This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
1. Through an impression on the mucous membrane, they call the peristaltic movement of the bowels into greater activity. in this respect they only imitate the normal action of the contents of the alimentary canal, through the influence of which upon the mucous surface, propagated to the muscular coat, the healthy evacuation of the bowels is effected. How it is that this impression on the mucous surface affects the muscular layer, whether through a direct propagation by contiguous sympathy, or through the spinal or ganglionic nervous centres, is not certainly known; but, from the regularity of the combined peristaltic movements, and from the association with them of the action of the abdominal muscles, when the evacuation takes place, it is highly probable that the organic nervous centres are concerned. To a certain extent, a direct stimulation of the muscular coat may be effected by mechanical weight or distension from the alvine contents; and it is probable that purgation may sometimes be produced or aided in this way; but that such an influence cannot be the chief cause is proved by the simple fact, that the cathartic often acts without adding anything appreciable to the weight or bulk of the contents of the bowels.
2. A second and very frequent method of action is by increasing the exhalation of serous liquid, and the secretion of mucus into the primae viae. The liquid thus poured out into the bowels stimulates the peristaltic movement, as the cathartic is itself supposed to do, according to the first method of operating.
3. Certain cathartics appear to operate mainly by stimulating the liver, and thus increasing the secretion of bile. it is probably through this secretion, operating on the mucous membrane, that the regular healthful action of the bowels is chiefly sustained. That the bile is actively purgative is proved by the fact that, when secreted in excess, it almost always occasions either cholera morbus, or bilious diarrhoea. Medicines, therefore, which stimulate the hepatic secretion, must necessarily act as cathartics, and I believe it is in this way that the mercurials mainly operate. it is supposed that a similar effect is produced by an increase of the secretion of the pancreas, but we have no satisfactory proof of the fact.
4. Still another method is probably through absorption. We know that certain cathartic medicines are either wholly or in part absorbed, and enter into the circulation. Thus rhubarb and gamboge impart colouring matter to the urine; sulphur exhales, in the form of sulphuretted hydrogen, from the surface of those who take it freely; and the purgative principle of senna is said to be secreted with the milk. But a still stronger proof is, that several of the cathartics, as senna, colocynth, and elaterium, operate on the bowels, when brought into contact with the serous or cellular tissue, or injected into the blood. Aloes operates upon the same portion of the bowels, and in the same characteristic method, whether taken into the stomach, injected into the rectum, or applied to a blistered surface denuded of the cuticle. Whether, after entering the circulation, they act directly on the muscular coat of the bowels, or first on the mucous coat or nervous centres, and secondarily on the peristaltic function, is uncertain; but as, in either mode of administration, the drastic substances irritate or inflame the lining membrane, it is probably upon this that the primary impression is made, as well when they operate through the circulation, as when by immediate contact.
Certain cathartics act exclusively in some one of the preceding methods; but the greater number, though they may be disposed to act in one rather than another of them, conjoin two or more methods in a greater or less degree.
Different individuals of the class act on different parts of the alimentary canal. Thus, gamboge, which is apt to vomit, operates probably more especially on the stomach and upper portion of the bowels; aloes is well known to direct a peculiar influence to the large intestines, including the colon and rectum; while the neutral salts, castor oil, senna, etc., if they have any preference, affect specially the small intestines, though probably operating also, in some degree, on the whole extent of the primae viae. This tendency to one part of the bowels rather than another has been ascribed to the different degrees of solubility in the acting principle of the medicine; the most soluble operating on the upper, and the least so on the lower portions of the track. it is probable that a medicine of very difficult solubility in the liquids of the stomach and bowels, whether originally so, or rendered so by the manner of preparing it, might pass the upper portion unchanged, and thus act more especially on the lower; but the fact is that, in relation to most of the cathartics in which this peculiar tendency is observable, there is no such peculiarity of solubility Thus, the active principle of aloes, which operates on the lower bowels, is readily soluble; and it shows the same disposition, whether given in pill, or in solution. Nay, even when applied to the denuded surface of the skin, it is asserted still to operate preferably on the rectum and colon. The result, then, must be ascribed to some other cause than any mere modification of solubility. There is nothing remarkable in the fact. Some medicines have a tendency to act especially on the kidneys, some on the skin, some on the brain, etc. Why should there not be a similar election in relation to different portions of the alimentary canal ? So far as any explanation can be given, in the present state of our knowledge, the fact must be ascribed to different susceptibilities of the several portions of the canal, arising from peculiarity of intimate constitution. The stomach and duodenum are so constituted as to be especially susceptible to the action of gamboge, while they do not feel the influence of aloes; and exactly the reverse may be said of the colon and rectum.