These experiments are in favour of my views, and seem, as far as they go, to cast discredit on the theory of Poisseuille. It will be very easy to multiply them, so as to place the fact beyond a doubt.[24]
This point being so far settled, and there being no physical reason why any other soluble mineral substance should not be absorbed—it being, moreover, proved, by repeated experiments, that they all pass into the blood, we therefore proceed to the second class of bodies capable of absorption.
2. Mineral substances soluble in acids.—It is apparent that if any insoluble substance will admit of easy solution in the fluids of the stomach and intestines, it may be brought to the condition of a soluble one and thus absorbed. Now the gastric juice is acid. This secretion has been variously ascribed to free hydrochloric, acetic, phosphoric, and lactic acids; also to an acid phosphate or lactate of magnesia. There seems to be most reason to conclude that it is lactic acid, which would be easily formed out of the constituents of the food.[25]
But we have now only to regard the gastric juice as containing a free acid. This would reduce to solution the carbonates and oxides of all the metals; less perfectly when anhydrous, more easily when in the hydrated form. In this manner are absorbed the oxides of Iron and Silver, the carbonates of Lime and Magnesia, and other medicines of the same kind. They would be slightly modified, being absorbed in combination with the (lactic) acid of the stomach. In this they would differ somewhat from the substances already soluble, which would probably be absorbed unchanged, unless we except the alkalies and their carbonates, which might be similarly decomposed. Even these, if given in solution, would probably be absorbed faster than the gastric acid could be secreted to neutralize them.
Other insoluble substances, such as calomel and metallic sulphurets, would be more or less dissolved by the action of this acid. But as a great part would necessarily escape its influence, being propelled onwards by the muscular contraction of the stomach, it follows that insoluble substances of this kind are much less active as medicines than similar compounds introduced in a soluble state. It is also likely that a certain quantity would be sufficient to exhaust for a time the acid gastric secretion.
We now pass on to a third class of mineral products, which, though insoluble both in water and in acid, yet, being soluble in alkalies, appear to be in this way prepared by absorption.
3. Mineral bodies insoluble in water and in acid, but soluble in alkalies.—These remain unaffected by the action of the gastric juice, but on passing on as far as the centre of the duodenum, they meet with two alkaline fluids. These are the bile and the pancreatic juice. We know that some such insoluble bodies pass into the blood. It will be shown in the proof of the next Proposition, that they cannot do so without being first dissolved. Therefore, there must be in the intestinal canal something capable of thus dissolving them. For this purpose these two alkaline secretions are well adapted. As an example of such mineral bodies, of which there are not many, I may adduce sulphur. It unquestionably passes into the blood, and is found there in combination with alkalies, as a sulphuret or sulphate. It has been detected in the urine of those who were taking it, by Dr. Ronalds and others. It is possible that iodine, bromine, and some of the hydrated metallic oxides; may enter the blood in the same way.
4. Vegetable substances soluble in water.—These would be absorbed with more or less readiness according to their degree of solubility. They would probably be taken up in great part by the stomach. Some vegetable matters—as Lignine, or woody fibre, and Resins—are insoluble in water. Starch even is comparatively insoluble, and thus comes under another division, being probably one of those vegetable substances which are rendered soluble by the aid of the pepsin of the gastric juice.
These soluble vegetable matters are of several kinds. Cane-sugar and grape-sugar are readily soluble. So also are the various vegetable acids. There is another class of bodies which is highly important in a medical point of view. These are the natural salts of the vegetable alkaloids, of which are constituted so many of the active principles of plants. In pharmaceutical operations we are enabled, by the judicious employment of different menstrua, to extract from the crude and inert mass these vegetable active principles, and thus to obtain in a concentrated form the medicinal power for which each plant is esteemed. This process is also performed in the stomach. By digestion and concoction, with or without the aid of acid, it is enabled to dissolve out these soluble and potent matters from the ligneous and bulky tissues which surround them. For though the alkaloids themselves are in general almost insoluble in water, yet their natural salts which occur in the vegetable kingdom are mostly very soluble. The most important of these salts are as follow. In Cinchona bark, the Kinates of Quina and Cinchonia. In Opium, the Bimeconates of Morphia and Codeia. In Nux Vomica, the Igasaurate of Strychnia. In Aconite, the Aconitate of Aconitina. In Colchicum and Sabadilla, the Supergallates of Colchicia and Veratria. There are some neutral soluble substances, not partaking of the nature of Alkalies. Thus we find in Ipecacuanha and Ionidium, Emetine; in Tea and Coffee, Caffeine; in Willow-bark, Salicine; and the soluble active principles of Senna, Aloes, and Gentian, are probably of the same nature. Many other alkaloids are known. Atropia in Belladonna, Daturia in Stramonium, and Hyoscyamia in Hyoscyamus, occur in combination with Malic acid. Conia in Hemlock, and Nicotia in Tobacco, are peculiar volatile alkalies containing no Oxygen: thus in two respects they resemble Ammonia. They too are soluble in water.