When the food is swallowed, it passes very rapidly down the first part of the alimentary canal, which is straight, and is then kept for some time in the stomach. The stomach differs from the rest of the canal in several particulars, among them the following: it is a _large_ cavity, and is closed at each end by a valve to keep the food in until it has been thoroughly treated, and it deals with the whole mass of food taken at a meal at one time, and yet has no contrivances for increasing its surface. Here the food is subjected to a most important and searching examination. Enclosed in this bag, it is thoroughly mixed with weak hydrochloric acid, secreted by numerous glands, and kept churning round and round by the muscular action of its walls, that the contents may be kept well mixed. The acid is just strong enough to kill protoplasm, and hence the putrefactive bacteria which were necessary in the mouth, but would be a very doubtful blessing in the interior of the body, are disposed of. Other things are also killed. Not only does the stomach execute intruding bacteria, but it also kills a good deal of our food. Fruit and salad consist largely of still living cells, and occasionally there is bigger game, _e.g._, oysters. One thing, however, the acid does not kill, and that is the cells lining the stomach, and it may as well be said here that the parts of the body exposed to ferments have the very necessary power of resisting them, so that a normal animal does not digest itself. The stomach, however, is a kitchen as well as a slaughter-house. The gastric juice, or secretion of all the glands opening into it, contains, besides the acid, two important ferments, both of which act on proteids. Carbohydrates are absorbed, but not digested, in the stomach, as acid destroys saliva. One of the ferments is rennet, an article familiar to the culinary profession, which solidifies milk. The other acts on proteids generally, converting them ultimately into a very simple form, peptone, which is absorbed at once. How much of the proteid in the stomach is converted into peptone is not known, for the action of acid alone is sufficient to enable it to be absorbed. A solution of proteid, _e.g._, white of egg, is quite altered if made slightly acid; it no longer coagulates when boiled, but the change of the most practical interest is that, if injected into the veins, it seems to become part of the blood, while ordinary proteids act as poisons. The peptonizing ferment, however, has one very important function: it digests the collagen of the connective tissue, the substance which becomes gelatin when boiled. The reason why this is so important is not only that nothing else in the body affects it, but that fat is enclosed in it, and if it were not thus set free would pass through the body unabsorbed.