The principal part played by gastric digestion consists in the conversion of albuminates into the more soluble forms of propeptones and peptones, which are the result of the combined action of hydrochloric acid and pepsin. The rennet ferment curdles milk. The gastric juice is also endowed with the property of converting cane sugar into grape sugar, and gelatin into a soluble form (a peptone) which no longer coagulates. Besides this, a small percentage of fat is split into fatty acids.

The work accomplished by the stomach in the act of digestion must also be ascribed to the active and passive movements that take place in this organ, in consequence of which certain physical changes are effected in the ingested food. Each particle of food is brought into more intimate contact with the stomach walls by these movements than would otherwise be possible. The food as a whole becomes more liquefied and passes, as chyme, through the pylorus into the small intestines. The pylorus is said to control the entrance of the more liquid chyme into the duodenum. It opens and closes at certain intervals. We are as yet not able to give a full explanation for this seemingly elective action of the pylorus, nor do we exactly know at what intervals the pylorus opens. It is only known that at certain times after certain meals (about two hours after a small meal, six to seven hours after a large meal) the stomach is completely empty.

Some of the substances contained in liquefied chyme are absorbed through the stomach wall, such as sugar, salts, peptone, perhaps propeptones; the rest passes into the small intestine, and is subjected to the action of several secretions that combine there in order to further change it and make it fit for absorption.