How the rabbit digests its food.—A rabbit needs food to repair the constant waste of substance which the activities of its life entail; and the same is true of every other living thing, be it plant or animal. Now, every part of a rabbit’s body is irrigated and drained by the finest branches of a system of pipes through which blood is always flowing; and it is in this blood-stream that the food is conveyed to the muscles and other organs which are to be repaired. The food finds its way into the blood when that fluid is flowing through the small vessels which lie in the thickness of the wall of the digestive tube. Before food can gain access to the blood it must be in a condition in which it is capable of diffusing through the thin membrane which separates them. Digestion is the process which renders food soluble and diffusible, and hence capable of passing into the blood. The food of animals is of several different kinds. A few of these are soluble and diffusible at the time they are eaten, but most of them are neither, and therefore require treating according to their nature. This is why so many different fluids are poured into the [Pg 234] rabbit’s digestive canal. Saliva, gastric juice, bile, and pancreatic juice are each capable of acting upon certain constituents of the food and rendering them soluble and diffusible. It would be beyond the scope of this book to consider the work of these fluids in detail, but the action of saliva is not only fairly typical, but it can easily be imitated outside the body. Starch is a very common constituent of vegetable foods, and its presence or absence is readily determined by the blue colour which it gives with a solution of iodine. Starch is quite insoluble in cold water, but when treated with hot water it swells up and, to a great extent, dissolves. But a mere solution of starch cannot get into the blood, for it is incapable (Expt. 45, 1) of passing through a thin membrane. On the other hand, if starch is mixed with saliva, and the mixture is kept at the temperature of the body, it is found in a short time that the starch has been changed into sugar, which is not only soluble, but readily diffuses through a thin membrane. In other words, starch as such is useless to the rabbit as food; only after it has been digested by conversion into sugar can it be used by the body. Something very similar often takes place in plants. It was seen (p. 33) that the cotyledons of a pea become sweet during germination. Starch is a convenient form of food for storing in the cotyledons of a pea, the endosperm of the maize seed, the short stem of the crocus-corm, and so on; but before the plant can use it as food the starch must be made soluble and diffusible by being changed into sugar. In plants the change is brought about, not by saliva, but by a substance known as diastase.