When more food is introduced into the stomach than can be readily digested, the ordinary action of the gastric fluids is checked, and chemical changes, including fermentation, advancing to putrefaction, result. This final change is more likely to occur among flesh-feeding animals than among those which live on grain or herbage. A vegetable food is also of course capable of undergoing decomposition and producing considerable derangement in the alimentary canal. In cases where any of the products of decomposition are absorbed into the blood, signs of blood-poisoning may result, with a possibility of a fatal termination. Short of this, however, excess of food may only produce more or less serious indigestion, ending in recovery under appropriate treatment. But the habitual consumption of excessive quantities of food, by horses which are thrown out of work without having the quantity of diet materially reduced, will result in the deposit of large quantities of fat in and upon various organs of the body, some of it on the surface of the heart, in the cellular structure of the liver, and in other organs. In the first instance fatty material may be merely laid up as a deposit in the texture of an organ, or the cells and fibres composing it may undergo complete degeneration, by which its natural structure is converted into particles of fat.
Insufficient food, on the other hand, produces extreme debility, and predisposes an animal to many diseases.
If the deficiency is chiefly in regard to nitrogenous materials, the muscular structures suffer most, and fatal results may follow. This is especially the case when a horse is compelled, notwithstanding the insufficiency of food, to continue its ordinary work.
Quality Of food has also a considerable influence on nutrition, and foods grown on certain soils are known to be extremely poor in this respect. Where food of inferior quality is supplied on the assumption that it is of good quality, the nutrition of the body will fail to be sustained, and wasting results. On certain soils the herbage produced, and even the hay made from it, exercise an injurious influence on the animals which partake of it; at the same time it may not be possible to decide what changes have occurred to render the food deleterious. The scouring lands of central Somerset are a case in point, and although investi-gations have been made repeatedly, including analyses of the soil, herbage, and water, the cause of the disease which attacks animals feeding on the pastures has never yet been accurately determined.
There are many cases in which there is no doubt at all of the fact that weakness results from a diseased condition of the food. It is generally understood that plants suffer from a large number of diseases, many of them being easily recognized, but there is every reason to believe that there are others the exact nature of which has not been determined.
Among the diseases which are known, those depending upon animal or vegetable parasites are most marked in their character. Plants are attacked by numerous fungi or moulds, which are not only injurious to the plant, but in many cases to the animals which partake of them. Mouldy hay, for example, produces derangement of the digestive organs. Ergotized grain, taken in large quantities, has an extraordinary action upon the animals which consume it, including nervous derangement, associated with mortification of the lower extremities, and occasionally sloughing of the extremities about the region of the fetlocks. A curious fungus, which is known as actinomyces, or rayed fungus (fig. 477), attacks certain grains and grasses, especially in river valleys, which, on being consumed, sets up centres of disease in different parts of the animals which feed on the infested plants. Sometimes it gets an entrance to the alveolar cavities by the side of a loose or decayed tooth, penetrating into the sinuses of the head and the cellular structure of the bones, causing enormous swellings, giving rise to the disease which is known in America as " Lumpy jaw". In other situations it causes large tumours in the neighbourhood of the throat, in the stomach, and also in the intestines.
Besides the vegetable parasitic fungi which attack food plants there are numerous animal parasites, and animals supplied with food so attacked suffer in various ways; in the first place from the damage done to the fodder by the ravages of the parasites, and in the second place from the direct attack of the parasites themselves, some of which produce irritation of the skin. An unfortunate circumstance connected with the effects of damaged food invaded by animal or vegetable parasites is the difficulty of connecting the disease of the animal with the disease of the plant. The tendency undoubtedly is to exonerate the food from suspicion, even to ignore it altogether as a possible cause of disease, until a considerable fatality has forced the owner to conclude that some common cause must be at work to produce it, and even then it frequently occurs that the investigation which is made is directed into the wrong channel by the history of the case which is presented to the investigator, who is very likely to be informed that exactly the same articles of food have been, and are still being, supplied to other animals on the same premises, which have not suffered in consequence. This is a source of error which is particularly likely to occur when the food which is suspected is some kind of cake. It is quite possible that a cake contaminated with mould may be part of a lot which was supplied at the same time as the cake on which other animals are feeding with impunity, and that the animals which are poisoned are the only ones which have eaten the fungus-infected food. The investigator, therefore, should not be satisfied until he has made an exhaustive enquiry into the origin and quality of the food which has been supplied, and the particular animals which have suffered in consequence.