Another set of physiologists imagined that _trituration_ would account best for all the changes occurring in the food during digestion; and consequently regarded the chyme as a sort of emulsion formed by the intimate mixture of the aliment with the juices of the stomach, just as an emulsion is formed by rubbing down almonds in a mortar. The advocates of this theory referred for proofs not only to the contractile motions of the stomach, already noticed, but to the muscular apparatus for trituration which forms so remarkable a feature in the gizzard of granivorous birds. But, in adopting this conclusion, they forgot that in birds the triturating apparatus does not digest, but serves, like the organs of mastication in man and quadrupeds, merely to bruise the grain on which the animal lives. In birds, in fact, digestion _begins_ only _after_ the trituration is finished. A more recent and much more accurate view of digestion, is that which considers it as neither more nor less than a _chemical solution_ of the food in the gastric juice. This theory is supported by a greater number of facts and experiments than any other; but although substantially correct, it is perhaps too exclusive and limited in its principles. It is true that, by the agency of gastric juice on food out of the body, a change very similar to chymification can be effected on it; but when we remember that _chyme_ or the _result_ of real digestion, is essentially the same in its elementary or component principles, whatever be the kind of food from which it is formed,--and that, as yet, we are acquainted with no purely chemical agent, which, applied to different substances, gives rise to the same uniform product,--we shall be more willing to believe that chymification is neither a purely mechanical nor a purely chemical operation; but the result of a vital process, to which both mechanical and chemical forces contribute, and which no action or combination of inanimate matter can either exactly imitate or supersede. To enable ourselves to appreciate correctly the nature of digestion, we must begin by considering the conditions essential for its performance, or without which it cannot be carried on. The FIRST indispensable requisite is an _adequate supply of gastric juice_, and its _thorough admixture with every particle of the food_ on which it is to operate. The SECOND is _a steady temperature of about 98° or 100° Fahr._; and the THIRD is _the gentle and continued agitation of the alimentary mass in the stomach while digestion is going on_.