Castor oil is obtained by expression from the seeds of Rici-nus communis, or palma Christi, a plant which, in its native climate of the East indies and Northern Africa, has the character of a tree, but, as grown in extra-tropical latitudes, is usually annual. it is cultivated largely in certain parts of the United States, for the sake of its oil. The seeds, which are contained in a roundish three-celled, glaucous capsule, from which they are expelled when ripe, are of about the size of a small bean, oval, very smooth and shining, mottled on the surface, and with a small projection at one end, giving to them somewhat the appearance of a tick, from which the plant has derived its generic name. They are powerfully cathartic and often emetic; three of them being generally sufficient to operate on the bowels, and seven or eight acting with great violence. This property depends on an acrid principle which pervades the whole kernel, and is in some degree no doubt extracted with the oil.

Castor oil, though capable of being separated by decoction, or by the agency of alcohol, is at present obtained for medical use, at least in this country, exclusively by expression. After having been expressed, it is clarified by boiling with a little water, by which the albumen is coagulated, and any gummy matter dissolved, and which answers another good purpose by driving off a portion of the acrid principle, and thereby rendering the oil milder. it is very important not to carry the boiling too far; as, if altered by heat, the oil acquires acrid properties, which wholly unfit it for the special purposes it is calculated to fulfil.

Sensible and Chemical Properties. Castor oil is a thick, viscid liquid, colourless when quite pure, with little odour, and A taste at first mild though somewhat nauseous, but afterwards slightly acrid. As kept in the shops, however, it is not unfrequently more or less yellowish or brownish, with an unpleasant odour, and a much stronger and more nauseous and acrid taste. it ranks among the drying oils; those, namely, which become hard and translucent upon exposure. it is not congealed by cold, but sometimes deposits a whitish matter in winter, which is redissolved in warm weather. Though lighter than water, it is heavier than most other fixed oils, from which it differs, also, in being soluble in all proportions in absolute alcohol; by which property its purity may be tested. it is dissolved also by ether. its ultimate constituents are carbon, hydrogen, and oxygen; but its proximate composition has not been definitively settled. it is supposed, however, to contain two or more distinct fatty principles, which yield as many distinct acids on saponification. These acids are more or less acrid; and as they are also partially developed by heat, the injurious influence of this agent on the oil may be readily understood.

It has been made a question, whether castor oil depends for its purgative property on the acrid principle of the seeds, or upon its own inherent constitution. From the peculiarity of its cathartic operation, I am disposed to think that the latter is the case. Fixed oils generally are somewhat laxative; and there is no reason why castor oil may not possess the same property in a higher degree. it has been urged, in relation to the coloured and acrid specimens of the oil, that they are really preferable to the pure, because more powerfully purgative; but they are also more acrid, and it is the characteristic mildness of the oil that gives it peculiar value. We have numerous active purgatives, but none which combine, in an equal degree, blandness with efficiency. The purer, therefore, and the more free from smell and taste the oil can be obtained, the better is it calculated to answer the purposes for which it is used.

Medical Effects and Uses

Castor oil is characterized, as a cathartic, by promptness, gentleness, and efficiency. it is among those which operate most quickly on the bowels, not unfrequently producing its effects in two or three hours; though, in certain conditions of the alimentary canal, and in certain individuals, it is retained long in the stomach; and I have known it to be thrown up twenty-four hours after it was taken, without having affected the bowels at all. Dr. Ward mentions the case of a woman in whom, instead of acting as a cathartic, it appeared to be absorbed, and exuded from all parts of the surface. (Lond. Med. Gaz., x. 311.) its operation is usually attended with little pain or uneasiness of the bowels; and the discharges obtained by it are mainly the feculent and liquid contents of the alimentary canal, with which the oil itself is generally to be seen mixed in various states. it is, therefore, inferred to act chiefly, if not exclusively, by promoting the peristaltic motion. it not unfrequently nauseates, and is sometimes thrown up from the stomach. This is particularly the case with the acrid oil; but it happens also with the purest. The vomiting, I believe, is generally ascribable, not to the direct influence of the oil on the stomach, but to associations previously made of its taste with the somewhat nauseating effect of former doses; so that the very thought of it becomes offensive. I have known individuals who had this repugnance so strongly that, though they might compel themselves to swallow the oil, it was rejected instantly by a sort of spasmodic effort, before it had the opportunity of making any impression whatever on the stomach. By young children it is almost uniformly retained in the ordinary state of the stomach; and it is very seldom rejected by adults, in a similar condition of that organ, who have not formed the nauseating association referred to. in regard to its mode of operation, I believe that it is exclusively local. This is to be inferred from the rapidity of its action when it does act; and when not, from its being retained so long in the stomach, showing that it is not absorbed. it is said, however, when thrown into the veins, to produce griping and purging, and to cause an oily taste in the mouth. (Pereira, Mat. Med., 3d ed., p. 1291.)