There have been many systems brought forward to facilitate the studying of drugs, and it is owing, possibly, to the number that authors observe little uniformity in the arrangement followed. It has been thought wise to outline the five most important, of which the first and fourth alone are by their nature sufficiently comprehensive to include all organic and inorganic drugs without any omissions. The others (three) are but scientific systems applicable only to the organic medicines, and, as such, are recommended mostly by pharmaceutical and chemical investigators.
I Arrangement By Alphabetic Sequence. This is the least scientific but the most popular; in fact, it is not a true system, as no tacit relationship in any particular exists between the associated subjects, save that of initial letter in spelling, which possibly can offer to the student only the trifling advantage of lexical convenience.
II Arrangement By Chemical Constituents. From a very early period organic drugs (vegetable and animal) were known to yield some of their activity to water and spirit, but the precise nature of the active constituents were little sought after until the beginning of the last century. The intelligent development of chemistry has been the means of separating these potentials and assigning their individual nature, to the extent of formulating a system or an arrangement of plants into groups dependent upon their chief constituent furnishing the medicinal properties, as: alkaloid, glucoside, fixed oil, volatile oil, resin, starch, etc. This classification, although best for the chemist in his laboratory investigation, somewhat assists the botanist, since certain genera and even families (natural orders) occasionally have similar constituents: Labiatae (volatile oil), Solanaceae (mydriatic alkaloids), Convolvulaceae (cathartic resin), etc. Its great drawbacks consist in the variability of the nature assigned these constituents from time to time through the advance of chemical science: thus, a neutral principle today may be an alkaloid tomorrow; and, again, nearly all drugs have more than one constituent, the most abundant often being the least active, and which should govern its classification is not always easy to decide. To the general student, however, it is of considerable value, as it furnishes a knowledge of the character, name, and number of the possible drug constituents, also impresses the great difference between the crude drug and its active principle, and beyond all - that in every case it is the latter that furnishes the drug's working capacity.
Amylaceous. Those containing starch as their chief medicinal principle: inula, lappa, cetraria, etc.
Acidulous. Those containing chiefly an organic acid: lemon, orange, tamarind, rubus, etc.
Oleaginous. Those containing: (a) Volatile or essential oil: (1) Terpenes, C10H16: turpentine, cubeb, juniper, etc. (2) Oxygenated, C10H18O: cinnamon, anise, fennel, etc. (3) Sulphurated, C12H22S2: mustard, asafetida, allium. (4) Nitrogenated, C7H6O (HCN): bitter almond, wild cherry, peach, etc. (b) Fixed oils (Compound ethers) which leave a permanent stain: olive, almond, castor oils, etc.
Balsamic. Those containing a liquid, semi-liquid, or solid vegetable product composed of a resin or oleoresin, an odorous principle, and either one or both benzoic and cinnamic acids: Peru, Tolu, storax, etc.
Glucosidal. These contain an organic principle converted by mineral acids, alkalies, or ferments into glucose and an allied organic compound; they may be neutral or acid, and sometimes form salts; nearly all are soluble in alcohol: salicin, gentiopicrin, cathartic acid, tannin, etc.
Neutral Principles. These, sometimes called bitter principles from their bitterness, are plant-constituents, either neutral or feebly acid, and, when possible, form salts with alkalies; they differ from glucosides in not splitting into glucose, and from alkaloids in not being precipitated by tannin or mercuric-potassium iodide: aloin, elaterin, picrotoxin, santonin, etc.
Alkaloidal. These contain alkaloids composed of carbon, hydrogen, and nitrogen alone, if liquid (amines), and additionally oxygen, if solid (amides'); sometimes they are called vegetable alkalies, and, in fact, are related to ammonia, as when heated with alkalies ammonia is given off: quinine, morphine, cocaine, atropine, nicotine, coniine, etc.