or, Oil of Turpentine, with the addition of three atoms of Oxygen, produces Pinic acid, i.e. Resin, and an atom of water.
Now Turpentine, when given in large quantities, irritates the surface of the intestinal canal in man, and is not absorbed. But in the horse very large doses are found to pass through the system into the urine. It is not likely that a large quantity should enter in solution in water. It seems more probable that it may first in some way become oxidized, and then dissolved as a resin. The action of turpentine resembles that of some other substances which contain resin, as Copaiba and Balsam of Peru.
The resins thus dissolved would pass, like other solutions, through the mucous membrane of the intestines into the Portal capillaries.
But of the fats and fixed oils it cannot be said to be proved that they are absorbed in a state of solution, although such a conclusion is almost forced upon us by a consideration of the laws of endosmosis. They do not pass into the veins, but are taken up by the lacteal absorbents. They are capable of solution, and are thus not in that sense an exception to the rule of Prop. II. But in another sense they are an exception to it; for they do not pass directly into the veins, but through the lacteal system. It seems that the sole purpose of these lacteal vessels is to absorb fats. Thus it appears that all soluble substances, whether in the food or given as medicine, and in whatever manner rendered soluble, whether by acid, by alkali, or by stomach digestion, are absorbed in the stomach and intestines. All of them, with the exception of fatty matters, pass directly into the blood, traversing the mesenteric and Portal veins, to reach the liver. From this organ they pass on into the heart through the Vena cava inferior. I have shown also that they are mostly absorbed without material change. Supposing the stomach acid to be lactic, it would be too weak to displace mineral acids. It would, however, decompose a few insoluble matters, and combine with alkalies and their carbonates, forming salts which in the blood would again change into carbonates. (Vide Prop. VI.)
Prop. III.—That those medicines which are completely insoluble in water, and in the gastric and intestinal juices, cannot gain entrance into the circulation.
It may at first sight be objected to this proposition, that fatty matters may probably enter the lacteals in an undissolved state. But this is not proved; and besides, whether dissolved or not, we know that they are soluble in one at least of the intestinal juices—viz. the Bile. So that they do not come under the above definition.
We have just seen that many medicines which are given in the insoluble form are capable of being dissolved in the fluids of the intestinal canal. This so much reduces the list of perfectly insoluble medicines, that it is difficult to find any that come under such a definition. But Charcoal, the simple metals, woody fibre, and Nitrate of Bismuth, will serve as examples.
Sulphate of Lead is often quoted as perfectly insoluble; but this is not the case. It is soluble in a solution of acetate of ammonia. This salt is contained in the perspiration. Thus the sulphate, when substituted for the carbonate in some lead works at Paris, proved fatal to the foreman, who died of colic. M. Flandin found that it poisoned a dog when rubbed into the skin as ointment. Even some metals may possibly be brought within the influence of weak acids when in a fine state of subdivision, as Mercury in blue-pill. Gold in a very fine powder has been used successfully in syphilis.
Thus the list of insoluble substances is still further reduced. But there is no doubt that many substances which are slightly soluble in the intestinal fluids may in great part escape this solution, and pass out with the fæces just as they went in.