The Action of Medicines in the System / Or, on the mode in which therapeutic agents introduced into the stomach produce their peculiar effects on the animal economy

by Frederick William Headland

Under the first proposition I have shown that the majority of medicines are absorbed into the system. It is now seen where and how they are absorbed. But it still remains to be seen what particular substances or classes of medicaments are thus taken into the blood, and whether while so passing they undergo any modifications.

The following, then, are the chief classes which I believe to undergo solution and absorption in the intestinal canal. They comprise the great majority of all substances used either as food or medicine:—

1. Mineral substances soluble in water.
2. Mineral substances soluble in acids.
3. Mineral substances soluble in alkalies.
4. Vegetable products soluble in water.
5. Animal and vegetable substances rendered soluble by the gastric juice.
6. Fatty and resinous substances soluble in alkali.

1. All soluble mineral substances—acids, alkalies, salts, and soluble compounds of the earths and metals—are absorbed in the stomach and intestines. The conditions and the process of absorption have been already stated. When solid they are first dissolved, when in a dense solution diluted down by the gastric fluid before they are absorbed. Thin watery liquids are quickly taken up. Motion on one side, and pressure on the other, hasten the process. I am not inclined to make any exception to this rule. This leads me to make some remarks on the Endosmotic theory of the action of saline purgatives.

M. Poisseuille found that the serum of the blood would pass through an animal membrane to a solution of Sulphate of Soda of greater specific gravity. He thus concluded that an ordinary saline purgative of greater specific gravity than serum (1.028) acted by causing the passage of this serum outwards into the intestine, producing a watery evacuation. The theory was taken up by Liebig and Matteucci, and by Laveran and Millon. Dr. Pereira and Dr. Golding Bird have drawn some practical inferences from it.

It is said that when a saline is absorbed it produces diuresis, and no purging; that when it purges it is not absorbed; that when given to act on the kidneys, the solution should be so diluted as to contain less than five per cent. of the solid; and when administered as a purgative, the specific gravity should be considerably above 1.028. Now I do not deny that a weak solution is more likely to pass off by the kidneys, and a dense one by the bowels; but I consider this due to the fact, that the kidneys are the proper emunctories of water, and of such matters as are soluble in it. But I believe that these solutions are equally absorbed, both heavy and weak, and that the purging or diuretic effect depends mainly on the quantity of the salt, as the kidneys will not eliminate beyond a certain amount of it. I must allege, to support this view of the question, firstly, some arguments à priori; and secondly, some experiments which I have made to satisfy myself. To consider first the former. Analogy is certainly against this theory. Other purgatives from the vegetable kingdom produce their effect when injected into the blood, and are yet often powerfully hydragogue. What effect on the process of endosmosis can be exerted by Castor or Croton oils, or by calcined Magnesia? On turning to what we know of the theory of digestive endosmosis, we find that the solid parts of the food are diluted down so as to be absorbed, and that the influence of pressure is to be taken into calculation. It was laid down by the great Boerhaave that a necessary requisite for the formation and absorption of chyle was the contractile force of the viscera. (Van Sweiten's Comment, vol. i. p. 290.) This, in fact, would press the fluid into the absorbent vessels, even if against the other endosmotic forces. Further, a fundamental principle was insisted on by Dutrochet—that even when the liquids on the two sides varied in density, they would both pass through to each other, though in different proportions. The ultimate result of this would be, that a heavy saline solution would be diluted down to the specific gravity of the serum, and would then pass through. This point I have illustrated in my first experiment. Thus it seems that my view is favoured by these physical laws. But even if it were not so, I should not be inclined to admit that all the endosmotic processes in the living body were regulated by merely physical tendencies. This cannot be the case with some of the secretions. The bile is of higher specific gravity than the serum of blood. Urine in health is lower, but in diabetes often much higher. The production of these secretions, differing in density from the serum of blood, would be inconsistent with the first law of endosmosis. Lastly, we find that in many known cases the theory which I am contesting does not hold good. Seidlitz water and sea-water are both known to be purgative. In both the specific gravity is below that of the serum of blood.

The first, according to Bergmann, has a density of 1.006. It contains in each pint 192.8 grains of solid matter. Out of this, 180 grains consist of Sulphate of Magnesia.

Sea-water has a specific gravity of 1.026, and its solid contents are about 3.5 per cent. But Dr. G. Bird says that less than five per cent. of saline matter constitutes a liquid diuretic.

These, then, are strong reasons against this idea considered merely as a theory. But it can readily be put to the proof. With this purpose I have made the following experiments. As far as they are concerned, they seem to show that salines are in all cases absorbed, and that whether they are subsequently excreted through the kidneys or by the bowels depends more on the quantity administered than on the degree of dilution of the dose.

Exp. 1.—A sufficient quantity of white sugar was dissolved in four ounces of water to raise its specific gravity to 1.028, that of the serum of the blood. Two drachms of sulphate of magnesia were dissolved in another ounce of water, so that the solution measured nine fluid drachms, and had a density of 1.060. This heavier solution was introduced into an open wide tube, closed completely below by a clean piece of bladder. It was introduced into a small vessel containing the solution of sugar, and arranged so that the height of the two liquids should correspond. After three days, the inner solution measured two drachms more, and the specific gravity had sunk to 1.050. The outer solution, after making up exactly the loss by evaporation, was found to have risen in density to 1.040. On adding a small quantity each of the solutions of phosphate of soda and carbonate of ammonia, a copious precipitate took place, indicating the presence of magnesia. Thus it appeared that the fluids passed both ways, some of the heavy solution of magnesia finding its way through to the lighter solution of sugar. The tendency of this process was evidently to an equalization of their densities, both by endosmose one way and by exosmose the other. Thus, apparently, would it be with a saline purgative, and with the serum of the blood.

Exp. 2.—Three drachms of Sulphate of Magnesia (a very mild purgative dose) were dissolved in ten ounces of water, and thus administered to a healthy young man. It produced, after some time, slight purging, and some diuresis. The urine, when tested, contained only a very little more than the usual quantity of magnesia. The quantity in the dose was less than five per cent. of the solution, and thus, according to the endosmotic theory, should have produced no purging.

Exp. 3.—Six drachms of the same salt were given in eighteen ounces of water to the same patient. After a few hours, it caused very copious and long-continued watery purging. The urine did not seem to be increased, and contained no excess of magnesia. It seemed, that in spite of the dilution, the quantity of the salt was so large, that it could not pass off by the kidneys, and so was eliminated from the blood by the bowels, in the same way as other purgative medicines. (Vide Chap. IV.)

Exp. 4.—This trial was the reverse of the last. Two scruples of Acetate of Potash were dissolved in three drachms of water, and thus administered. The solution then contained about twenty per cent. of the salt. According to the endosmotic theory, it should have caused only slight purging, on account of its density. It did not do so, but produced diuresis. The dose was so small, that after absorption it was easily eliminated by the kidneys.