In 1837 a Frenchman named Dutrochet de scribed sonic remarkable experiments made by him. He found that if he took a tube open at both ends, one end being of a bulb shape, and if he closed the bulb-shaped end with a piece of animal membrane, a piece of bladder, for instance, then filled the bulb and tube with a strong solution of salt, and dipped it into a glass jar containing water, two currents were set tip through the membrane. A current of water passed from outside through the membrane into the salt solution, so increasing the quantity of liquid on that side that it rose in the tube. At the same time salt in solution passed through the membrane into the water outside and could be detected there very soon. Anyone can re peat this experiment for himself, and, pro vided the solution be strong enough, and the bulb be kept dipping in the water outside, the liquid will continue to rise in the tube of the bulb for days, so that many feet of tubing have to be added, end to end. At the close of the experiment a very large quantity pf the salt will be found to have passed into the water outside. The process by which the water passes through the membrane into the bulb is called endosmosis. Since the days of Du trochet many experiments have been made of a similar kind. Sugar and salts of all kinds are capable of producing the currents and of pass ing in solution through animal membranes. In stead of the water, a solution of salt may be placed in the jar outside, and, provided the strength of the outer and inner solutions is different, the same interchange will go on through the mem brane, or solutions of two different substances may be used with a like result. The general result may be put in this way : whenever there are two different solutions separated only by an animal membrane an interchange will take place between them through the membrane.
Now let this be applied. In the stomach and intestinal canal there is a quantity of liquid food, to a great extent a liquid containing many substances in solution. In the walls of the stomach and bowels there is flowing a stream of blood, another liquid containing many sub stances in solution. These two liquids are separated from one another by the extremely thin walls of the channels along which the blood flows, and by a thin portion of the wall of the intestinal canal, in short, by an animal membrane. The liquid in the alimentary canal oontains a much larger quantity of dissolved substances than the blood. The inevitable result will be that an interchange will take place between the blood and the contents of the stomach and bowels, resulting in the pass ing through the wall of the intestinal canal into the blood of the dissolved substances of the food. A continuation of the experiments of Dutrochet, especially those made by Graham, the late Master of the Mint, throws further light on this subject.
The purpose of experi ments show that while substances like sugar and salt readily pass through the animal mem brane, other substances, like albumin, starch, gum, and fat pass through with great difficulty, indeed hardly at all. Suppose into the bulb
closed by the animal membrane a solution of salt, starch, sugar, and white of egg be placed, and then the bulb dipped into the jar of water. After some time, if the water outside be in suf ficient quantity, all the salt and sugar will be found to have passed out of the bulb, but none of the starch or white of egg, which are still retained. Accordingly, when one takes a meal of bread and meat the contents of the stomach will consist of a liquid containing albumin ob tained both from the meat and bread, fat from the meat, starch and sugar from the bread, and salts of various kinds from both. The sugar and salt will readily pass through the animal membrane of the intestinal walls into the blood, but the albumin, fat, and starch never will. Yet as it is absolutely necessary that they also gain entrance to the blood, it is ob vious that they must undergo some change that will confer on them the power of passing through animal membranes. Now starch can be converted into sugar, and sugar can pass through membranes, and albumin can be con verted into a substance called peptone, which also can pass through membranes. The power of converting starch into sugar is possessed by the saliva from the salivary glands, and by the juice poured into the bowel from the pancreas (sweet-bread), and the power of converting albumin into peptone is possessed by the gas tric juice poured out from glands in the walls of the stomach and by the juice from the pan creas as well. Fat, again, cannot pass through a membrane, but fat and soda make a soap. Now the bile is an alkaline fluid, it contains a large quantity of soda salts. It mixes with the fat in the small intestine, and so saponifies it —makes it soap-like—that it becomes possible for it also to pass through a membrane. The story of digestion is practically this, then, that the food we take must get into the blood; but to get there it must pass through the walls of the stomach or bowel interposed between it and the blood. To pass through these walls it must first be made into a solution, so the food is broken down by the teeth and mixed with fluids poured into it from various glands. With no further change the salts, sugar, and similar substances can pass at once through the animal membrane into the blood, but the albumin, fat, and starch cannot: they are, therefore, acted on by the juices till they are converted into substances that can pass. The whole purpose of digestion is, therefore, to make the food into a condition that will enable its nourishing elements, albumin, fat, starch, sugar, and salts, to pass into the current of blood circulating in the walls of the stomach and intestines.
juices that the food is mixed with in the alimentary canal which transform it into absorbable material, are produced by glands, salivary, gastric, to be described, and are called secretions.