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.
These have already been defined. They are indicated generally whenever the object, in the administration of cathartics, is simply to keep the bowels open, so as to imitate the healthful condition as nearly as possible. They are essentially bland in their operation; differing in this respect from most of the purges and drastics, which, however the dose may be diminished, even though the amount of purgative effect may be less than that from the laxative, exhibit generally some evidence of an irritant character, however slight. Thus a grain or two of aloes will often produce a stool; but a slight sense of irritation may be felt in the rectum during the operation, while an equivalent quantity of manna, magnesia, or sulphur may act in a manner, not distinguishable in this respect from a healthy natural opening. Hence the importance of the laxatives in those affections in which there is any existing irritation of bowels, especially of the rectum.
The laxatives may be subdivided into two sets, one operating physically or mechanically, the other dynamically.
Substances consisting of coarse grains, or of minute irregular fragments, not capable of solution in the gastric juice, often by their mere shape become a mechanical source of irritation to the alimentary mucous membrane, sufficient to stimulate the muscular coat into increased action. They may be considered as acting in the same manner as sand acts on istered by the rectum. A fluidrachm of the camphorated tincture of opium will generally be found to answer the purpose.
the conjunctiva, though with infinitely less severity. Again, there are others which appear to operate mainly by their weight, putting the bowel somewhat as it were on the stretch, and stimulating the muscular fibre to contract, by the very tendency to lengthen it. Mustard seed are an example of the first, fluid mercury of the second. I shall treat first of the former set, and afterwards of the latter.