absence of pretension in Dr Beaumont’s work is one of its most pleasing characteristics. CHAPTER V. THEORY AND LAWS OF DIGESTION. Different theories of Digestion--Concoction--Fermentation--Putrefaction--Trituration--Chemical solution--Conditions or laws of digestion--Influence of gastric juice--Experiments illustrative of its solvent power--Its mode of action on different kinds of aliment--beef, milk, eggs, soups, &c.--Influence of temperature--Heat of about 100° essential to digestion--Gentle and continued agitation necessary--Action of stomach in admitting food--Uses of its muscular motion--Gastric juice acts not only on the surface of the mass, but on every particle which it touches--Digestibility of different kinds of food--Table of results--Animal food most digestible--Farinaceous next--Vegetables and soups least digestible--Organs of digestion simple in proportion to concentration of nutriment--Digestibility depends on adaptation of food to gastric juice more than an analogy of composition--Illustrations--No increase of temperature during digestion--Dr Beaumont’s summary of inferences. Before entering upon the consideration of the theory of digestion which naturally evolves itself from the facts expounded in the preceding chapter, it may be of advantage to turn for a moment to the various theories which have prevailed since the subject first attracted the attention of the learned. Hippocrates regarded digestion as a kind of _concoction_ or _stewing_; and many of his followers believed that it is effected in the stomach by the agency of heat alone, much in the same way as food is cooked over a fire. It is quite ascertained that heat favours the process, but it is pure absurdity to maintain that that agent alone will accomplish digestion. Others of the older physiologists contended, that chymification results from simple _fermentation_ of the alimentary mass, and referred to the gas disengaged during difficult digestion, as a proof that the process of fermentation goes on. But it is now demonstrated, that the tendency of healthy digestion is rather to arrest than to induce fermentation, and that the latter takes place only when disease exists, or when more food has been swallowed than the quantity of gastric juice secreted by the stomach is able to dissolve. Moreover, the products of digestion and of fermentation are so extremely different, that it is impossible to believe them to originate from the same chemical action. The next theory which prevailed considered digestion to be the result of the _putrefactive_ process. The single fact that the gastric juice not only arrests putrefaction, but even restores to sweetness meat in which that process is begun, is sufficient to demonstrate the wildness of such a supposition.