It is probable that the presence of saliva, which is an alkaline fluid, is beneficial in checking undue acidity in the stomach. Respecting the action of a cell-membrane dissolving ferment, see page 68.

The digestion of the nitrogenous matter of the food begins in the stomach and is carried on by the action of pepsin, which is a ferment found in the gastric juice, and which has the property of changing nitrogenous matter into a soluble form (peptone). It also serves to split up vegetable fat into a fine state of division by dissolving the nitrogenous envelopes which enclose the globules of fat. The natural and healthy acidity of gastric juice is chiefly due to the presence of hydrochloric acid, which confers antiseptic properties on gastric juice, as we may see by the immunity with which many races of men eat putrid flesh and decomposing fish. Gastric juice contains about 97.3 per cent. of water, and about .3 per cent. of hydrochloric acid (Schmidt).

The presence of food in the stomach acts as a stimulant to the secretion of gastric juice, which at first flows slowly into that organ, but more quickly later on. Its secretion is liable to become checked by exercise, by excitement, or by the stomach becoming unduly distended. It is evident from our knowledge of the distribution of blood in the body (p. 7), that stimulation of the muscles and organs of breathing from exercise, or of the brain from excitement, will produce more or less congestion of blood in these parts, and anaemia in other organs, which cannot perform their work efficiently without a copious supply of blood. Undue distension of the stomach or intestines causes anaemia by pressure, as for instance, when the animal consumes large quantities of boiled food. Drinking freely of water while digestion is going on in the stomach is liable, by largely diluting the gastric juice, to arrest its action for the time being. The lower the temperature of water that is drunk while digestion is going on in the stomach, the more liable will such water be to check the action of the gastric juice, which is unable to perform its functions at a temperature much below blood heat. When the supply or action of the gastric juice becomes more or less arrested, the food which is in the stomach is apt, in the absence of its natural antiseptic, to become decomposed, with the probable result of indigestion and flatulent colic caused by the evolution of gas. Hence we may conclude that horses should not be worked, watered, or subjected to excitement soon after they have been liberally fed; and that they, as already stated, should not be given large quantities of food which they can readily swallow. The fact of a horse's intestines being of great volume shows that his food should be of a bulky nature. But as his stomach is small and simple in structure, we may assume that the material from which he derives his nourishment should be easy of digestion, and that he should be frequently fed. We know that the stomach of an animal is large and complex in proportion to the amount of preparation its natural food requires in that organ. Thus we find that the stomach of weasels and ferrets, whose natural food (blood) needs but slight modification before being digested, is little more than an enlarged prolongation of the gullet. The stomach of cattle is, on the contrary, very complex, consisting as it does of one true and three preparatory stomachs, and is consequently far better suited for the digestion of coarse and innutritious fodder (such as hay made from grass in seed, and straw) than that of a horse. As the material from which a horse derives his nourishment should be easy of digestion, we are forced to infer that the remainder should be made up of indigestible matter; for it would be unreasonable to imagine that a horse could digest all, or even a large part, of the food which his intestines are capable of containing. We may see by referring to pages 96 and to 98, that this indigestible matter consists chiefly of vegetable fibre, which, although it contributes little or nothing to the direct nourishment of the system, greatly aids the digestion of the nutrient portion of the food.

The conclusion as to the advisability of frequent feeding, is in agreement with the manner a horse eats when at grass, namely, that of consuming small quantities of food at short intervals of time; in fact, feeding more or less continuously. When a horse is fed during frequent intervals between labour, the digestion of comparatively small quantities of food in the stomach is not as a rule followed by digestive disturbance. For instance, although strong exercise immediately after feeding would be dangerous for a horse that has a liberal daily ration divided into only three or four feeds; it produces no bad effect on an ordinary London cab horse, which on every available occasion during his tour of duty has the feeding bag put on his head.

Under ordinary conditions, as soon as a horse's stomach becomes nearly filled while he is eating, food, more or less digested, begins to pass from his stomach into the intestine at the same rate as fresh food enters into his stomach from his mouth and gullet. The food that is then in front, on its way out of the alimentary canal, gets to a greater or less extent pushed out of the stomach by the food behind it. When the animal stops eating, the rate at which the food leaves the stomach, gradually decreases, and even after the horse has been fasting for a considerable time, a certain residue of food will be found in that organ. Hence, the larger the quantity of food consumed at one time, other things being equal, the quicker will it pass through the stomach, and the less perfectly will it be digested. During the digestion of food in the stomach, the action of the gastric juice increases in activity the longer the food remains under its influence; probably because the gastric juice contained in the stomach during the later periods of digestion, is not so much diluted with saliva as the gastric juice of the earlier periods. As corn requires to remain for a comparatively long time in the stomach to become properly digested, we should evidently refrain from giving hay to a horse soon after he has eaten his grain. In fact, in giving hay and corn we should adopt the opposite procedure to that usually practised in stables. If our object is to get a horse to eat as much corn (including chop, in the event of its being used) as he can consume, we may give him the corn in the ordinary way, and after he has eaten it, wait for at least an hour before giving him hay. A better plan I have found in such cases, supposing that the animal gets on chop in his grain, is to give a small amount of hay (say, 1 or 2 lb.) before the corn, and after the corn has been consumed, to give no more hay for an hour or longer. When a horse has a full allowance of hay and a moderate amount of grain (say, 14 lb. of the former and 10 of the latter), we might with advantage devote, say, two-thirds of the hay to be divided into portions to be given before the feeds of corn, and keep the remaining one-third for night use.

Experiment has shown that the average period of retention of food in the stomach is probably not longer than an hour and a-half.

Under healthy conditions, the opening which forms the exit from the stomach does not permit food to enter the small intestine before the nitrogenous matter is fairly well dissolved. If, however, the passage be effected before that desirable result is obtained, the irritation to the intestine caused by the presence of an excess of unprepared material is apt to give rise to indigestion, the usual symptoms of which will be pain and flatulent colic in aggravated cases, or merely diarrhoea.

As a rule, the cause of such digestive disturbance is exercise or water given too soon after the horse has had a full feed. Water in such a case is apt to carry the food out of the stomach into the small intestine; because water stays only a very short time in the stomach on its way to the caecum.

The movements of the muscular coat of the stomach cause the food contained in that organ to become saturated with gastric juice, and to become slowly carried towards the opening from the stomach into the small intestine. The muscular coat of the intestines gives their contents a wormlike motion, which enables them to expel the residue of food onwards.