Appetite (hunger or craving for food) is the instinct which prompts an animal to eat. As it has become developed in the species under the natural condition of the animal having to find his own food with more or less difficulty, we may conclude that it is a more reliable guide to the selection of suitable food when the horse is in full exercise, than when he is idle. Hence we find, that although in almost all cases it is safe to allow a horse an unlimited supply of oats and hay, with free choice between the two, when he is in very hard work, it is highly dangerous to be equally indulgent when his physical powers are not taxed. We should here take into consideration the nature of the forage; for his food in the open is not presented to him in a form so concentrated, and consequently so liable to upset his digestion, as it is in the stable; and besides, it is very rarely as palatable as thrashed oats, for instance. The general wants of the system and a more or less empty condition of the stomach appear to be the essential stimuli of hunger (appetite), which can be allayed, as has been shown by experiment, by the introduction of food into that organ through an artificial opening. Appetite is also excited by external impressions connected with food, as we may note when the idea of food presents itself to the horse by his sense of smell, taste, feeling, seeing, and even hearing, which excites the cavalry trooper to neigh and paw the ground when the trumpet sounds "feed!" In such cases, the stimulus is not confined only to the appetite, but also extends to the entire digestive apparatus; hence the value of flavour by odour and taste.

In order to avoid excess under the promptings of appetite, we should try to make our horses eat slowly. Carpenter remarks : "To eat when we are hungry, is an evidently natural disposition; but to eat as long as we are hungry, may not always be prudent. Since the feeling of hunger does not depend so much upon the state of fulness or emptiness of the stomach, as upon the condition of the general system, it appears that the ingestion of food cannot at once produce the effect of dissipating it, though it will do so after a short time; so that, if we eat with undue rapidity, we may continue swallowing food long after we have taken as much as will really be required for the wants of the system; and every superfluous particle is not merely useless, but injurious." These observations apply with greater force to horses than to ourselves; for the size of the stomach as compared to that of the intestines is much smaller in them than in us, and besides, they are unable to relieve by vomiting, an overcharged condition of the stomach.

Appetite guides ourselves as well as horses, not only as regards the amount, but also in the selection of food suitable to the requirements of the body. Thus, men who habitually work hard, such as navvies, miners, and sailors, can eat with benefit a quantity of fatty food, from which a person leading a sedentary life would turn with loathing. We here see the good effect of healthy exercise in promoting the power of digestion, and in increasing the appetite, the activity of both of which functions is lowered by severe work, as we may judge from the fact that the appetite of racehorses in training generally becomes capricious when they are kept at "concert pitch" for too long a period.

The following results were obtained by Grandeau and Leclerc from carefully made experiments on horses :

In ministering to the requirements of the system, we should be careful to act in agreement with the order in which the functions of nutrition stimulate each other. Thus, if the power of taking up nourishment by the body be more or less in abeyance from the effects of fatigue or illness, it would be manifestly injurious to tempt the appetite or to excite the digestion. On the contrary, the only safe procedure in such a case will be to promote the removal of the cause of the functional torpidity, and when that has been accomplished, the healthy stimulation of digestion and appetite by the demands of the tissues will put the animal machine into perfect working order, supposing that the food and the manner of its supply are suitable. To briefly illustrate this point, to which I shall refer more fully later on, I may say that instead of forcing food on a horse which is "off his feed" from overwork, and giving him stimulants, it is much better to content ourselves with allowing him a full supply of drinking water and some green food or carrots, so as to facilitate the removal from his system of the deleterious products that are the cause of his depression. When they have been excreted, his tissues will demand materials for repair, with the result of healthy general stimulation.