But although, in every class of living beings, the gastric juice is constituted with a direct relation to its natural food, still its qualities may be so much modified by a very gradual change of diet, as to fit it for digesting aliment of a very dissimilar or opposite kind. Thus, in the natural state, the stomach of a sheep exerts scarcely any action on beef or mutton; but if the change from the one kind of food to the other be made by slow degrees, the gastric juice will in the end become so essentially altered, as to enable it to digest both. In this way, as is mentioned by Delabere Blaine,[29] a horse at the Veterinary College was supported for some time by animal matter alone; while others have subsisted on dried fish, or on milk. It has been shewn, also, by John Hunter, Spallanzani, and others, that eagles, falcons, owls, pigeons, and domestic fowls, may for a time be fed on aliments altogether foreign to their natural habits. But these facts only chew the extent to which Nature will go, on an emergency, for the preservation of life; and no more indicate the equal fitness of both kinds of aliment, than the fact of some men being able to stand for a few minutes on their heads proves an inverted position to be the natural attitude of the human race. In consequence of this adaptation of the gastric juice to the nature of the food, it is obvious that _sudden and extreme changes from one kind of diet to another must be injurious_, because the stomach has not time to modify its secretions sufficiently to meet the altered demand made upon its powers. This, accordingly, is one of the reasons why so much caution is used in bringing horses into condition after having been for some time in the pasture-field. When they have previously been on dry food in the straw-yard, corn may be given with greater safety: so that it is the change, not so much in quantity as in _kind_ of aliment, which causes the risk. And, on this account, when a horse is to be put upon hard food, after having been fed on grass or other succulent vegetables, Blaine recommends, not only that hay and corn should be given in very small quantity at first, but that the hay should be moistened, and the corn _mixed with bran and mashed_; by which means having acquired a greater analogy to grass, it will be more easily acted upon by the gastric juice, which has been previously adapted for green food.