This section is from the book "Materia Medica And Therapeutics: An Introduction to the National Treatment of Disease", by John Mitchell Bruce. Also available from Amazon: The pharmacology and therapeutics of the materia medica.
a. Drastics.-These cause the vessels to dilate, and retard the blood current, so that the fluid and part even of the solid constituents of the blood escape into the walls and cavity of the bowel. In other words, they establish an inflammation of the mucosa, somewhat resembling a common "cold" in the nose. The result is similar in the two cases: there is a profuse discharge from the mucous membrane, of the watery part of the blood, with a certain amount of solid elements, constituting a "catarrh," and producing in the case of the bowel a very liquid stool. The drugs which act in this way are obviously powerful or even dangerous, and comprise chiefly Croton Oil, Elaterium, Gamboge, and Colocynth. They constitute a group of purgatives known as drastics ( I act) or drastic cathartics.
b. Intestinal Astringents.-Opposed to these measures we possess certain substances which contract the walls of the intestinal vessels, reduce the quantity of watery exudation, prevent the escape of solid elements, and thus diminish the liquidity of the fasces. Such substances, include Lead, Silver, and the Diluted Mineral Acid, and constitute the first group of intestinal astringents, called intestinal vascular astringents.
c. Intestinal Constringents.-The substances thus named possess the property of coagulating or otherwise condensing the gelatiniform and albuminous tissue supporting the small vessels of the mucosa, increasing its compactness, diminishing the freedom of the circulation, and thus reducing the amount of exudation through the vessel walls. Intestinal Constringents are a very large group, including Persalts of Iron, Alum, Sulphate of Copper, Oxide of Zinc, Tannin, and the numerous vegetable products which yield it or some of its modifications, such as Catechu, Kino, Krameria, and Cinnamon.
Measures Which Influence Absorption And Excretion. a. Saline Purgatives.-Certain salts possess the property of greatly disturbing the process of osmosis in the intestinal wall, such as the Sulphates of Magnesia, Soda, and Potash; Phosphate of Soda; Tartrate, and Acid Tartrate of Potash; and the Tartrate of Soda and Potash. These produce two effects, namely, first, increased flow of water from the intestinal vessels into the cavity of the bowel, and consequently increased liquidity of the stools: and secondly, a flow of the salts, with a certain amount of water, from the cavity of the bowel into the blood-vessels, whence it is partly carried away into the general circulation, and partly again excreted into the bowel by the intestinal glands, once more to be absorbed. The result is an abundant liquid stool; in the case of Acid Tartrate of Potash, or very large doses of the other salts, almost entirely watery. The precise way in which these effects are produced by saline substances is still obscure. They appear to be due in part to the difference in specific gravity between the watery materials in the bowel and the liquor sanguinis, in part to some specific action of the salts upon the structures of the walls through which they pass, depending on their chemical constitution and affecting dialysis. According to some authorities, saline purgatives act in a measure by stimulating peristalsis.
These salts furnish us with a ready means of increasing the liquidity of the motions and the frequency of the stools, and constitute the group called saline purgatives, the most powerful of which are called hydragogue salines.
Saline Astringents.-A sufficient amount of salts, and (within broad limits) a particular strength of solution, are required to secure an abundant excretion; otherwise their absorption in watery solution is stimulated beyond their excretion, and constipation instead of relaxation is the result. The same effect is liable to be produced by their habitual employment. We do not use this group of measures therapeutically.
Measures Which Influence The Intestinal Glands. a. The secretions of the intestinal glands are moderately increased by Mercurial preparations; greatly increased by Croton Oil, Elaterium, Colocynth, Jalap, Scammony, and Podophyllin, which no doubt act also upon the vessels and muscles. Jalap and Scammony require to be dissolved in the bile. We have just seen that the saline purgatives are also glandular stimulants, being no sooner absorbed than they are again excreted. This class of purgatives may be called cathartics ( I cleanse); such of them as produce very watery motions, hydragogue cathartics.
b. Opium, Lead, and Lime directly diminish the intestinal secretions and promote constipation. Alkalies, Alkaline Earths and their Carbonates interfere with the acidity of the chyme when given in full doses, and thus indirectly arrest the intestinal secretions; whilst, by conversion into sulphates in the bowel, they may become active purgatives. Thus certain saline substances may not only be purgative in more than one way, but may even be purgative and astringent at the same time ; the one effect or the other occurring according to the dose, the patient, and other circumstances which are often obscure.
Measures Which Influence The Nervo-Muscular Structures. Many of the materiae medicae influence the bowels through the muscular coat, the nerves, or both. Thus drastics excite intestinal peristalsis and griping even before they have left the stomach, as is seen in Croton Oil. Saline purgatives are believed to have the same effect. It is practically convenient to arrange in a special class those substances which act entirely or chiefly upon the intestinal muscles.
a. Nervo-muscular Stimulants. - These include Rhubarb, Senna, Aloes, Castor Oil, Sulphur, Sugars, Nux Vomica, Rham-nus Frangula, Cascara Sagrada, and Belladonna, and many others. They are best given with carminatives, to prevent the intestinal pain caused by excessive or spasmodic muscular contraction, popularly known as "griping," which they readily induce. Belladonna appears to act in a different way from the others, by removing the inhibition of the splanchnic; and ergot by causing anaemia of the muscles. The stool which follows the action of a muscular stimulant is much less watery than that produced by saline or cathartic purgatives, being chiefly the ordinary contents of the small bowel hurried down, unless the drug be given in large doses. For the same reason the disturbance of the portal circulation, liver, the general circulation, and the system as a whole, is less marked. The nervo-muscular purgatives are commonly known as simple purgatives; and the mildest of them, such as Castor Oil and Sulphur, Figs and the like, are classed by themselves as aperients (aperio, I open), or laxatives (laxo, I loose), as inducing a simple opening or relaxation of the bowels.