carbonic acid, and ammonia. Water is composed of the elements oxygen and hydrogen; carbonic acid is a compound of oxygen and carbon; and ammonia is formed of hydrogen and nitrogen. These four substances are termed the organic elements, because they form by far the larger portion—sometimes the whole—of organic bodies. The combustible portion of plants and animals is composed of the organic elements; the incombustible part is made up of potassium, sodium, and the various other elements enumerated in another page. The organic elements are furnished chiefly by the atmosphere, and the incombustible matters are supplied by the soil.
Water in the state of vapor forms, according to the temperature and other conditions of the atmosphere, from a half per cent. to four and a half per cent. of the weight of that fluid—about 1·25 per cent. being the average; carbonic acid exists in it to the extent of 1⁄2000th; and ammonia forms a minute portion of it—according to Dr. Angus Smith, one grain weight in 412·42 cubic feet of air (of a town), or 0·000453 per cent. It is remarkable that the most abundant constituents of atmospheric air—oxygen and nitrogen—are not assimilable by plants, although these elements enter largely into the composition of vegetable substances. In the soil, also, the part which ministers to the wants of vegetables is relatively quite insignificant in amount.
Plants are unendowed with organs of locomotion, their food must therefore be within easy reach. Every breeze wafts gaseous nutriment to their expanded leaves, and their rootlets ramify throughout the soil in search of appropriate mineral aliment. But no matter how abundant, or however easy of reach may be the food of plants, the vegetable organism is incapable of partaking of it unless under the influence of light. Exposed to this potent stimulus, the plant collects the gaseous carbonic acid and the vaporous water, solidifies them, decomposes them, and combines their elements into new and organised forms. In effecting these changes—in conferring vitality upon the atoms of lifeless matter—the plant acts merely as the mechanism, the light is the force. As the work performed by the steam-engine is proportionate to the amount of force developed by the combustion of the fuel beneath its boiler, so is the rapidity of the elaboration of organic substances by plants proportionate to the amount of sunlight to which they are exposed. It is an axiom that matter is indestructible; we may alter its form as often as we please, but we cannot destroy a particle of it. It is the same with force: we may convert one kind of it into another—heat into light, or magnetism into electricity—but our power ends there; we can only cause force, or motion, to pass from one of its conditions to another, but its quantity can never be diminished by the power of man.
The principle of the Conservation of the Forces gives us a clear explanation of the fact that animals can obtain their food only through the medium of the vegetable kingdom. Plants are stationary mechanisms; they have no need to develop motive power, as animals have, in moving themselves from place to place. Their temperature is, we may say, the same as that of the medium in which they exist. Such beings as plants do not, therefore, require the expenditure of force to maintain their vitality; on the contrary, their mechanisms are, for a beneficent purpose, constructed for the accumulation of force. The growing plant absorbs, together with carbonic acid, water, and ammonia, a proportionate amount of light, heat, and the various other subtile forces which have their abiding place in the sun-beam—
"That golden chain,
Whose strong embrace holds heaven and earth and main." Co-incidentally with the conversion of the mineral constituents of the food of plants into organised structures—albumen, fibre, and such like substances—the light, and the heat, and the various other forces likewise suffer a change. Although the precise nature of the new force into which they are converted is still a mystery—one, too, which may never be revealed to us—still we know sufficient of it to satisfy us that it can only exist in connection with organic or organised structures. It is owing to its presence that the elements of these structures (the natural state of which is mineral) are bound together in what may be aptly designated a constrained state; or, as Liebig aptly expresses it, like the matter in a bent spring. So long as the organic structure retains its form, it will be a reservoir of latent force—which will manifest itself in some form during the recoil of the atoms of the matter forming the structure to their original mineral, or statical condition: so the bent spring, when the pressure is removed, returns to its original straight form.
Animal Life.—The chief manifestation of the life of a plant is the accumulation of force; very different are the functions of animal life. It is only by the continuous expenditure of force that the vitality of animals is preserved; the heat of a man's body, his power of locomotion, the performance of his daily toil, even his very faculty of thought, are all dependent upon, and to a great extent proportionate to, the amount of organised matter disorganised in his body. It is by the conversion of this organised matter into its original mineral state of water, carbonic acid, and ammonia, that the force originally expended in arranging, through the agency of plants, its atoms, is again restored, chiefly in the form of heat and animal motive power.
Animals, as a class, are completely dependent upon vegetables for their existence. There is every reason to believe that the most lowly organised beings in the scale of animal life, even those of so simple a structure as to have been long regarded as vegetables or as plant-animals, are incapable of organising mineral matter. The so-called vegetative life of animals—for I believe the term to be exceedingly inexact—is applied to their growth, that is, to the increase in their weight. This increase takes place by their power of reorganising, or of assimilating to the nature of their own organisms, certain of the substances elaborated by plants, and destined to become food for animals.
SECTION II.
COMPOSITION OF ORGANIC SUBSTANCES.
Elements of Organic Bodies.—The number of distinct kinds of substances—each distinguishable from all the others by the peculiarity of its properties, taken as a whole—is exceedingly great, yet all these substances are resolvable into a very small number of bodies. As an illustration, I shall take a well-known substance, common green copperas, or, as the chemists term it, protosulphate of iron. By submitting this compound to the process termed chemical analysis, two other kinds of matter may be obtained from it, namely, oxide of iron and oil of vitrol, or sulphuric acid. If we continued this process—if we submitted the acid and the oxide to analysis—we could separate the former into sulphur and oxygen, and the latter into iron and oxygen. Now, by these means we could demonstrate the compound nature of copperas; we could prove that it was proximately composed of sulphuric acid and oxide of iron; and, ultimately, of iron, sulphur, and oxygen.
Iron, sulphur, and oxygen, are elementary, or simple bodies. They cannot be decomposed; they cannot be analysed. Torture them as we will in our crucibles; expose them as we please to the highest temperature of a wind furnace, or to the more intense heat evolved by a powerful galvanic battery; subject them to the influence of any agent, or force, or process we may choose, and still they will yield nothing but iron, sulphur, and oxygen: hence these undecomposable bodies are regarded as elements, or simple substances. So far as our knowledge extends, there are about sixty-six of these undecomposable bodies, of which about one half occurs in but exceedingly minute quantities, and a considerable number of the others exists in comparatively small amounts. As by far the greater proportion of compounds is made up of two or more of about a dozen elementary bodies, it would at first sight appear as if the distinct kinds of compounds which exist, or which may be called into existence by the chemist, must be limited to, at most, a realisable number; but the fact is there is no practical limit to the variety of substances which may be artificially formed. Every difference in the mode of the arrangement of the constituent atoms of a compound, causes its metamorphosis into another kind of substance. To prove that the number of these changes is bounded by no narrow limits, I need but refer to the rules of Permutation, which demonstrate that twelve letters of the alphabet may be arranged in no fewer than 479,000,000 different ways.[!--1--][1] The elements are the letters of Nature's alphabet, their compounds are the words of the language of Creation. The combinations of sounds and of signs which express the ideas and sensations of man may be limited to millions; but numberless are the hieroglyphs by which the Divine wisdom and beneficence is inscribed on the pages of the magnificent volume of Nature.
Of the sixty-six elementary bodies, not more than a dozen occur commonly in animal and vegetable substances; these are Oxygen, Hydrogen, Nitrogen, Carbon, Sulphur, Phosphorus, Chlorine, Silicium, Potassium, Sodium, Calcium, Magnesium, and Iron. In addition to these, Iodine, and sometimes Bromine, are found in plants which grow in or near the sea; and the former element has also been detected in some of the lower animals, and in land plants. Manganese, Lithium, Cæsium, Rubidium, and a few others of the simple bodies, occasionally occur in plants and animals, but I believe their presence therein is always accidental.
Proximate Composition of Animal Substances.—The differences between vegetable and animal substances are often more apparent than real. Indeed many of the more important of these substances are almost identical in composition. The albumen which coagulates when the juices of vegetables are boiled, is identical with the albumen of the white of eggs; the fibrine of wheat is in no respect chemically different from the fibrine, or clot, of the blood; and, lastly, the legumine, or vegetable caseine, of peas is almost indistinguishable from the curd of milk, or animal caseine. But not only has chemical research demonstrated the identity of the albumen, fibrine, and caseine of vegetables with three of the more important constituents of animals, it has gone a step further, and proved that they differ from each other in but a few unimportant respects. They are unquestionably convertible into each other[!--2--][2] within the animal organism; and their functions, as elements of nutrition, are almost, if not quite, identical.
Exclusive of the blood, which contains the elements of every part of the body, the animal organism is composed of three distinct classes of substances—namely, nitrogenous, non-nitrogenous, and mineral. All of these constituents, or substances capable of being converted into them, must exist in the food. Certain articles, for example, milk, contains all of them; but in others, for instance, butter, only one of these substances is found. The nitrogenous part of the body embraces the muscles, or lean flesh, the gelatine of the bones, and the skin and its appendages—such as hair and horns; the non-nitrogenous constituents are its fat and oil; and its mineral matter is found chiefly in the bony framework. These constituents are not, however, isolated: the mineral matter, no doubt, accumulates in certain parts, but in small quantities it is found in every portion of the body; and although the fat forms a distinct tissue, the muscles of the leanest animal are never free from a sensible proportion of it.
Albumen, fibrine, and caseine are the principal nitrogenous constituents of food, and as they are employed in the reparation of the nitrogenous tissues of the animal body, they have been termed flesh-formers.
The fat and oil of animals are derived either from vegetable oil and fat, or from some such substance as starch or sugar. The constituents of food which form fat are termed fat-formers, and sometimes heat-givers or respiratory elements, from the notion that their slow combustion in the animal body is the chief cause of its high temperature.
The mineral elements of the body are furnished principally by the varieties of food which contain nitrogen. The whey of milk is rich in them; but they do not exist in pure butter, in starch, or in sugar.
Fat is a much more abundant constituent of the animal body than is generally supposed, That this substance should constitute the greater portion of the weight of an obese pig seems probable enough; but few are aware that even in a lean sheep there is 50 per cent. more fat than lean.
For a very accurate knowledge of the relative proportions of the fatty, nitrogenous, and mineral constituents of the carcasses of animals used as human food, we are indebted to Messrs. Lawes and Gilbert. Before these investigators turned their attention to this subject, it had scarcely attracted the notice of scientific men; but a notion appears to have been current, amongst non-scientific people, at least, that in all, save the fattest animals, the lean flesh greatly preponderated over the fat. That this idea was unsustained by a foundation of fact, has been clearly proved by the results of an investigation[!--3--][3] undertaken a few years ago by Messrs. Lawes and Gilbert—an investigation which I cannot avoid characterising as one of the most laborious and apparently trustworthy on record. The mere statement of the results of this inquiry occupies 187 pages of one of the huge volumes of the Transactions of the Royal Society—a fact which best indicates the immensity of the labour which these gentlemen imposed upon themselves, and which, independently of their other and numerous contributions to scientific agriculture, entitles their names to most honourable mention in the annals of science.
I shall now briefly advert to a few of the more important facts established by Lawes and Gilbert. From a large number of oxen, sheep, and pigs, on which feeding experiments were being conducted, ten individuals were selected. These were, a fat calf, a half-fat ox, a moderately fat ox, a fat lamb, a store sheep, a half-fat old sheep, a fat sheep, a very fat sheep, a store pig, and a fat pig. These animals were killed, and the different organs and parts of their bodies were separately weighed and analysed. The results were, that, with the exception of the calf, all the animals contained, respectively, more fat than lean. The fat ox and the fat lamb contained each three times as much fat as lean flesh, and the proportion of the fatty matters to the nitrogenous constituents of the carcass of the very fat sheep was as 4 to 1. In the pig the fat greatly preponderated over the lean; the store pig containing three times as much, and the fat pig five times as much fat as lean.
That part of the animal which is consumed as food by man, is termed the carcass by the butcher, and contains by far the greater portion of the fat of the animal. The offal, in the language of the butcher, constitutes those parts which are not commonly consumed as human food, at least by the well-to-do classes. In calves, oxen, lambs, and sheep, the offal embraces the skin, the feet, and the head, and all the internal organs, excepting the kidneys and their fatty envelope. The offal of the pig is made up of all the internal organs, excepting the kidneys and kidney fat. It is the relative proportion of fat in the carcasses analysed by Lawes and Gilbert that I have stated; but as the nitrogenous matters occur in greatest quantity in the offal, it is necessary that the relative proportions of the constituents of the body, taken as a whole, should be considered. On an average, then, it will be found that a fat fully-grown animal will contain 49 per cent. of water, 33 per cent. of dry fat, 13 per cent. of dry nitrogenous matter—muscles separated from fat, hide, &c.—and 3 per cent. of mineral matter. In a lean animal the average proportions of the various constituents will be 54 per cent. of water, 25½ per cent. dry fat, 17 per cent. of dry nitrogenous substances, and 3½ per cent. of mineral matter. In the following table these proportions are set forth.
| SUMMARY OF THE COMPOSITION OF THE TEN ANIMALS—SHOWING THEPER-CENTAGES OF MINERAL MATTER, DRY NITROGENOUS COMPOUNDS,FAT, TOTAL DRY SUBSTANCE, AND WATER. | ||||||||||||||||
| 1st. In Fresh Carcass. 2nd. In Fresh Offal (equal Sum of Parts,excluding Contents of Stomachs and Intestines). 3rd. In EntireAnimal (Fasted Live-weight, including therefore the weight ofContents of Stomachs and Intestines). | ||||||||||||||||
| Description of Animal. | Per cent. in Carcass. | Per cent. in Offal. | Per cent. in Entire Animal. | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A. | B. | C. | D. | E. | A. | B. | C. | D. | E. | A. | B. | C. | D. | F. | E. | |
| Fat calf | 4·48 | 16·6 | 16·6 | 37·7 | 62·3 | 3·41 | 17·1 | 14·6 | 35·1 | 64·9 | 3·80 | 15·2 | 14·8 | 33·8 | 3·17 | 63·8 |
| Half-fat ox | 5·56 | 17·8 | 22·6 | 46·0 | 54·0 | 4·05 | 20·6 | 15·7 | 40·4 | 59·6 | 4·66 | 16·6 | 19·1 | 40·3 | 8·19 | 51·5 |
| Fat ox | 4·56 | 15·0 | 34·8 | 54·4 | 45·6 | 3·40 | 17·5 | 26·3 | 47·2 | 52·8 | 3·92 | 14·5 | 30·1 | 48·5 | 5·98 | 45·5 |
| Fat lamb | 3·63 | 10·9 | 36·9 | 51·4 | 48·6 | 2·45 | 18·9 | 20·1 | 41·5 | 58·5 | 2·94 | 12·3 | 28·5 | 43·7 | 8·54 | 47·8 |
| Store sheep | 4·36 | 14·5 | 23·8 | 42·7 | 57·3 | 2·19 | 18·0 | 16·1 | 36·3 | 63·7 | 3·16 | 14·8 | 18·7 | 36·7 | 6·00 | 57·3 |
| Half-fat old sheep | 4·13 | 14·9 | 31·3 | 50·3 | 49·7 | 2·72 | 17·7 | 18·5 | 38·9 | 61·1 | 3·17 | 14·0 | 23·5 | 40·7 | 9·05 | 50·2 |
| Fat sheep | 3·45 | 11·5 | 45·4 | 60·3 | 39·7 | 2·32 | 16·1 | 26·4 | 44·8 | 55·2 | 2·81 | 12·2 | 35·6 | 50·6 | 6·02 | 43·4 |
| Extra fat sheep | 2·77 | 9·1 | 55·1 | 67·0 | 33·0 | 3·64 | 16·8 | 34·5 | 54·9 | 45·1 | 2·90 | 10·9 | 45·8 | 59·6 | 5·18 | 35·2 |
| Store pig | 2·57 | 14·0 | 28·1 | 44·7 | 55·3 | 3·07 | 14·0 | 15·0 | 32·1 | 67·9 | 2·67 | 13·7 | 23·3 | 39·7 | 5·22 | 55·1 |
| Fat pig | 1·40 | 10·5 | 49·5 | 61·4 | 38·6 | 2·97 | 14·8 | 22·8 | 40·6 | 59·4 | 1·65 | 10·9 | 42·2 | 54·7 | 3·97 | 41·3 |
| Means of all | 3·69 | 13·5 | 34·4 | 51·6 | 48·4 | 3·02 | 17·2 | 21·0 | 41·2 | 58·8 | 3·17 | 13·5 | 28·2 | 44·9 | 6·13 | 49·0 |
| Means of 8 of the half-fat, fat, and very fat animals | 3·75 | 13·3 | 36·5 | 53·6 | 46·4 | 3·12 | 17·4 | 22·4 | 42·9 | 57·1 | 3·23 | 13·3 | 29·9 | 46·4 | 6·26 | 47·3 |
| Means of 6 of the fat, and very fat animals | 3·38 | 12·3 | 39·7 | 55·4 | 44·6 | 3·03 | 16·9 | 24·1 | 44·0 | 56·0 | 3·00 | 12·7 | 32·8 | 48·5 | 5·48 | 46·0 |
| KEY: | A.—Mineral matter. B.—Dry nitrogenous compounds. C.—Fat. | D.—Dry substance. E.—Water. F.—Contents of viscera. | ||||||||||||||
SECTION III.
USE OF FAT IN THE ANIMAL ECONOMY.
As fat forms so large a portion of the body, it is evident that the part it plays in the animal economy must be a most important one. The general opinion which prevails amongst scientific men as to its physiological functions was originated by the celebrated Liebig. According to his theory, the food of animals includes two distinct kinds of substances—plastic[!--4--][4] and non-plastic. The plastic materials are composed of carbon, hydrogen, oxygen, nitrogen, and a little sulphur and phosphorus. Albumen, fibrine, and casein are plastic elements of nutrition; they form the lean flesh, or muscles, the membranes, and cartilages, the gelatine of the bones, the skin, the hair, and, in short, every part of the body which contains nitrogen. The non-plastic elements of nutrition include fat, oil, starch, sugar, gum, and certain constituents of fruits, such as pectine.
All non-plastic substances—and of each kind there are numerous varieties—are capable of conversion, in the animal mechanism, into fat and oil. The non-plastic food substances do not contain nitrogen, hence they are commonly termed non-nitrogenous elements. The oily and fatty matters contain a large proportion of carbon, their next most abundant component is hydrogen, and they contain but little oxygen. Unlike the plastic elements, they are—except the fats of the brain and nervous tissue—altogether destitute of sulphur and phosphorus. The starchy, saccharine, and gummy substances are composed of the same elements as the fatty bodies, but they contain a higher proportion of oxygen. According to Liebig, fat is used in the animal economy as a source of internal heat. We all know that it is a most combustible body, and that during its inflammation the most intense heat is developed. It is less evident, but not less true, that heat is evolved during its slow oxidation, or decay.
The more rapidly a body burns, the greater is the amount of heat evolved by it in a given time; but the total amount of heat developed by a specific weight of the body is the same, whether the combustion takes place rapidly or slowly. An experiment performed with phosphorus illustrates the case perfectly. If we burned two pieces of equal weight, the one in oxygen, the other in atmospheric air, we should find that the former would emit a light five times as brilliant as that evolved by the latter, for the simple reason that its combustion would be five times as rapid. The white, vapor-like matter into which phosphorus is converted by its combustion, is termed phosphoric acid. It is composed of phosphorus and oxygen. In forming an ounce of this compound, by the direct oxidation, or combustion of phosphorus, the amount of force, either as heat, or as heat and light, evolved is precisely the same, whether the time expended in the process be a minute or a month.[!--5--][5] If, in the experiment I have described, we were to substitute two pieces of fat for the fragments of phosphorus, the results would be precisely similar. The fat burned in oxygen gas would emit intense light and heat; but the total amount of these forces evolved would be neither greater nor less than that developed during the slower and therefore less brilliant combustion of the fat in ordinary atmospheric air. Now, as we can demonstrate that an ounce of fat will emit a certain amount of heat, if burned within a minute of time, and that neither a larger nor a smaller amount will be developed if the combustion of the fat extend over a period of five minutes, I think we may fairly assume that the amount of heat evolved by the complete oxidation of a specific quantity of fat is constant under all conditions, except, as I have already explained, at high temperatures, when a portion of the heat is converted into light.
In the animal organism fat is burned. The process of combustion no doubt is a very slow one, but still the total amount of heat evolved is just the same as if the fat were consumed in a furnace. When the fat constituting a candle is burned, what becomes of it? Its elements, carbon and hydrogen (we may disregard its small amount of oxygen) combine with the oxygen of the air, and form carbonic acid gas and water. What becomes of the fat consumed within the animal body? It also is converted into carbonic acid gas and water. It is not difficult to prove these statements to be facts. A candle will not burn in atmospheric air which has been deprived of its oxygen, because there is no substance present with which the elements of the taper can combine, consequently the process of combustion cannot go on. Now, a man may in one respect be compared with this taper. He is partly made up of fat; that fat is consumed by the oxygen of the air, and the heat developed thereby keeps the body warm. In the process of respiration oxygen is introduced into the lungs, and from thence, by means of the blood vessels, is conveyed throughout every part of the body. In some way, at present not thoroughly understood, the elements of the fat combine with the oxygen, and are converted into carbonic acid gas and water, which are exhaled from the lungs and from the surface of the body.
Fat is a constituent of both animals and plants. The animal derives a portion of its fat directly from the vegetable; but it possesses the power of forming this substance from other organic bodies, such, for example, as starch. Plants elaborate fat directly from the minerals—carbonic acid gas, and water.
I have already explained that the growth of plants is, cæteris paribus, directly proportionate to the amount of sunlight to which they are exposed. Not less certainly is the force which constitutes the sun-beam expended in grouping mineral atoms into organic forms, than is the heat which converts water into steam. But in neither case is the force destroyed. When the vaporous steam is condensed into the liquid water, all the heat is restored, and becomes palpable. By the ultimate decomposition of vegetable substances all the force expended on their production is liberated, and, in some form, becomes manifest.
When the fat formed in the mechanisms of plants is decomposed in the animal organism, two results follow:—The atoms of the fat are re-converted to their original mineral, or statical conditions of carbonic acid gas and water; and the force which maintained them in their organic state is set free as heat, and its equivalent, motive power.
One of the most useful instruments which the ingenuity of man has devised, is the Thermometer. It is so familiarly known that I need not describe it. This instrument does not enable us to estimate the actual quantity of heat contained in a substance, but it indicates the proportion of that subtile element which is sensible—that is recognisable by the sense of touch. The dusky Hindu, clad in his single cotton garment, and the Laplander in his suit of fur, are placed under the most opposite conditions in relation to the heat of the sun—the Indian is exposed during the whole year to Sol's most ardent beams, whilst but a scant share of its genial rays goes to warm the body of the Laplander. Now, if we placed the bulb of a thermometer beneath the tongue of a Hindu, we would find the mercury to stand at 98 degrees on Fahrenheit's scale, and if we repeated the experiment on a Laplander, we would obtain an identical result. Numerous experiments of this nature have been made on individuals in most parts of the world, and the results have proved that the temperature of the blood of man is 98 degrees Fahrenheit, whether he be in India or at Nova Zembla, on the steppes of Russia, or the elevated plateaus of America. This invariability[!--6--][6] of the temperature of the bodies of men and of all other warm-blooded animals, appears the more wonderful when it it is considered that the range of the temperature of the medium in which they exist exceeds 200 degrees Fahrenheit. In India, the mercury in the thermometer has been observed to stand at 145 degrees in the direct sunlight, and at 120 degrees in the shade. In high latitudes the temperature is sometimes so low as 100 degrees below zero. A Russian army, in an expedition to China, in 1839, was exposed for several successive days to a temperature of 42 degrees below zero, and suffered severely in consequence.
The facts which I have cited clearly prove that the animal body possesses the power of generating, or, to speak more correctly, liberating heat, either from portions of its own mechanism or from substances placed within that mechanism.
At one time it was the general belief amongst physiologists that one portion of the food consumed by an animal was employed in repairing the waste of its body, and the remaining part was burned as fuel, evolving heat just in the same way as if it had been consumed in a furnace. It was this theory that led to the classification of food into flesh-formers, and heat-givers. It is now doubted if any portion of the food be really burned in this way; and I, for one, think it far more probable that, before its conversion into carbonic acid gas and water (whereby, according to this theory, it develops the heat which keeps the body warm), it first becomes assimilated, that is, becomes an integral part of the animal body—blood, fat, muscle. Perhaps we would be nearer the truth if we were to assume that heat is evolved during the decomposition of both the nitrogenous and fatty constituents of the body.
The constantly recurring contractions of the muscles must alone be a source of much heat. The development of animal motive power is said to be strictly proportionate to the amount of muscular tissue decomposed. As the nitrogen of the latter is almost completely excreted under the form of urea, the quantity of the latter daily eliminated from the body of an animal is a measure of the decomposed muscular tissue, and consequently of the amount of muscular power generated in the animal organism.[!--7--][7] The correspondence between the amount of the motive power of an animal, and the quantity of effete nitrogen excreted from the body, is limited to laboring men and to the lower animals. Strange as it may appear, it is an incontrovertible fact that men whose pursuits require the constant exercise of the intellectual faculties—lawyers, writers, statesmen, students, scientific men, and other brain-workers—excrete more urea than do men engaged in the most physically laborious occupations. An activity of thoughts and ideas involves a corresponding destruction of the tissues, and these require, for their reparation, the consumption of food. Here, then, we have a physical meaning for the common expression—"food for thought."
That the amount of heat developed in the animal organism, is proportionate to the quantity of fatty matters (or of substances capable of forming them) supplied to it in the shape of food, is a proposition which admits of easy demonstration. The natives of warm regions do not require the generation of much heat within their bodies, because the temperature of the medium in which they exist is generally as high as, or higher than, that of their blood. But as they must consume food for the purpose of repairing the waste of their nitrogenous tissues, and as every kind of food contains heat-producing elements, an excess of heat is developed within their bodies, which, if allowed to accumulate, would speedily produce fatal results. The means by which nature removes this superabundant heat are admirably simple, as indeed all its contrivances are. The skin is permeated with millions of pores, and through these openings a large quantity of vapor is given off, and carries with it the surplus heat. The pores are the orifices of minute convoluted tubes which lie beneath the skin, and when straightened measure each about the tenth of an inch, or, according to a writer in the British and Foreign Medico-Chirurgical Review (1859, page 349), the one-fifteenth of an inch in length. According to Erasmus Wilson, the number of these tubes which open into every square inch of the surface of the body is 2,800. The total number of square inches on the surface of an average sized man is 2,500, consequently the surface of his body is drained by not less than twenty-eight miles of tubing, furnished with 7,000,000 openings. The cooling of the body, by the evaporation of water from it, admits of explanation by well-known natural laws. Water, in the state of vapor, occupies a space 1,700 fold greater than it does in its liquid condition. It is heat which causes its vaporous form, but it ceases to be heat when it has accomplished this change in the condition of the liquid; for, suffering itself an alteration, it passes into another form of force—mechanical, or motive power. The heat generated within the body is absorbed by the liquid water, the conversion of the latter into vapor follows, and both the heat and the water, in their altered forms, escape through the pores.
Fatty food necessary in cold climates.—As a grave objection against the chemical theory of heat, it has been urged that rice—the pabulum of hundreds of millions of the inhabitants of tropical regions—contains an exceedingly high proportion of heat-giving substances. I have, however, great doubt as to rice ever forming the exclusive food of those people, without their health being impaired in consequence of the deficiency in that substance of the plastic elements of nutrition. Indeed I believe it is a great mistake to assert that the natives of India live almost exclusively on rice. This article, no doubt, forms a large proportion of their food, but it is supplemented with pulse (the produce of leguminous plants), which is rich in flesh-forming materials, also with dried fish, butter, and various kinds of vegetable and animal food rich in nitrogen. The innutritious nature of rice is clearly shown by its chemical composition, and so large a quantity of it must the Hindu consume in order to repair the waste of his body, that his stomach sometimes acquires prodigious dimensions; hence the term "pot-bellied," so often applied to the Indian ryot. I doubt very much, however, if the stomach of the Hindu, large as it is, could accommodate a quantity of rice, the combustion of which would produce a very excessive development of heat. This substance, when cooked, contains a high proportion of water, the evaporation of which carries off a large amount of the heat generated by the combustion of its respiratory constituents. The amount of motive power developed by the Hindu is small as compared with that which the European is capable of exerting; hence he has less necessity for a highly nitrogenous diet. On the whole, then, I am disposed to think that the food of the natives of tropical climates contains sufficient nitrogenous matters to effectually build up and keep in repair their bodies; it also appears clear to me that the amount of heat developed in their bodies is not excessive, and that it is readily disposed of in converting the water, which enters so largely into their diet, into vapor. The proportion of plastic to non-plastic elements in the diet of the Hindu and of the well-fed European, is probably as follows:—
| Nitrogenous. | Non nitrogenous (calculated as starch.) | ||
|---|---|---|---|
| Hindu | 1 | to | 9 |
| European | 1 | to | 8 |
This statement does not quite correspond with Liebig's, who estimates the proportion of nitrogenous to non-nitrogenous substances in rice as 10 to 123, in beef as ten to seventeen, and in veal as ten to one. The results of Lawes and Gilbert's investigations, already alluded to, have, however, dispelled the illusion that the plastic constituents of flesh exceed its non-plastic. In the potato, which at one time constituted more of the food of the Irish peasantry than rice does that of the Hindu, the proportion of plastic to non-plastic materials is as 10 to 110. The results of some analyses of the food grains consumed in the Presidency of Madras, made by Professor Mayer, of the University of Madras, clearly prove that the food of the inhabitants of that part of India is of a far more highly nitrogenous character than is generally supposed. That the Hindu, who subsists exclusively on rice, exhibits all the symptoms of deficient nutrition, is a fact to which numerous competent observers have testified.
A slight consideration of the facts which I have mentioned leads to the conclusion that the food of the inhabitants of very cold regions is required to produce a large amount of heat. Melons, rice, and other watery vegetable productions, however delicious to the palate of the Hindu, would be rejected with disgust by the Esquimaux, whilst the train oil, blubber, and putrid seal's flesh which the children of the icy North consider highly palatable, would excite the loathing of the East Indian. On this subject I may appositely quote the following remarks by Dr. Kane, the Arctic explorer:—"Our journeys have taught us the wisdom of the Esquimaux appetite, and there are few among us who do not relish a slice of raw blubber, or a chunk of frozen walrus beef. The liver of a walrus (awuktanuk), eaten with little slices of his fat—of a verity it is a delicious morsel. Fire would seem to spoil the curt, pithy expression of vitality which belongs to its uncooked juices. Charles Lamb's roast pig was nothing to awuktanuk. I wonder that raw beef is not eaten at home. Deprived of extraneous fibre, it is neither indigestible nor difficult to masticate. With acids and condiments, it makes a salad which an educated palate cannot help relishing; and as a powerful and condensed heat-making and anti-scorbutic food, it has no rival. I make this last broad assertion after carefully considering its truth. The natives of South Greenland prepare themselves for a long journey, by a course of frozen seal. At Upper Navik they do the same with the narwhal, which is thought more heat-making than the seal; while the bear, to use their own expression, is 'stronger travel than all.' In Smith's Sound, where the use of raw meat seems almost inevitable from the modes of living of the people, walrus holds the first rank. Certainly this pachyderm (Cetacean?) whose finely condensed tissue and delicately permeating fat (oh! call it not blubber) assimilate it to the ox, is beyond all others, and is the best fuel a man can swallow." The gastronomic capabilities of the Esquimaux and of other northern races, and their fondness for fatty food, are exhibited in a sufficiently strong light in the following statements:—
Captain Parry weighed and presented to an Esquimaux lad the following articles:—
| lb. | oz. | |
|---|---|---|
| Frozen seahorse flesh | 4 | 4 |
| Wild seahorse flesh | 4 | 4 |
| Bread and bread dust | 1 | 12 |
| Rich gravy soup | 1 | 4 |
| Water | 10 | 0 |
| Strong grog | 1 tumbler. | |
| Raw spirits | 3 wine glasses. | |
This large quantity of food, which the lad did not consider excessive, was consumed by him within twenty-four hours. According to Captain Cochrane a reindeer suffices but for one repast for three Yakutis, and five of them will devour at a sitting a calf weighing 200 lbs. Mr. Hooper, one of the officers of the Plover, in his narrative of their residence on the shores of Arctic America, states that "one of the ladies who visited them was presented, as a jest, with a small tallow candle, called a purser's dip. It was, notwithstanding, a very pleasant joke to the damsel, who deliberately munched it up with evident relish, and finally drew the wick between her set teeth to clean off any remaining morsels of fat."
The partiality for certain kinds of food, and disgust at other varieties, which particular races of men exhibit, is an instinct which they cannot avoid obeying. Instead of exciting our disgust, as it too frequently does, it should exalt our admiration of the infinite wisdom of the Creator, who by simply adapting man's desire for particular kinds of food to the external conditions under which he is placed, enables him to occupy and "subdue the earth" from the Equator to the Poles.
The food of human beings and of the lower animals who inhabit cold countries is nearly exclusively composed of animal substances. The flesh, fat, and oil of animals occupy less space than do the corresponding elements of vegetables; consequently the nutriment they afford is more concentrated, and a larger quantity can be stowed away without inconvenience in the stomach. The heat-forming constituents of these substances constitute not only the chief part of their bulk, but they are also capable of evolving a greater amount of heat than any other of the respiratory elements. One pound of dry fat will develop as much heat as two and a half pounds of dry starch, and the fattest flesh includes four times as much plastic materials as rice. The diet of people all over the world, unless under circumstances which prevent the gratification of the natural appetite, establishes the intimate relation which subsists between cold and food. The appetite of man is at a minimum at the Equator, and at a maximum within the Arctic circle. The statements as to the voracity of Hottentots and Bosjesmans, recorded in the narratives of travellers, do not in the slightest degree affect the general rule that more is eaten in cold climates than in hot regions. These are mere records of gluttony, and it would not be difficult to find parallel cases in our own country. Gluttony is an abnormal appetite, and the greater part of the food devoured under its unnatural, and generally unhealthy stimulus is not applied to the wants of the body.
The bodies of animals are heated masses of matter, and are subject to the ordinary laws of radiation. Every substance radiates its heat, and receives in return a portion of that emitted from surrounding bodies. If two bodies of unequal temperature be placed near each other, the warmer of the two will radiate a portion of its heat to the colder, and will receive some of the heat of the latter in return; but as the warmer body will emit more heat than it will receive, the result will be, that after a time, the length of which will depend on the nature of the bodies, both will acquire the same temperature. In very warm climates the bodies of animals derive from the sun, and from the heated bodies surrounding them, more heat than they give in return; and were it not for their internal cooling apparatus, which I have described, the heat so absorbed would prove fatal. In every climate, on the contrary, where the temperature is lower than 98°, or "blood heat," the bodies of animals lose more heat by radiation than they receive by the same means. The philosophy of the clothing of men and the sheltering of the lower animals is now evident. It is not only necessary that heat should be developed within the body, but also that its wasteful expenditure should be prevented. The latter is effected by interposing between the warm body and the cold air some substances (such as fur or wool) which do not readily permit the transmission of heat—non-conductors as they are termed. The close down of the eider duck is destined to protect its bosom from the chilling influence of the icy waters of the North Polar Sea, and the quadrupeds of the dreary Arctic Circle are sheltered by thick fur coverings from the piercing blasts of its long winter.
Fat Equivalents.—Whilst it is quite certain that neither nerves nor muscles can be elaborated exclusively out of fat, starch, sugar, or any other non-nitrogenous substance, it is almost equally clear that fat may be formed out of nitrogenous tissue. The quantity of fat, however, which is produced in the animal mechanism, from purely nitrogenous food appears to be relatively very small. No animal is capable of subsisting solely on muscle-forming materials, no matter how abundantly supplied. The food of the Carnivora contains a large proportion of fat, and the nutriment of the Herbivora is largely made up of starch and other fat-formers. Dogs, geese, and other animals fed exclusively upon albumen or white of egg rapidly decreased in weight, and after presenting all the symptoms of starvation, died in three or four weeks.[!--8--][8] The fat of the bodies of the Carnivora is almost entirely formed—and probably with little if any alteration—from the fatty constituents of their food. Herbivorous animals, on the contrary, derive nearly all their fat from starch, sugar, gum, cellulose, and other non-nitrogenous, but not fatty, materials.
Although starch is convertible into fat, it is not to be understood that a pound weight of one of these bodies is equivalent to an equal quantity of the other. During the conversion of starch into fat, the greater number of its constituent atoms is converted into water and carbonic acid gas. The greater number of the more important metamorphoses of organised matter, which take place in the animal organum, is the result of either oxidation or fermentation: in the conversion of starch or sugar into fat or oil, both of these processes, it is stated, take place; a portion of the hydrogen is converted by oxidation into water, and by fermentation carbonic acid gas is formed, which removes both oxygen and carbon. Perhaps in the formation of fat fermentation is alone employed—a portion of the oxygen being removed as water, and another portion as carbonic acid. The chief difference between the ultimate composition of starch and fat is, that the latter contains a much larger proportion of hydrogen and carbon. The knowledge of the exact quantity of starch required for the formation of a given amount of fat is of importance in enabling us to estimate the relative feeding value of both substances. Certain difficulties stand in the way of our acquiring an accurate knowledge on this point. Not only are there several distinct kinds of fat, but the precise formula, or atomic constitution of each, is as yet veiled in doubt. There are three fats which occur in man and the domesticated animals, and in vegetables. These are stearine, margarine, and oleine. The relative proportions of these vary in each animal: thus, in man and in the goose margarine is the most abundant fat, whilst oleine[!--9--][9] exists in the pig in a greater proportion than in man, the sheep, or the ox. The composition of the animal fats does not, however, vary much; and this fact, together with other considerations, have led chemists to assume that two-and-a-half parts of starch are required for the production of one part of the mixed fats of the different animals. Grape sugar and the pectine bodies—substances which form a large proportion of the food of the Herbivora—contain more oxygen and hydrogen than exist in starch, and, consequently, are not capable of forming so large an amount of fat as an equal weight of starch. We may assume, then, that 2·50 parts of starch, 2·75 parts of sugar, or 3 parts of the pectine bodies, are equivalent to 1 part of fat.
SECTION IV.
RELATION BETWEEN THE COMPOSITION OF AN ANIMAL AND THAT OF ITS FOOD.
I have already stated that the results of the admirable investigations of Lawes and Gilbert prove that the non-nitrogenous constituents of the carcasses of oxen, sheep, and pigs exceed in weight their nitrogenous elements. This fact is suggestive of many important questions. What relation is there between the composition of an animal and that of its food? Should an animal whose body contains three times as much fat as lean flesh, be supplied with food containing three times as much fat-formers as flesh-formers? To these questions there is some difficulty in replying. There is a relationship between the composition of the body of an animal and that of its food; but the relationship varies so greatly that it is impossible to determine with any degree of accuracy the quantity of fat-formers which is required to produce a given weight of fat in animals, taken in globo. If, however, we deal with a particular animal placed under certain conditions, it is then possible to ascertain the amount of fat which a given weight of non-plastic food will produce. For the greater part of our knowledge on this point, as on so many others, in the feeding of stock, we are indebted to Lawes and Gilbert. In the case of sheep fed upon fattening food these inquirers found that every 100 lbs. of dry[!--10--][10] non-nitrogenous substances consumed by them produced, on an average, an increase of 10 lbs. in the weight of their fat. In the case of pigs, also, supplied with food, the proportion of non-nitrogenous matters appropriated to the animal's increase was double that so applied in the bodies of the sheep. As the food supplied to these animals contained but a very small proportion of ready-formed fat, it was inferred that four-fifths of the fat of the increase was derived from the sugar, starch, cellulose, and pectine bodies.
These tables exhibit in a condensed form the results of one of the elaborate series of experiments in relation to this point carried out by Lawes and Gilbert:—
| ESTIMATED AMOUNT OF CERTAIN CONSTITUENTS STORED UP IN INCREASE, FOR 100 PARTS OF EACH CONSUMED IN FOOD BY FATTENING SHEEP. | ||||||||||
| General Particulars of the Experiments. | Amount of each Class in Increase for 100 of the same consumed in Food. | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Breed. | A. | Duration. | Description of Fattening Food. | B. | C. | D. | E. | |||
| Given in limited quantity. | Given ad libitum. | |||||||||
| Class I. | ||||||||||
| wks. | dys. | |||||||||
| Cotswolds | 46 | 19 | 5 | Oilcake and clover chaff. | Swedish turnips. | 3·98 | 4·43 | 11·6 | 9·60 | |
| Leicesters | 40 | 20 | 0 | " | " | 3·15 | 3·39 | 12·0 | 9·48 | |
| Cross-bred wethers | 40 | 20 | 0 | " | " | 3·24 | 3·60 | 11·6 | 9·31 | |
| Cross-bred ewes | 40 | 20 | 0 | " | " | 3·25 | 3·60 | 11·8 | 9·40 | |
| Hants Downs | 40 | 26 | 0 | " | " | 3·40 | 4·28 | 10·3 | 8·49 | |
| Sussex Downs | 40 | 26 | 0 | " | " | 3·30 | 4·16 | 10·3 | 8·44 | |
| Means | 3·39 | 3·91 | 11·3 | 9·12 | ||||||
| Class III.—(Series 1.) | ||||||||||
| Hants Downs | 5 | 13 | 6 | Oilcake. | Swedish turnips. | 4·16 | 4·01 | 11·1 | 9·33 | |
| 5 | 13 | 6 | Oats. | " | 5·73 | 7·07 | 10·0 | 9·45 | ||
| 5 | 13 | 6 | Clover chaff. | " | 3·98 | 7·44 | 9·0 | 8·49 | ||
| Means | 4·62 | 6·17 | 10·0 | 9·09 | ||||||
| Class IV.—(Series 2.) | ||||||||||
| Hants Downs | 5 | 19 | 1 | Oilcake. | Clover chaff. | 1·69 | 2·20 | 6·3 | 5·07 | |
| 5 | 19 | 1 | Linseed. | " | 1·81 | 2·32 | 6·2 | 5·19 | ||
| 5 | 19 | 1 | Barley. | " | 1·75 | 2·82 | 5·7 | 5·00 | ||
| 5 | 19 | 1 | Malt. | " | 1·46 | 2·17 | 5·3 | 4·61 | ||
| Means | 1·68 | 2·38 | 5·9 | 4·97 | ||||||
| Class V.—(Series 4.) | ||||||||||
| Hants Downs | 4 | 10 | 0 | Barley ground. | Mangolds. | 3·80 | 5·65 | 9·8 | 8·91 | |
| 5 | 10 | 0 | Malt, ground, & malt dust. | " | 4·04 | 6·18 | 10·4 | 9·49 | ||
| 4 | 10 | 0 | Barley ground and steeped. | " | 3·72 | 6·35 | 8·9 | 8·28 | ||
| 4 | 10 | 0 | Malt, ground and steeped, & malt dust. | " | 2·95 | 4·34 | 9·3 | 8·23 | ||
| 5 | 10 | 0 | Malt, ground, & malt dust. | " | 3·46 | 5·46 | 9·1 | 8·25 | ||
| Means | 3·59 | 5·60 | 9·5 | 8·63 | ||||||
| Means of all | 3·27 | 4·41 | 9·4 | 8·06 | ||||||
| KEY: | A.—No. of Animals. B.—Mineral matter (ash).[!--11--][11] C.—Nitrogenous compounds (dry). | D.—Non-nitrogenous substance. E.—Total dry substance. | ||||||||
| ESTIMATED AMOUNT OF CERTAIN CONSTITUENTS STORED UP IN INCREASE, FOR 100 OF EACH CONSUMED IN FOOD, BY FATTENING PIGS. | |||||||||
| General Particulars of the Experiments. | Amount of each Class in Increase for 100 of the same consumed in Food. | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| A. | Duration. (weeks) | Description of Fattening Food. | B. | C. | D. | E. | F. | ||
| Given in limited quantity. | Given ad libitum. | ||||||||
| The Analysed "Fat Pig."[!--12--][12] | |||||||||
| 1 | 10 | Mixture of bran 1, bean and lentil-meal 2, and barley-meal 3 parts, ad libitum. | 2·66 | 7·76 | 17·6 | 14·9 | 405 | ||
| Series I. | |||||||||
| 3 | 8 | None. | Bean & lentil-meal. | 0·68 | 4·88 | 25·3 | 17·5 | 621 | |
| 3 | " | Indian-meal. | " | 1·86 | 6·39 | 23·7 | 17·9 | 477 | |
| 3 | " | Indian-meal and bran. | " | 0·33 | 5·02 | 21·1 | 16·1 | 362 | |
| 3 | " | None. | Indian meal. | 2·09 | 9·28 | 20·9 | 18·6 | 300 | |
| 3 | " | Bean and lentil-meal. | " | 0·99 | 9·18 | 20·9 | 18·4 | 324 | |
| 3 | " | Bran. | " | 2·35 | 12·10 | 20·3 | 18·7 | 300 | |
| 3 | " | Bean, lentil-meal, and bran. | " | 2·71 | 10·03 | 21·3 | 18·5 | 307 | |
| 3 | " | Bean, lentil-meal, Indian-meal, bran, ad libitum. | 0·22 | 5·65 | 21·1 | 16·8 | 362 | ||
| Means | 0·74 | 7·82 | 21·8 | 17·8 | 382 | ||||
| Series II. | |||||||||
| 3 | 8 | None. | Bean & lentil-meal. | 3·20 | 3·12 | 26·5 | 18·2 | 801 | |
| 3 | " | Barley-meal. | " | 0·16 | 4·65 | 19·2 | 14·7 | 575 | |
| 3 | " | Bran. | " | 0·16 | 3·99 | 21·2 | 15·2 | 547 | |
| 3 | " | Barley-meal and bran. | " | 0·75 | 4·57 | 20·1 | 15·6 | 514 | |
| 3 | " | None. | Barley-meal. | 0·56 | 10·09 | 18·5 | 16·9 | 574 | |
| 3 | " | Bean and lentil-meal. | " | 0·53 | 6·57 | 21·1 | 17·5 | 620 | |
| 3 | " | Bran. | " | 0·49 | 9·79 | 18·9 | 16·9 | 506 | |
| 3 | " | Bean, lentil-meal, and bran. | " | 4·33 | 4·49 | 22·7 | 18·0 | 578 | |
| 6 | " | Mixture of bran 1, barley-meal 2, and bean lentil-meal 3 parts, ad libitum. | 0·27 | 5·65 | 20·4 | 16·1 | 495 | ||
| 6 | " | Mixture of bran 1, bean lentil-meal 2, barley-meal 3 parts, ad libitum. | 1·58 | 8·10 | 21·1 | 17·6 | 515 | ||
| Means | 0·59 | 6·10 | 21·0 1 | 6·7 | 572 | ||||
| Series III. | |||||||||
| 4 | 8 | Dried Cod Fish. | Bran & Indian-meal. (equal parts). | 1·06 | 5·06 | 24·3 | 18·1 | 315 | |
| 4 | " | " | Indian-meal. | 0·26 | 8·16 | 25·6 | 20·9 | 352 | |
| Means | 0·66 | 6·61 | 24·9 | 19·5 | 333 | ||||
| Series IV. | |||||||||
| 3 | 10 | Lentil-meal & bran. | Sugar. | 3·07 | 9·30 | 19·4 | 16·9 | ||
| 3 | " | " | Starch. | 3·18 | 9·36 | 19·4 | 16·9 | ||
| 3 | " | " | Sugar & starch. | 4·06 | 10·78 | 17·7 | 16·1 | ||
| 3 | " | Lentils, bran, sugar, starch, ad libitum. | 4·80 | 9·96 | 18·7 | 16·5 | |||
| Means | 3·78 | 9·85 | 18·8 | 16·6 | |||||
| Means of all | 0·58 | 7·34 | 21·2 | 17·3 | 472 | ||||
| KEY: | A.—No. of Animals. B.—Mineral matter (ash). C.—Nitrogenous compounds (dry). | D.—Non-nitrogenous substance. E.—Total dry substance. F.—Fat. | |||||||
The larger appropriation of the non-nitrogenous constituents of its food by the pig, as compared with the sheep, must not be attributed solely to its greater tendency to fatten, but partly to the far more digestible nature of the food supplied to it.
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--][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--][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 7½ per cent. of the plastic materials of their food, whilst sheep accumulated somewhat less than 5 per cent. That is, 92½ 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.
| Cases. | Estimated per cent. in Increase whilst Fattening. | |||
|---|---|---|---|---|
| Mineral matter (ash.) | Nitrogenous matter (dry). | Fat (dry). | Total 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--][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 £600.
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. | ||||
| Description of Food. | Estimated Money Value of the Manure from One Ton of each Food. | |||
|---|---|---|---|---|
| 1. | Decorticated cotton-seed cake | £6 | 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.
([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.
([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.
([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.
([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.
([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.
([6]) This statement is not absolutely correct, but the range of variation is confined within such narrow limits as to be quite insignificant.
([7]) Doubt has recently been thrown on the truth of this belief by Frankland, Fick, and Wislicenus.
([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.
([9]) So termed because it is the basis of the common oils; the fluid portion of fat is composed of oleine.
([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.
([11]) The amounts of "mineral matter" are too high, owing to the adventitious matters (dirt) retained by the wool.
([12]) This pig was completely analysed by Lawes and Gilbert.
([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.
([14]) As ammonia, urea, uric acid, or hippuric acid; all of which are nearly or perfectly mineralised substances.
([15]) The excrements of animals are capable of evolving, by combustion, enormous amounts of heat.