This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
(a) The fixed oils and fats are mixtures of the three bodies, olein (liquid), palmitin (semisolid), and stearin (solid), or close relatives of these, and in addition usually small amounts of other bodies. Olein, palmitin, and stearin are compounds of glyceryl, C3H5≡, with radicles of the various fatty acids. With alkalies they form soaps and glycerin. Castile soap, for example, is made by the action of sodium hydroxide on olive oil, which is nearly pure olein:
The oils differ from the fats only in the relative proportions of these basal ingredients, the oils having more of the olein, which gives them a liquid consistence at ordinary temperatures, and the fats more of the stearin and palmitin, which make them solid or semisolid.
The fats and fixed oils have a greasy feeling and are nonvolatile, so that they leave a permanent grease-spot. They cannot be distilled, for by heat they are decomposed, with the generation of disagreeable acrid vapors (the familiar odor of burning grease). They are insoluble in water and alcohol (except castor oil and croton oil, which dissolve in alcohol), and are readily soluble in ether, chloroform, and benzin. They are almost all bland, non-irritating substances with nutrient and emollient properties; but on exposure to the air they gradually become rancid by the liberation of odorous and irritating fatty acids. Linseed oil (oleum lini), if exposed to the air in thin layers, will dry like varnish, but most of the oils are of the non-drying type. A few of the fats and oils are of animal origin, e. g., butter, lard (adeps), tallow, suet (sevum), and cod-liver oil (oleum morrhuae); but the majority are of vegetable origin, as almond, cottonseed, cocoanut, linseed, olive and peanut oils, and cocoa-butter. These are found chiefly in seeds or in fruits, the best qualities being usually obtained with the least compression necessary and in the cold; the poorer qualities by expression between heated plates. They may also be extracted by a suitable solvent, such as benzin, which is afterward removed by distillation.
Cocoa-butter or cacao-butter (oleum theobromatis) is obtained from chocolate-seeds by compression between hot or cold plates. The fat is the cocoa-butter, and the residue constitutes "cocoa." This fat has a very slight odor and taste of chocolate, is firm and rather brittle at ordinary temperatures, melts at the temperature of the body, and does not readily become rancid. It is used as a basis for the manufacture of suppositories, these retaining their shape at ordinary temperatures and quickly melting when inserted into a body orifice, such as the rectum.
Castor oil (oleum ricini) and croton oil (oleum tiglii) differ from the other fixed oils in being soluble in alcohol and in possessing special cathartic properties. (See Part II.) Castor oil is sometimes added to alcoholic hair lotions to prevent drying of the scalp (about 10 minims to a 3-ounce bottle).
Glycerin (glycerinum) is a product of the saponification of fats or fixed oils. (See Reaction, page 29;) It is thick and viscid, has a sweet taste, mixes freely with water and alcohol, and has great affinity for water. It has extensive employment in pharmacy as a solvent, as a softening agent and preservative, and as a means for increasing the viscosity of liquids.
Applied in concentrated form to mucous membranes, it is astringent, causing the superficial cells to shrink by abstraction of water. For this reason it is used as an application to a relaxed uvula or pharynx. Diluted with water or rose-water, as in "rose-water and glycerin" (two parts to one) and in "calamine lotion" (see Zinc Carbonate), it is used upon the skin as an emollient, serving to prevent the drying of the epithelium. With lemon-juice or rose-water it is also used as an application to the dry tongue of fever patients. In mixtures for internal use it serves as a sweetening agent and is slightly laxative. In diabetes it tends to increase the glycosuria. For use in the rectum as a mild irritant and lubricant it may be added to an ordinary enema, or used in the form of glycerin suppositories (suppositoria glyc-erini), which hold 95 per cent. of glycerin. To soften hard feces, 1/2 ounce (15 c.c.) may be added to half a pint of soapsuds. Hertz, in "The Sensibility of the Alimentary Canal," 1911, states that glycerin acts as an irritant to the anal canal, but not to the rectum. The glycerites are a class of official preparations in which glycerin is the solvent.