THIRDLY.--The necessity of _gentle_ and _continued agitation_ for the accomplishment of digestion, is so obvious from the preceding exposition, that it requires no direct experiments to establish it. When portions of meat were suspended in the stomach, by a string so short as to prevent them from being fully subjected to the motion already described as always going on during digestion, _the action of the gastric juice was confined almost entirely to their surface_, and a longer time was consequently required for their solution than when they were left at liberty. In like manner, when meat out of the stomach was placed in a vial containing gastric juice, its solution was uniformly accelerated by gentle agitation, which acted simply by removing the coating of chyme as it formed on the surface, and thus affording to the gastric fluid an easier access to the undigested portions below. Accordingly, when in one of Dr Beaumont’s experiments two ounces of unmasticated roasted beef were introduced through the external aperture into the stomach, and held by a string, only one-half of it was digested in four hours, evidently from the want of mastication confining the action of the gastric juice to the surface of the mass, and because the string prevented it from following the regular motions of the stomach. * * * * * Having now made the reader sufficiently acquainted with the agents concerned in, and the conditions essential to the performance of digestion out of the body, we have next to exhibit the same agents and the same conditions in their ordinary operation in the living being, and to describe the beautiful arrangements by which they are respectively and unerringly regulated. It has been already shewn, that, in endowing us with appetite, Nature has intended both to insure by its means a timely provision for the wants of the system, and to guard against our eating more than enough to supply them. We have also seen that, within certain limits, the quantity of gastric juice secreted bears a direct relation to the quantity of food consumed; that when the food exceeds considerably the real necessities of the system, a part of it remains undigested, because the stomach is unable to secrete a sufficiency of fluid for the solution of the whole; and that, as a necessary consequence, indigestion follows. This being the case, we may expect to find all the arrangements of Nature made with a view to prevent us from hastily filling the stomach to repletion, without being fully warned beforehand of the error we are committing. And such accordingly is the fact.