Alcoholic and Ethereal fluids may be enumerated here as soluble products of the vegetable kingdom. Alcohol mixes with water to any extent; and one part of Ether is soluble in ten parts of water. Nitric Ether and Chloroform are also sufficiently soluble. So is Creosote, 1.25 parts of which dissolve in 100 of water. But this may be absorbed in another way, as will be seen presently. Volatile Oils and Turpentine come also under this head. They are all slightly soluble in water. The former, when given in small doses, are probably absorbed in this way. Turpentine, when given in large doses, may perhaps, by undergoing a change, come under the head of vegetable substances dissolved by alkalies. Camphor may be included here: one part is soluble in 1000 parts of water.
The soluble gummy matters of plants, when added to the substances enumerated above, constitute an aggregate which is called the watery extractive of a vegetable product: i.e. that part which is capable of being dissolved out of it by pure water.
But there are other active parts of vegetables, such as oily and resinous matters, and some neutral acrid principles, which, before they can be dissolved by water, require the aid of an alkali. These will be considered separately.
We now arrive at a fifth class of matters which are taken up by absorption.
5. Animal and vegetable products dissolved by the gastric juice.—The most important of these are the nitrogenous and nutritive constituents of the flesh of animals and of the parts of vegetables. Albumen, Glutine, Fibrine, and Caseine, are connected together as compounds of Proteine. Animal Fibrine, and the analogous Glutine of vegetables, are quite insoluble in water. Albumen and Caseine, though soluble, are immediately precipitated by acids. This is known to be at first effected by the gastric juice on their entry into the stomach. But the action of the gastric juice which contains an acid, and a peculiar nitrogenous material called Pepsin, —together with the temperature of the body, which is about 100°,—causes at length the gradual solution of these previously insoluble matters. This is found to take place out of the body when the above conditions are imitated with an artificial gastric fluid. The result of the process is a viscid fluid, which is then absorbed. The hard Gelatine of gristle and bone is not soluble in water at this temperature, but is readily soluble in the acid gastric juice. The Pepsine seems to be an important agent in this process, for an acid by itself is found to produce an imperfect solution. The nitrogenous matters thus digested and absorbed constitute that portion of the food which is of most use in the nutriment of the system; for the starchy compounds cannot be appropriated to the more solid tissues, although in some cases they may be converted into fat, as in herbivorous animals. (Liebig's Animal Chemistry, p. 113.)
Starch itself is one of this class. Tiedemann and Gmelin found that by the action of the gastric juice it was slowly converted into Dextrine, which afterwards changed into grape sugar. Both of these are very soluble. From the researches of Bouchardat and others, it appears that other fluids, as the Saliva, the secretion of Brunner's glands, and the Pancreatic juice, possess also this power of converting starch into a more soluble compound. So that if any of it escape the action of the stomach-secretion, it is probably reduced to solution and absorbed in the small intestine.
This change of starch is the first of a series of transformations, now ascertained, the ultimate result of which is its combustion and resolution into carbonic acid. Thus the nitrogenous compounds are called the nutritive, and the starchy materials the calorifacient elements of the food. Considered as medicines, these substances belong to the division of Aliments.
6. We have already considered some few mineral substances which are absorbed by the aid of the free alkali contained in the Biliary and Pancreatic secretions.[26] The sixth kind of absorbed matters consists of some vegetable and animal products which can only be rendered soluble by a similar agency. Fats and oils, resinous matters, and some principles resembling resins, come under this head. Fats and fixed oils consist of acids, as Stearic, Margaric, and Oleic, insoluble in water, in combination with a base, Glycerine, which, when isolated, is soluble. With a free alkali such a fat forms a soluble salt, called a soap, and the base Glycerine is set free.
In Man the Bile and Pancreatic juice are discharged together into the middle of the Duodenum. The fatty matters of the food are not absorbed before they meet with these secretions. But, after they have mixed with them, a milky fluid, called chyle, is formed, which is then taken up by the lacteal absorbents. It passes thence into the Thoracic duct, meeting there with an albuminous lymph, and is discharged at length into the general circulation at the junction of the left jugular and subclavian veins.
M. Bernard, in some papers laid lately before the French Academy, states, as the result of his experiments, that the function of the Pancreatic juice is to reduce the fat to the condition of a white emulsion. He states that no milky chyle is formed when the Pancreatic ducts are tied in dogs. He considers it absolutely necessary that saccharine and albuminous matters should be absorbed by the capillaries of the Portal veins, and then pass through the liver; and believes that the sole function of the lacteals is to take up fat thus emulsified. His experiments and inferences have received the high sanction of M. Magendie.