Phosphate of Lime.—The mineral components present in the animal body in the largest quantities are lime and phosphoric acid. They make up the skeleton of the body and form about 80 per cent of its entire ash content. In the case of suckling or young growing animals it is important that the feed shall contain a sufficient amount of lime and phosphoric acid. A deficiency of these components in the feed will give rise to serious disorders in the bone structure of the young, as shown in the case of pigs fed wholly or largely on Indian corn (pp. 300-301). Under ordinary conditions, when mixed rations are fed, there is generally little danger of not furnishing enough of these two ingredients. The matter should, however, receive attention in feeding pregnant or nursing animals, dairy cows, or young growing animals; these should always receive feeds high in calcium and phosphorus in their rations.

Among the feeds low in lime (calcium) may be mentioned : Straw and chaff, the cereals and their by-products, roots, and molasses. Feeds high in lime, on the other hand, are: Hay from legumes and grasses and many leguminous seeds. Straw, distillery feeds, molasses, mangels and potatoes are deficient in phosphoric acid, while the small grains, wheat bran, malt sprouts, brewers' grains and linseed meal, are high in phosphoric acid. If the rations fed are rather low in either or both of the ash materials mentioned, the deficiency may be made up by the addition of small amounts of bone meal, calcium phosphate or floats (ground phosphate rock).

Potassium and Iron.-Other essential mineral elements in the animal body are potassium, which is found especially in the cell walls, muscles, and blood; and iron, mainly found in the red coloring matter of the blood (hæmoglobin, p. 21). There is no danger that the rations fed farm stock will be deficient in these or other physiologically important elements, like fluorin and silicon, since only very small amounts of them are required and an ample supply is as a general rule present in the feeds.

QUESTIONS 1. Name the various groups of substances in the animal body and describe CHAPTER III

THE DIGESTION OF FEEDS

The farm animals that chew their feed a second time are known as ruminants. Cattle, sheep, and goats belong to this class. The non-ruminants, on the other hand, are represented among the farm animals by the horse and the pig. The two kinds of animals differ radically in the anatomy of their digestive apparatus; the stomach of the ruminants consists of four divisions or sacs, of which the first three are mainly reservoirs for softening and holding the feed till it is returned to the mouth to be chewed again, while the fourth one is the true stomach, where a digestive fluid is secreted. The non-ruminating animals have only one stomach, into which the feed passes directly from the mouth and the gullet (æsophagus), and is acted upon by the digestive fluid secreted there. We shall consider separately the digestive apparatus of ruminants and nonruminants.

The digestive apparatus of ruminants consists, as already stated, of four separate compartments that are connected with one another, viz.:

a. The rumen or paunch.
b. The reticulum or honeycomb.
c. The omasum or manyplies.

d. The abomasum or the true stomach (Fig. 7). The first three stomachs are mainly enlargements of the ali. mentary canal and serve as reservoirs for the feed before it is chewed the second time. The rumen or paunch is by far the largest one of the four stomachs and, in the case of grown cattle, holds about nine-tenths of the total capacity of them all. The abomasum, or fourth stomach, corresponds to the single stomach of the non-ruminants, and, like this, contains a digestive fluid which acts upon the feed. When the cow swallows her feed, which is partly chewed and well mixed with saliva, it passes down the gullet and partly into the paunch through a slit in the gullet, partly into the second stomach (honeycomb). It remains here for a time and is softened by the saliva and the watery secretions of the paunch wall. The contents of the paunch are given a churning motion which gradually forces it toward the funnel-shaped orifice of the gullet through compression of the paunch by the diaphragm

and the abdominal muscles. A portion of the softened mass is pressed at a time and conveyed into the mouth by a reverse, socalled peristaltic motion of the gullet. In the mouth it is chewed a second time and swallowed again. By the second chewing the cud or "bolus" is reduced to a pasty pulp, and it now passes directly through the oesophagus groove into the third stomach, the manyplies, without opening the slit in the gullet leading into the paunch. The manyplies has numerous hard, fleshy leaves, between

act on the starch. In the lining of the right side, on the other hand, there are numerous glands which secrete gastric juice, as in the case of the fourth stomach of ruminants. From this point on, the digestion of the feed takes place in essentially the same manner in non-ruminants as in ruminants.

The Digestion of Feed.—The various digestive fluids which act on the feed during its passage through the body are (1) saliva, (2) gastric juice, (3) pancreatic juice and the bile, and (4) intestinal juice.

Saliva.—The first step in the digestion of feed occurs in the mouth. When an animal is eating, the feed is crushed and ground by the teeth, and at the same time mixed with saliva. This is a digestive fluid secreted by several glands located beneath or at the base of the tongue. The secretion of saliva is stimulated by the presence of feed in the mouth, and the saliva is intimately mixed with the feed in the process of mastication, especially in the case of coarse and dry feeds.

This insalivation of the feed serves two purposes: First, it moistens and softens the feed so that it may be readily swallowed. Second, saliva contains a digestive ferment, called ptyalin, which acts upon the starch of the feed, changing it to sugar (maltose, the same sugar as is found in malted barley). Saliva is an alkaline, viscous fluid which is secreted in immense quantities in the case of the large farm animals. A horse fed on hay has been found to secrete over 10 pounds of saliva per hour. Oats require a little more than their own weight of saliva, and dry, coarse feed requires four times its weight. As a horse or cow will consume at least 24 pounds of dry feed in a day, it follows that the quantity of saliva secreted daily by these animals may reach or even go beyond 100 pounds (over 12 gallons).

The Gastric Juice.—The digestion of the protein substances of the feed is commenced in the true stomach (abomasum) of the ruminants, or in the single stomach of the non-ruminants, where the feed comes into contact with the gastric juice, which is secreted here. This digestive fluid contains two ferments, pepsin and rennin, and also an appreciable quantity of free hydrochloric acid (about 0.5 per cent). Both rennin and pepsin have the faculty of coagulating the casein of milk, a wonderful provision of nature which insures that milk will remain in the stomach long enough to be acted upon by the stomach ferments, and its nutrients thus fully utilized by the animal. In young calves and other young ruminants the first three stomachs are not fully developed, and the milk passes directly into the fourth stomach, where it is curdled by the rennin and subjected to the action of pepsin. The latter ferment acts only in an acid medium and on protein substances, which it causes to break up into soluble compounds, known as proteoses and peptones. Since the ptyalin of the saliva acts only in an alkaline solution, its action on the starch of the feed is stopped when the feed reaches the fourth stomach and is mixed with the gastric juice.

From the fourth stomach the feed passes through a valve, called the pylorus, into the small intestine. This is a long, tortuous tract, about 120 feet long in cattle, in which three different digestive fluids are secreted or emptied: The pancreatic juice, the bile, and the intestinal juice.

The pancreatic juice is secreted by the large gland called pancreas (or sweetbread) and is emptied into the small intestine near its upper end, through a duct leading from the pancreas. This digestive fluid contains three specific ferments: Trypsin, amylopsin, and lipase. Trypsin converts protein into soluble compounds, mainly peptones, but also compounds of simpler molecular structure than those resulting from pepsin digestion, viz., amino acids. Amylopsin changes starch into sugar, and lipase (formerly called steapsin) acts upon the fats, splitting these up into their component parts, free fatty acids and glycerin (see p. 23).

The bile plays an important part in the digestion of fats. It is a strongly alkaline, yellowish-green digestive fluid secreted by the liver and stored in the gall-bladder attached to the same. The bile acts upon the fats of the feed that are still unchanged, emulsifying these; i.e., separating them into very minute drops or globules that may be either absorbed through the intestinal wall or readily acted upon by the fat-splitting ferment lipase of the pancreatic juice. It also aids in the absorption of the fatty acids. The bile contains a number of characteristic components whose importance in the digestion of feeds is not clearly understood, but it has several regulative and digestive functions besides those mentioned; it acts as a natural laxative and prevents an accumulation of waste materials in the intestines, changing poisonous decomposition products of protein into harmless compounds that are excreted through the kidneys.

The intestinal juice is sècreted by numerous small glands in the mucous lining of the intestines, especially in the lower part of the tract.