From the foregoing observations we come to the important law, that the percentage of fibre in a horse's hay should be proportionate to that of the easily digestible nutritious matter of the corn. Highly-fed hunters have so clearly demonstrated the correctness of this principle, by manifesting their appreciation of a feeding material rich in fibre, that "hard hay" and "hunters' hay" have become synonymous terms. As the digestion of hay demands the expenditure of more energy than that of grain, it is better for a working horse to obtain the nutritious portion of his food from the latter than from the former kind of fodder. Hence the larger the ration of corn, the richer in fibre (i.e. the more innutritious) should be the hay or chop. When the supply of corn, and consequently that of easily digestible nutritious matter, is small, the hay given to a working horse should be rich in nourishment and comparatively poor in fibre, as we find with hay which has been made from young, good grass, and which has been well saved.

The amount of fibre, like that of water in food, greatly increases the weight as well as the volume of the contents of the stomach and intestines, as Wolff shows us by the following results of experiments with sheep : -

From this we may reasonably infer that the difference between the weight of the contents of the stomach and intestines of a horse fed on food rich in woody fibre, like hay and straw, and that of the same animal when fed principally on corn, might easily amount to 8 per cent. of the entire bodily weight, which difference in the case of a horse weighing 1,000 lb. would be 80 lb. The woody-fibre method of feeding would therefore force the animal to carry this amount of extra weight without any corresponding gain. In fact, as we have already seen, the advantage in muscular strength would be all the other way. Also the work of digesting hay, to take it as an appropriate instance, necessitates, according to Zuntz and Lehmann, the expenditure of 11.2 per cent. of the energy produced by the food; and oats, only 2.8 per cent. Furthermore, the large size of the stomach and intestines would greatly interfere with the animal's powers of breathing during severe labour.

The fact that food rich in fibre necessitates for its mastication the secretion of much more saliva, and consequently the drinking of more water than does corn, accounts to a great extent for the marked effect which its consumption has in increasing the weight of the contents of the stomach and intestines.

Among mineral matters, phosphoric acid, potash, soda (mostly as common salt), lime, and iron play the most important part in nutrition. Phosphoric acid, potash, and iron are plentiful in hay and corn, in both of which the percentage of soda is less than the requirements of the system. In the entire body of the higher animals, the quantity of potash is about equal to that of soda. Blood contains much more soda than potash; and bloodless meat, more potash than soda. Lehmann tells us that the mixture of blood and flesh which carnivorous animals consume, contains about three times more potash than soda. On this diet, these animals show no craving for common salt; neither do human beings who live almost exclusively on meat. But persons who live principally on potatoes, which contain a large percentage of potash and little or no soda, eat salt with great relish and find that it is necessary to their health. Horses, whose ordinary mixture of hay and corn contains a comparatively small amount of soda, also crave for salt, and salt added to their food has an obviously good effect on their condition. In the food of plant-eaters (horses, for example), there appears to be an excess of potash, which tends to irritate the mucous membrane of the alimentary canal. A portion of the common salt in the system is used up in eliminating potash from the body. Thus, as explained by Bunge, if a salt of potash, such as carbonate of potash, meets common salt (chloride of sodium) in solution, a partial exchange will take place by the formation of chloride of potash and carbonate of soda; the chloride of potash being excreted by the kidneys, as it does not enter into the composition of the tissues or fluids of the body.

Common salt stimulates digestion and all the other vital processes of the body. The soda of the body, of which the common salt of the food is the chief supplier, performs many offices in the nourishment and purification of the system. By combining with carbonic acid in the blood, it forms bicarbonate of soda, and thus helps to prevent the blood from becoming overcharged with that hurtful gas. Lehmann points out that an excess of carbonic acid in the blood tends to re-dissolve the lime salts deposited at the growing ends of bones, or prevents the precipitation of the lime salts. In agreement with this theory, Wachsmuth contends that an accumulation of carbonic acid in the blood is a cause of rickets. As cartilage is much richer in soda than bone, and as young animals have much more cartilage in their bodies than mature ones; they will need more soda in their food, and consequently more common salt. According to Gabriel, bones and teeth respectively contain 8 to 12 and I to 3 per cent. of soda and potash. They require a large amount of soda to neutralise the acids which might attack them. We all know that the formation of bones and teeth is particularly active in young animals.

The consumption of too much salt causes a large discharge of urine, irritation to the kidneys, and abnormal thirst, with a consequent watery condition of the blood, from which the nutrition of the system suffers. If drinking water be withheld, water will be drawn from the tissues, a portion of which will on that account become broken down. These untoward results will occur only when the quantity of salt which has been consumed is very large; for with a plentiful supply of drinking water, the kidneys rapidly eliminate salt from the system.