A medical prescription is a formal expression of the physician's instructions addressed to the authorized compounder of medicines—the apothecary. The word "prescription" is derived from the Latin "prae," before, and "scribo," I write, or "scriptum," written. Prescriptions are written in Latin, as are also the formulae of the present Pharmacopoeia. The reasons for the continued use of Latin are long custom, and because Latin, a dead language, is not subject to the variations of form and meaning—the fluctuations—of a living language, which is frequently adopting new words and terms, and changing the meaning of the old.

Long usage has given a special form to medical prescription. It is made up of the basis, or chief ingredient ; the adjuvant, or modifying material ; the corrigent, or correcting substance ; the excipient, for the proper combination of the whole into a uniform body ; and the vehicle, for containing all in a uniform solution, mass, or mixture. In actual practice this is much simplified.

In writing a prescription it is usual to begin with the letter R crossed thus,Rx, and this signifies "recipe," "take," and is an imitation of the invocation to Jupiter, with which the ancient physicians began their prescriptions. In constructing a prescription the proper Latin form, so far as necessary, should be adhered to. The word "recipe" governs the object in the accusative, and here the object is the quantity of the several ingredients composing the prescription. The quantity in grains, drachms, or ounces governs the medicament in the genitive, represented by the English preposition "of." Thus, Rx, "recipe," take, for example, one scruple, "scrupulum unum," of sulphate of iron, ferri sulphatis, "ferri" and "sulphatis" being the genitives of "ferrum" and "sulphas." The same construction obtains for the adjuvant, the corrigent, and for the vehicle. In prescriptions as usually written, the excipient is rarely given, the addition of this being left to the judgment of the compounder. When the several parts or ingredients have been written, then follows the instructions as to form. If all ingredients are liquid, the prescriber may content himself with simply writing under their names and proportions the first letter of the word mix, Latin "misce." If pills or powders are intended, then, in addition to M., should be written M. div. in. pil. no. xii, or M. ft. pil. or pulv. no. x, etc. When the form and number of the pills or powders are determined on, then follows the instructions directed by the word "signa" or the letter S., which signifies "write." There are a few rules for determining case, which the prescriber should be familiar with, as follows :

Rule 1

The quantity is put in the accusative case, governed by the word "recipe."

Rule 2

If no quantity is expressed, the substance ordered is put in the accusative.

Rule 3

Adjectives agree with nouns in gender, number, and case.

The substance in the genitive case is governed by the word for quantity, as already mentioned.

The terminations of nouns and adjectives vary with the declension. Thus :

Nouns ending in "a" are of the first declension, and have their genitive in "se" ; as aqua, aquae.

Nouns ending in "us," "um," "os," "on" have their genitive in "i" ; as hydrargyrum, hydrargyri, etc.

All other nouns make their genitive in "s" or "is," as chloral, chloralis; but in some the termination is lengthened. Thus: "as," genitive "atis," as acetas acetatis. "is," genitive "idis," as anthemis, anthemidis. "o," genitive "onis," as pepo, peponis. "x," genitive "cis," as cortex, corticis.

A few prepositions are used in the construction of prescriptions. They are the following :

"Ad," to; "ana," of each; "in," into; "cum," with. Of these, "ad" and "in" govern the accusative, "cum" the ablative, and "ana" the genitive.

Other phrases with their abbreviations follow:

Quantum sufficiat............ q. s., a sufficient quantity.

Granum.................... gr., a grain.

Gutta...................... gtt., a drop.

Semissis.................... ss., a half.

Pilula ..................... pil., a pill.

Chartula.................... chart., a powder.

Mistura.................... mist., a mixture.

Liquor..................... liq., a solution.

Pulvis...................... pulv., a powder.

Tinctura................... tinct., a tincture.

Syrupus.................... syr., a syrup.

Aqua...................... aq., water.

Fluidus, adjective........... fl., fluid.

Dilutus, adjective........... dil., diluted.

Ana....................... aa., of each.

In writing the directions for a prescription, many of the verbs employed are abbreviated.

Thus, "fiant" and "fiat," make, "ft."; "misce," mix, M. "Divide," Div., "Signa," write, S.

Prescriptions are extemporaneous or official. The latter are contained in the United States Pharmacopoeia. For example, the compound cathartic pill. In directing an official combination of this kind, we proceed as follows:

R,Pil. cathartic, comp. (U. S. P.) no. xii. S. One or two pills at night.

These abbreviations are sufficiently clear to permit no mistake. If written out they would be:

Rx, Pil. catharticse composite no. xii.

There are but fifteen formulae of official pills in the Pharmacopoeia, and nine of these are purgative combinations. In prescribing them, the official title should be made use of. For example, Pilulae ferriiodidi. To avoid the necessity of changing the termination, the word pilulae is abbreviated into pil.

Among the products of the manufacturing pharmacist's art are compressed pills or "pellets," "tablets," or "tabloids," in which the pill mass is formed by the compressing machines. Although these preparations are not "official," rightly speaking, the official forms may be thus prepared.

In prescribing pills extemporaneously certain considerations must have due weight. The pill mass must not be too large. By common consent, the pill should not exceed five grains in weight. Deliquescent substances, those too great in bulk, oils, emetics, and stimulants are not suitable for this mode of administration. When the pill mass is bulky and the taste disagreeable, capsules are substituted, as in the examples given below. A few formulae will suffice to show the mode in which extemporaneous prescriptions are constructed.

Rx Ext. colocynth. comp...................... Э j ;

Ext. rhei................................. gr. xij ;

Ext. belladonnae.......................... gr. iij.

Misce et ft. pil. no. xii. S. : One pill at night.

Or, instead of pills, the mass may be put into gelatine capsules, as follows :

Rx Quininae sulphatis........................ 3 j ;

Ferri sulphatis...........................gr. xij ;

Extracti nucis vomicae....................gr. iij.

M. Ft. capsulae no. xii.

S. : A capsule morning and night.

In these prescriptions the material used for forming the pill mass is left to the discretion of the pharmacist, whether he employs for this purpose syrup, gum, or acid.

A powder (chartula) is made use of when the medicament is bulky, not deliquescent, and not disagreeable to the taste. They are of various sizes, according to number and weight of the ingredients. When the taste is bad, or for the sake of convenience, the contents are put in a wafer instead of in a paper. The following is the mode in which a prescription for powders is written :

Rx Hydrarg. chlor. mitis..................... gr. j ;

Sodii bicarb............................. gr. xij ;

Pulv. aromatic...........................gr. vj.

M. div. in pulv. vel chartulas no. xii.

Sig. : One powder or wafer three times a day.

Mixtures (misturae) are made by mixing fluid preparations, salts that can be dissolved in a suitable menstruum or held in suspension by agitation, or substances that may be diffused and held by the use of suitable excipients. Mixtures to be of proper consistency should hold certain proportions. For example, one fluid ounce should contain about one drachm of a vegetable powder and one scruple of a vegetable extract.

Emulsions are mixtures made by suspending an oil or resin—an oil by means of gum, or yolk of egg, or tragacanth ; a resin by means of an alkali ; and if a gum resin, the proportion of gum present suffices to suspend the resin. The IT. S. Pharmacopoeia recognizes four mixtures and four emulsions. Of the latter, two are made merely by triturating thoroughly with water, as emulsum asafoetidae and emulsum ammoniaci. The emulsion of chloroform is made by triturating chloroform with oil of almond and gum tragacanth.

The following forms will show the manner of prescribing a mixture and an emulsion :

Rx Misturae ferri composite.................... oz iv.

S. : A dessertspoonful three times a day.

Rx Emulsi ammoniaci........................ oz viij.

S. : A tablespoonful as required.

Rx Bismuthi subnitratis....................... 3 iij ;

Pulvis aromatici.......................... 3 j ;

Mucilaginis acaciae........................ q. s. ;

Aquae menthae pip..................... ad f oz iv.

S. : Two teaspoonfuls every four hours.

Suppositories (suppositoria). The Pharmacopoeia recognizes but a single formula—suppositoria glycerini—beside the model. But it prescribes the size and form of the several kinds. Thus, "rectal suppositories" should be cone shaped and of a weight of about one gramme (15.4) ; "urethral suppositories" should be pencil shaped and of a weight of about one gramme (15.4) ; "vaginal suppositories" should be globular (or ovoid), and of a weight of about three grammes (46 grains).

The following prescription may serve as a model :

Rx Acidi tannici............................... 3 j ;

Olei theobromae............................. q. s.

M. et ft. suppositoria vaginale no. vi.

Linimenta (liniments) are preparations for topical application, to be rubbed in with friction, and sufficiently active to cause some rube-faction, or they may have anodyne and vesicating properties. The United States Pharmacopoeia recognizes nine liniments as official, and they are so various in composition and quality that it is rarely necessary to write an extemporaneous prescription calling for some other combination. In directing an official liniment, the nomenclature of the Pharmacopoeia should be adhered to. For example :

Rx Linimenti ammoniae......................... q. s.

S. : Ammonia liniment for external use.

Plasters (emplastra). There are thirteen official combinations contained in the United States Pharmacopoeia (1890). The materials are of such consistency and firmness as to be spread on lead or resin plaster. When a plaster is ordered by prescription, the following procedure should be made use of :

Rx Emplastri picis cantharidati...... quantum sufficiat fiat emplastrum ii. S. : Warming plaster.

When a blistering plaster is desired, the following method is used:

Rx Cerati cantharidis.......................... q. s.

ft. emplastrum 11 X 11. S. : Apply to seat of pain.

Besides the foregoing there are other forms for the prescription of medicinal agents, but as they are seldom used, and the principles governing them are the same as those above given, it is the less necessary to enter into further details.