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from 24 to 42 per cent. As these percentages increase, those for the other components decrease; e.g., the water contents in the body of a steer decrease from 66 to 49 per cent, in sheep from 67 to 43 per cent, and in swine from 58 to 44 per cent. In all cases except where the animals are very fat, their bodies consist of more than one-half water; the body of a lean animal or a fat calf (as of all young animals) is made up of nearly two-thirds water. As the animal grows toward maturity, and especially during fattening, the proportion of water in its body tissue becomes generally smaller, and that of fat increases. This is because the increase in body

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Fig. 6.-Composition of live animals less contents of stomach and intestines, in per cent.

weight of animals with increasing age or during fattening is composed of more dry matter and less water than when the animal is young or has not been fattened, and not because the fat replaces the water in the body tissues (Fig. 6).

The composition of the increase of live weight in fattening has been calculated by Lawes and Gilbert for steers, sheep, and hogs. They found that if a steer, for example, gained 100 pounds during fattening, these 100 pounds would be composed, on the average, as follows:

Water, 23.8 pounds, and total dry matter, 76.2 pounds, made up of:
Fat, 67.8 pounds,
Protein, 7.3 pounds,
Ash, 1.1 pounds.

While lean animals consist of nearly two-thirds water and less than one-tenth fat, the increase in body substance during fattening is over two-thirds fat and less than one-fourth water, and protein makes up only about 7 per cent of the increase. It is easy to understand, in view of these figures, why fat and older animals can command higher prices than young or lean animals, and why the percentage dressed weight of cattle, for example, is higher in the case of the former kind of animals than with the latter.1

Components of the Animal Body.-We shall now briefly consider the chemical composition and main characteristics of the various groups of components found in the animal body.

Protein.—Protein substances are found in all parts of the animal body, in the blood, lymph, muscles, connective tissues, milk, etc.

The blood is the vehicle by which the digested and absorbed nutrients are distributed throughout the body, and which supplies its different parts with the substances necessary for growth and the exercise of vital functions. Blood makes up about 8 per cent of the body weight of horses, cattle, and sheep, and less than 5 per cent of that of the pig. It is composed of a liquid portion called plasma, in which the blood-cells or corpuscles are floating. The plasma makes up about two-thirds of the blood; it contains three protein substances in solution, viz., fibrinogen, serum globulin, and serum albumen. On clotting of the blood or when it is whipped, the fibrinogen is changed, through a special ferment called thrombin, into fibrin, which entangles the blood-corpuscles and holds them in a solid clot. The liquid that separates from clotted blood on standing is called blood-serum.

There are two kinds of blood-corpuscles, red and white. The red corpuscles are minute, round dises, that vary in shape and size with different animals. They are composed of a spongy albuminoid substance which holds in its meshes the red coloring matter called hæmoglobin. This is a very complex protein substance and contains about one-half of one per cent of iron, in addition to the ordinary components of protein. Hæmoglobin is a dark, purplish red, crystalline substance which has great affinity for oxygen. It absorbs oxygen in the lungs, forming oxyhæmoglobin; this again readily gives up its oxygen in the cells of the different body tissues where the oxidation (combustion) of nutrients takes place. The chemical changes that occur in the cells and are necessary for the continuance of life and for growth are dependent on this supply of oxygen and on the nutrients which are carried to the different parts of the body by the blood.

1

* See live weight and dressed weight of steers of different breeds and ages, Woll, “ Handbook for Farmers and Dairymen,” 6th ed., p.

206.

The white blood-corpuscles (so-called leucocytes) are of larger size than the red ones, and are found in only small numbers compared with red corpuscles. The leucocytes have the power of going through the walls of the capillaries (p. 31), and can pass with the lymph in between the cells of the tissues. In case some part of the body is injured or diseased, they collect there in large numbers, and on breaking down form pus. Their main function appears to be to destroy disease germs.

The muscular tissues in animals consist approximately of 75 per cent water, 20 per cent protein, largely myosinogen (myosin), belonging to the globulin group, 3 per cent fat, less than 1 per cent carbohydrates (glycogen and dextrose), 1.0 to 1.5 per cent salts and 0.2 per cent nitrogenous extractives, mainly creatin and creatinin. The mineral matter in the muscle consists largely of potassium phosphates; small amounts of salts or sodium, calcium, magnesium, and iron are also present.

Connective tissues (tendons, ligaments, cartilage, skin, horns, hoofs, etc.) are all insoluble in water or salt solutions, and are only slightly attacked by acids or alkalies. Only two substances of this group need be mentioned here, collagen and keratin. The former is the main organic component of cartilage and bone, and also makes up a large proportion of tendons and ligaments. On long boiling with water, collagen is dissolved and forms gelatin, which solidifies on cooling. Keratin is the main component of skin, hoofs, horns, wool, hair, and feathers, all substances that offer the greatest resistance to the action of solvents. Keratin contains 4 to 5 per cent sulfur in addition to the elements ordinarily found in protein compounds. On treatment with steam under pressure it is rendered soluble and forms glue. The manufacture of this material is an important side-line of the large packing houses.

Milk contains two important protein substances, casein and albumen. Casein belongs to the so-called nucleoproteins, combinations of albumen and phosphoric acid. It is suspended in a colloidal state in milk, and is not in perfect solution, hence may be separated out by means of centrifugal force.la On addition of an acid to the milk, or through the action of enzymes, like rennin or pepsin, casein is precipitated, and the milk “curdles.” The manufacture of cheese from milk depends on this property of casein. Milk albumen is soluble in water, and, like other albumens, is coagulated on heating above 80° C. (176° F.). Milk contains about 3.2 per cent casein and albumen, ranging between 2.5 and 4.6 per cent according to the quality of the milk; about 80 per cent of the total milk proteins is composed of casein; the rest is largely albumen.

а.

1a Wisconsin Report, 12, p. 93. State publications referred to in this book, unless otherwise specified, are those issued by the agricultural experiment stations.

Fats may be present in animals as body fat, in the marrow of bones, and in milk. They occur in the former two as oval or round cells that are composed of a nitrogenous membrane filled with fluid fat in live animals. The body fat is similar in composition to the vegetable fats, being largely composed of glycerides of the fatty acids, stearic, palmitic, and oleic acids, but the proportions of the different glycerides vary from that of plant fats, and there are also characteristic components of animal fat which are not found in the vegetable kingdom. Milk fat is composed of the three glycerides mentioned and, in addition, of about 8 per cent of glycerides of volatile fatty acids (mainly butyric acid), which give the characteristic fine flavor to fresh butter and, on decomposition, a rancid flavor to old butter. On account of the presence of these volatile fatty acids in butter it is possible to distinguish, by means of chemical analysis, between pure and artificial butter.

Body fat may be deposited in animals receiving an abundant supply of feed; it is stored either between the layers of muscular tissue, about the internal organs, or directly beneath the skin, especially on the backs of animals. The body fat forms a reserve material that the animal can draw upon in time of a scarcity of feed. Through systematic liberal feeding and other favorable conditions the faculty to lay on body fat has been greatly developed in fattening animals, especially pigs. Adipose tissue of pigs consists of about 92 per cent of pure fat, the balance being 6.4 per cent water and 1.35 per cent nitrogenous substances (membrane).

Lipoids are organic substances closely related to the fats and found mixed with these in various tissues and organs of the body. The two most important lipoids are; lecithin found in egg yolks, blood, lymph, and cholesterol, present in bile, liver, eggs and wool fat; both are also normal constituents of brain and nerve tissue.

Vitamins are important organic substances of unknown composition that have been recently identified in minute quantities in many feed materials. One vitamin, which is soluble in fat, is found associated with certain animal and vegetable fats, like butter fat, fat of egg yolk, kidney and liver fat, as well as in the leaves of plants and, in small amounts, in cereal grains. Another vitamin is soluble in water and is found in the leafy portion of plants and vegetables, in seeds, milk, eggs, etc. A third vitamin is present in vegetables and fresh fruits, especially in orange juice and tomatoes; it prevents the development of scurvy and is therefore called antiscorbutic.

The importance of vitamins lies in the fact that body growth can only take place when they are present in the food. Animals fed all necessary food components in pure form (protein, fat, carbohydrates, mineral matter) will not make a normal body growth until some vitamin-containing food is added to the diet. A number of diseases, as scurvy, beriberi, pellagra, etc., will also occur whenever vitamins are not supplied in the diet. Since milk contains ample amounts of vitamins, the importance of this food, and of dairy products in general, in the feeding of man, especially for infants and children, is readily seen.”

Ash Materials. The animal body contains the same ash materials as are found in plants, and as a general proposition the elements essential to the life of plants are also essential to animal life. In the case of two elements, sodium and chlorin, it is a question whether they are absolutely essential to plants, but since they are present in all soils, plants always contain an ample supply of both elements. It is definitely known, however, that both sodium and chlorin are essential to the growth of animals and to the continued exercise of their vital functions. We shall see that gastric juice, one of the digestive fluids of the body, contains free hydrochloric acid; this acid comes from the sodium chloride (common salt) found in the feed of the animals or eaten directly by them. Animals fed largely coarse feeds receive a sufficient amount of salt in the feed to supply their wants, but when fed much grain or other concentrates low in mineral matter, they need more salt than that contained in the feed; all farm animals relish salt greatly, and the practice of “salting” livestock has, therefore, become quite general.

Salt improves the appetite of the animals and increases the flow of digestive juices; it promotes and regulates digestion and should, therefore, be furnished in ample amounts. In the case of milch cows at least, a supply of salt in addition to that in the feed is essential to their continued health, both because of their large feed consumption, especially grain feed, and because of the amount of chlorin removed in the milk. It is a general practice among dairyfarmers to supply about an ounce of salt daily per cow, placing it before them in the mangers or giving them free access to salt. Unless milch cows receive salt, abnormal conditions will soon appear and there will be a gradual reduction in vitality of the animals which will result in a general breakdown after a period varying with different cows from a month to more than a year.3

? McCollum, The Newer Knowledge of Nutrition," New York, 1918; Sci. Monthly, 1918, p. 179; Jr. Soc. Chem. Ind., 1918, p. 53; Gue. Breeders' Jr., XVI, p. 234.

3 Wisconsin Report 22, p. 154.

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