The physical and chemical phenomena of digestion, relate chiefly, 1st. To the action of heat; 2ndly. To the dissolution of the alimentary substances. The heat of the animal is such, as is well fitted to promote solution.
That digestion is performed by solution, is evident, from several experiments, particularly those made by Dr. Stevens, who enclosed different alimentary substances in hollow spheres of silver, pierced with small holes. These were swallowed, and after remaining some time in the stomach, the contents were found dissolved. The great agent of solution is the gastric juice, which possesses a very strong solvent power. This juice is secreted by the arteries of the stomach; it may be collected in considerable quantity, by causing an animal that has been fasting for some time, to swallow small hollow spheres, or tubes of metal filled with sponge.
This liquid does not act indiscriminately upon all substances; for if grains of corn be put into a perforated tube, and a granivorous bird be made to swallow it, the corn will remain the usual time in the stomach without alteration; whereas if the husk of the grain be previously taken off, the whole of it will be dissolved. There are many substances likewise which pass unaltered through the intestines of animals, and consequently are not acted upon by the gastric juice. This is the case frequently with grains of oats, when they have been swallowed by horses entire, with their husks on. This is the case likewise with the seeds of apples and other fruits, when swallowed entire by man; yet if these substances have been previously ground by the teeth, they will be digested. It would appear therefore, that it is chiefly the husk or outside of these substances which resists the action of the gastric juice.
This juice is not the same in all animals; for many animals cannot digest the food on which others live. Thus sheep live wholly on vegetables, and if they are made to feed on animals, their stomachs will not digest them: others again, as the eagle, feed wholly on animal substances, and cannot digest vegetables.
The accounts of the experiments made on gastric juice are very various: sometimes it has been found of an acid nature, at other times not. The experiments of Spallanzani show, however, that this acidity is not owing to the gastric juice, but to the food. The result of his experiments, which have been very numerous, prove, that the gastric juice is naturally neither acid nor alkaline. No conclusion, however, can be drawn from these experiments made out of the stomach, with respect to the nature of the gastric juice; nor do the analyses which have been made of it throw any light on its mode of action. But, from the experiments which have been made on digestion, in the stomach, particularly by Spallanzani, the following facts have been established.
The gastric juice attacks the surfaces of bodies, and combines chemically with their particles. It operates with more energy and rapidity, the more the food is divided, and its action increased by a warm temperature. By the action of digestion, the food is not merely reduced to very minute parts, but its chemical properties become changed; its sensible properties are destroyed, and it acquires new and very different ones. This juice does not act as a ferment; so far from it, it is a powerful antiseptic, and even restores flesh which is already putrid.
When the alimentary substances have continued a sufficient time in the stomach, they are pushed into the intestines, where they become mixed with the bile and pancreatic juice, as was before observed. What changes are caused by these substances, we have yet to learn; but there is no doubt, that they serve some important purposes. By the peristaltic motion of the bowels, the alimentary matters thus changed are carried along, and applied to the mouths of the lacteal vessels, which open into the intestines, like a sponge, and by some power, not well understood, absorb that part which is fitted for assimilation, while the remainder is rejected as an excrement.
The lacteal vessels are furnished with valves, which allow a free passage to the chyle from the intestines, but prevent its return. The most inexplicable thing in this operation, is the power which these vessels possess of selecting from the intestinal mass, those substances which are proper for nutrition, and rejecting those which are not.
These lacteal vessels, as was before observed, pass through the mesentery, and their contents seem to undergo some important change in the mesenteric glands. The chyle which passes through vessels, appears to be an oily liquor, less animalised than milk, and its particles seem to be held in solution by the intermedium of a mucilaginous principle. It is conveyed along the thoracic duct in the manner already described, and enters the blood slowly, and, as it were, drop by drop, by the subclavian vein; in this way it becomes intimately mixed with the blood, and combining with oxygen in the lungs, it acquires a fibrous character, and becomes fit to nourish the body.
We have now seen how the process of digestion is performed, at least, so far as we are acquainted with it, and how its products are conveyed into the blood. But to what purposes the blood is employed, which is formed with so much care, we have yet to discover. It seems to answer two purposes. The parts of which the body is composed, namely, bones, muscles, ligaments, membranes, &c. are continually changing: in youth they are increasing in size and strength, and in mature age they are continually acting, and, consequently, continually liable to waste and decay. They are often exposed to accidents, which render them unfit for performing their various functions; and even when no such accidents happen, it seems necessary for the health of the system that they should be perpetually renewed. Materials must therefore be provided for repairing, increasing, or renewing all the various organs of the body. The bones require phosphate of lime, and gelatine, the muscles fibrine, and the cartilages and membranes albumen; and accordingly we find all these substances contained in the blood, from whence they are drawn, as from a storehouse, whenever they are wanted. The process by which these different parts of the blood become various parts of the body is called assimilation.