If we regard chymification as going on at the surface of every individual portion of the food, we at once see that the constant motion of the stomach serves, not only to affect the original mixture of the food with the gastric juice, but to remove the chyme from the surface of each little fragment of the alimentary mass in proportion as it is formed, and by this very removal to expose a fresh surface of the fragment to the contact of the mucous membrane, thereby enabling it to excite a farther secretion of the gastric juice, where, as sometimes happens, the stomach is unable to provide a sufficient quantity from the beginning. In this way the formation and removal of chyme go on from the very first, although, of course, more slowly than after the gastric solvent has had time to act. It is generally said that an hour elapses before any chyme is formed; but Dr Beaumont has detected its existence at a much earlier period, and is of opinion that, from the uniform and constant decrease in the contents of the stomach, which begins as soon as the food is swallowed, chymification commences almost immediately. This decrease, though slow at first, becomes gradually accelerated, till the whole mass is converted into chyme. Apparently in harmony with this more energetic action, the acidity of the gastric fluid also becomes greater, and affords a greater stimulus, in proportion as digestion advances. As formerly explained, the thickish, semi-fluid, greyish, chyme, into which the aliment is converted, is gradually impelled towards the pyloric extremity of the stomach. On its arrival there, the pylorus, or valve between the stomach and the intestine called the _duodenum_, opens and allows the chyme to pass into the intestine. But, by a curious mode of sensibility, if any portion of undigested food be mixed with it, the pylorus contracts upon it, refuses it egress, and throws it back into the stomach for farther digestion. If, however, any thing really indigestible finds its way into the stomach, and presents itself at the pylorus--or if the stomach has temporarily lost its digestive power, and the food remains in it for many hours unchanged--then the pylorus, after repeatedly refusing egress, at last opens and allows it to pass into the gut. So marked is the contractile impulse towards the pylorus when digestion is going on, that Dr Beaumont found even the bulb of his thermometer carried down with a steady and considerable force.