(γαστρικός), the name given to a transparent and colourless fluid which is the principal agent in affecting those changes that constitute digestion, that is, which convert the aliment into chyme. This secretion has a saline and somewhat bitter taste, occasionally possessing acid properties, but probably in its natural and healthy condition being neither acid nor alkaline. It contains a small proportion of albumen, together with a matter which is either gelatin or mucus. But while it thus differs to all appearance in so trifling a degree from many of the other secretions, it yet possesses very extraordinary solvent powers over the substances usually employed as food. Even when made to act upon these substances in vessels out of the body, provided they are kept in a temperature equal to that of the human body, it will reduce them in a few hours to the state of a soft pulp, producing apparently the very same change which is induced upon the same species of aliment by the digestive process within the stomach. There are three ways in which the gastric juice acts on alimentary matter; the first is that of coagulation, which is exerted on all the fluid forms of albumen, whether existing in the serum of the blood, or the white of the egg, or in different secretions, more especially milk. It is by means of this property, indeed, that cheese is obtained from the coagulation of its albuminous portion by the addition of rennet, which is an infusion of the digestive stomach of a calf. The object of this coagulation appears to be to detain the substance for a longer time in the stomach, and subject it more completely to the solvent power of the same fluid, by its previous reduction to a solid form, which prevents its escape by the pylorus. The second kind of action is that of counteracting the tendency to putrefaction, and even to the acsequent fermentation. This effect takes place in a remarkable degree in many carnivorous animals, which frequently take their food in a half putrid state, and in which the first operation of the gastric juice is to remove from it all putrescence; showing that this secretion possesses the property not only of preventing putrefaction from taking place, but also of suspending its further progress when it has actually commenced. The third species of chemical action exhibited by the gastric juice is that of solution. That this effect takes place independently of any concurrent mechanical operation of the muscular powers of the stomach has been very decisively proved by the experiments of Réaumur, of Stevens, and of Spallanzani. Those of Stevens in particular are highly valuable, as having been made on a man who was in the habit of swallowing stones, which he could afterwards, by a voluntary effort, reject by vomiting from his stomach. Taking advantage of this power, Stevens induced him to swallow hollow metallic spheres perforated with holes, and filled with different alimentary substances, which, after being allowed to remain a sufficient time in the stomach, were returned, and their contents examined. It was invariably found that the food under these circumstances of exposure to the gastric fluid alone, with protection from external pressure of a mechanical nature, was more or less completely dissolved, and reduced to the state of pulp. He afterwards pursued a similar train of experiments on dogs, causing them to swallow the perforated spheres, and after a certain time destroying the animals, and examining the changes effected in their contents. Spallanzani has also varied and multiplied experiments of this kind in a manner that leaves no room to doubt the truth of the conclusion deduced from them as to the solvent power of the gastric secretion. Dense membranes and even bones are reduced into a pulpy mass by this fluid in many animals, while at the same time many bodies of comparatively delicate textures, such as the skins of fruits, and the fibres of flax or cotton, are not in the slightest degree affected by it. This difference of action on different substances is analogous to the operation of chemical affinity, and corroborates the theory that digestion is effected principally by chemical agency. The results of these experiments have been fully confirmed by experiments made on the stomachs of persons who, in consequence of a wound, had a permanent opening into that organ from the exterior of the abdomen.

Portions of the stomach are sometimes found dissolved after death, more especially when death has occurred suddenly during the act of digestion. This effect can never take place during life, because the living structures resist the solvent power of the gastric juice. Acid is frequently developed during imperfect digestion; but it appears from the experiments of Dr Prout and others, that this effect is also attendant upon healthy digestion, and that it is principally the muriatic acid which is thus disengaged from its combinations, and makes its appearance in a free state. The lactic acid, which appears to be a modification of the acetic, is also present in considerable quantity. Tiedemann and Gmelin found the acetic acid always present in the gastric juice. They observe that many of the substances employed as food which are not soluble in water are so in the diluted muriatic and acetic acids at a high temperature, and they are inclined to ascribe to a chemical solution of this kind the principal change effected by digestion. Among the agents concerned in the digestion of the aliment, the high temperature at which the contents of the stomach and intestines are retained must be considered as one of the most important. While digestion is taking place, both orifices of the stomach are closed, and there often comes on a feeling of chilliness, especially in a weakly constitution, in consequence of the demand which the stomach makes upon it for an additional supply of heat to assist in the process. There is also disinclination to exertion, and frequently a tendency to sleep. Yet the indulgence in this disposition, as well as violent exercise immediately after a meal, tend equally to retard the formation of chyme. The circumstances most favourable to perfect digestion are gentle exercise, with cheerfulness, and moderate mental exertion.