It contains three different ferments: (1) Erepsin, which acts upon the décomposition products of the proteins, albumose and peptones, ‘in the same way as trypsin; (2) an amylolytic ferment which converts starch to sugar (maltose), and (3) invertases (sucrase, maltase, and lactase), changing the di-saccharides, cane-, malt-, and milk-sugar, into monosaccharides (dextrose or glucose, see p. 14).

From the small intestines the undigested material passes into the large intestine, where the formation of the solid excrements takes place. The ferments and bacteria, present here in immense numbers, continue their action until the mass has assumed the consistency peculiar to each species and is voided as feces.

Summarizing the various steps in the digestion of the different components of feed, we note that

Starch is changed into sugar (maltose) by the ptyalin of the saliva, the amylopsin of the pancreatic juice, and an amylolytic ferment in the intestinal juice.

Fats are changed into free fatty acids and glycerin by the lipase of the pancreatic juice, and by the bile into emulsified fats or soaps.

Proteins are changed into proteose and peptones by the pepsin of the gastric juice, and into amino bodies by the trypsin of the pancreatic juice and the erepsin of the intestinal juice.

In addition to the action of these various digestive ferments the feed is subjected to bacterial action in the paunch and the intestines. Through the fermentation processes caused by bacteria, the cellulose and considerable quantities of other carbohydrates are decomposed and converted into gaseous products, like marsh gas, carbon dioxide, and free hydrogen, which are of no value to the animals except incidentally through the heat generated in these processes; this may be of benefit in aiding to maintain the body temperature of the animals.

Digestion of Feed by Non-ruminants. The digestion of feed by the non-ruminating farm animals takes place, as previously suggested, in much the same way as in the case of the ruminants.

The same digestive fluids act on the feed of the horse and other animals of this class as in the case of the ruminants, viz., saliva, gastric juice, pancreatic juice, bile, and intestinal juice. These differ from the corresponding fluids secreted by ruminants mainly in point of concentration. The protein compounds are broken up into simple soluble substances, like peptones and amino acids; the starch is changed into maltose, and the fats into free fatty acids, emulsified fats, or soaps. The stomach of the horse has a capacity of 12 to 15 quarts, while the paunch of a cow or steer holds over 100 quarts. The horses cannot, therefore, eat as much of bulky feeds at a time aş cattle, nor can they digest coarse feeds containing considerable cellulose (fiber) so completely as the ruminants.

To make up in a measure for its small stomach, the horse has a large sac, cæcum, which is about a yard long and corresponds to the vermiform appendix in man. The large intestine is also of considerable size and has several enlargements. The entire length of the alimentary canal of the horse is about twelve times the length of the body, that of the pig fourteen times and that of cattle and sheep twenty times or more the length of the body. The length of time during which the feed remains in the alimentary canal and is exposed to the action.of the digestive enzymes (or to bacterial action in the intestines) will, therefore, vary in different animals. In cattle and sheep the undigested part of the feed is voided in three to five days, and in horses and pigs in one and one-half to two days.

Absorption of Digested Materials.—The soluble materials and those that have been broken down into simpler, largely soluble compounds through the various processes of digestion are absorbed to some extent by the mucous membrane of the stomach, but for the most part pass through the walls of the intestines. The intestines are lined with innumerable fine projections called villi, inside of which are found microscopic branches of two systems of vessels, the capillaries of the blood-vessels, and the lacteals belonging to the so-called lymphatic system. The digested materials in the form of sugar, salts, soluble proteose and peptones, and similar compounds pass over into the capillaries by the process of osmosis. The capillaries are exceedingly fine blood-vessels that converge to a large vein called the portal vein, through which the materials absorbed by the blood are taken into the liver. They are here distributed through a second set of capillaries and then reunited, passing into the hepatic vein which leads to the heart.

The emulsified fats and free fatty acids, or combinations of these with alkali (soaps), on the other hand, are taken up by the lacteals in the villi of the intestines. From these they pass into the lymphatic system and are later emptied into the thoracic duct which leads to one of the large veins before this enters the heart.

The nutrients thus taken into the blood circulation come into contact with the oxidizing agent of the blood, the oxyhæmoglobin, and are either directly oxidized in the blood or carried to the body tissues to repair waste and supply materials for the formation of new tissues. Very likely, both these processes occur simultaneously. Some of the digested and assimilated nutrients, especially sugar and lactic acid, soon disappear from the blood through oxidation, and the carbon-dioxide and water formed in the process of oxidation are excreted through the lungs and skin. Other nutrients, like the mineral salts and soluble protein compounds, pass into circulation and are brought to the parts of the body where they are needed for building materials. The proteins are decomposed chiefly into amino-acids in the process of digestion, and pass through the intestinal wall as such, or possibly in part, as groups of amino-acids; they appear to be synthesized through the action of the living cell walls into more complex substances, from which the body is able to build its various protein tissues or fluids. In the same way the free fatty acids, and the soaps formed from these in the digestion of fats, are changed in their passage through the intestinal wall into neutral fats which enter the lacteals and pass into the circulation through the lymphatics.

The carbohydrates of the feed, as we have seen, are changed to sugar in the process of digestion and enter the capillaries as such; from these the sugar passes into the blood circulation and enters the liver, along with all other nutrients except the fats. In the liver the sugar is changed into a carbohydrate of the same composition as starch, called glycogen or animal starch, and is deposited as such in the cells of the liver. By this provision an accumulation of sugar in the blood is prevented, and the body has a base of supply of a readily available and oxidizable carbohydrate which can be drawn upon as needed. The liver normally contains only about 2 per cent of glycogen, but after heavy feeding with starchy feeds the content may rise as high as 10 per cent. Aside from furnishing material for production of heat and muscular energy, glycogen may also serve as supply material for the formation of body fat and butter fat, in the case of fattening animals and milch cows, respectively.

The fats may be stored between the muscular fibers or deposited as adipose tissue, or, in the case of females giving milk, may be changed into butter fat. We have seen that the muscular tissues of the body consist largely of protein substances, and that they are the form in which protein is stored in the animal's body. This can take place only in the growing animal. Oxidation of body tissues continues in the animal cells so long as life exists. The final oxidation products of protein substances in the body are carbon-dioxide and water (as in the case of carbohydrates and fat), and, in addition, urea, which is excreted through the kidneys in the urine. As there are no gaseous nitrogenous decomposition products formed, and urea represents the most important and, practically speaking, the only nitrogenous decomposition product in the oxidation of protein substances in the body, it becomes a measure of the protein decomposition in the body. By determining the amount of urea excreted in the urine, say during a day, we are able to ascertain the amount of protein substances in the feed or of body tissues that have been decomposed during the day (see

p. 39).

Metabolism.-The chemical changes that occur within the body incident to the exercise of vital functions and to growth are included under the general term metabolism. Metabolic processes in the animal body are of two kinds: Katabolic or destructive, those by which the food materials are broken into compounds of simpler structure, and anabolic or constructive, by which these simpler compounds are again built up into complex substances. The formation of peptones and amino acids from the proteins is a katabolic process, while the reverse change, the building up of these simpler compounds into body protein, albumen, globulin, etc., is an anabolic process. Both kinds of processes take place continuously in the living body, as we have seen; they are essential to life, and are discontinued only when life ceases.

QUESTIONS 1. Give the various groups of substances found in the animal body and CHAPTER IV

DETERMINATION OF THE NUTRITIVE VALUE OF

FEEDING STUFFS The nutritive value of different feeding stuffs may be determined by two different methods: First, by chemical analysis and digestion trials with farm animals; second, by trials with animals in a respira

a tion apparatus or respiration calorimeter. The former method shows the proportions of the feeds that are dissolved in the digestive processes, while the latten method furnishes direct information as to the energy values of the feeds or rations and shows the uses which the animal makes of the feed eaten. Both these methods will be discussed in the following pages.

A. THE DIGESTIBILITY OF FEEDING STUFFS Digestion Trials. The digestibility of feeding stuffs is determined in so-called digestion trials with animals. Numerous such trials have been conducted with ruminants during the past halfcentury in this country and abroad, and a number of trials have also been conducted with horses, pigs, and poultry. In these trials the animals experimented with are fed the feeding stuff whose digestibility is to be determined, for a period of about a week, and the solid excrements voided by the animal are then collected for another week. Samples of both the feed eaten and of the feces are taken for chemical analysis, and by a comparison of the total amounts of feed components in each the proportion of each component retained or digested by the animal may be determined and calculated on a basis of percentage digestibility.

An example will readily explain the method of calculation.

In an experiment by the author, in which the digestibility of corn silage was to be determined, a cow was fed, on the average, 55.0 pounds of silage per day; a small amount, 0.71 pound, was refused. She voided 58.8 pounds of dung daily during the trial. Chemical analyses were made of both the silage fed and that refused, as well as of the dung voided. The digestion coefficients for the silage were then calculated as shown below: Digestion Trial with Corn Silage

Nitrogen-
Dry
Protein,

Ash,
pounds
pounds
pound pounds

pound pounds

Fat,

Fiber,

matter,

free extract,