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 are the seeds of Sinapis alba, and will be more particularly described hereafter. They are noticed here merely in reference to their laxative property. They are of a roundish-elliptical shape, about as large as a middling sized shot, yellowish, inodorous, and of little taste when unbroken. Their exterior coating abounds in a mucilaginous matter, which is extracted by hot water. in the dose of a tablespoonful, they operate as a mild laxative, having a slightly stimulant influence on the mucous membrane, on account of which they have been recommended, and considerably used, in habitual constipation with a torpid or languid condition of stomach and bowels, as in dyspepsia, feeble gouty, rheumatic, and paralytic cases, and often in old people without special disease. it is possible that they operate in part by a dynamic property, stimulating the alimentary canal; but their effects are chiefly ascribable to the mechanical cause above referred to. They may be taken in molasses, or after immersion for a short time in hot water, by which the mucilage contained in their outer coating is drawn to the surface, and thus renders them slippery. if they should not operate, the risk is that they may accumulate in the bowels, and produce injury through obstruction, or by exciting inflammation. They are said to have proved fatal in the latter mode. Should, therefore, any considerable quantity have been taken, either without operating, or with insufficient operation on the bowels, they should be thoroughly evacuated by castor oil, or one of the saline cathartics. Cullen is said to have been the first writer who called attention to this use of mustard seed.
Other seeds would probably have a similar effect; and I have been informed that flaxseed are used by some habitually as a laxative;. but the same objection lies against them all, of liability to accumulate in the bowels, and especially in the caecum or appendix, producing obstruction, or inflammation, ulceration, etc.