PHYSIOLOGICAL CLASSES OF MEDICINES. 71

deserving of notice that each atom of the alkaloid contains preciselythe quantity of nitrogen which exists in one atom of ammonia.

The vegetable alkaloids act powerfully on the animal economy; butthey present too much diversity in their mode of operation to allow ofany general remarks being made thereon. Some are most energeticpoisons; for example, strychnia and aconitina: others, which cannotbe called poisonous, are powerful and valuable remedies, as quinia.

Class 2. Stimulants,Incitants, or Excitants. —An agent which in-creases the vital activity of an organ is tenned a stimulant (from stimulus,a goad or spur), or sometimes an incitant (from incito, to incite or spuron), or excitant. Those which affect all the organs or functions of thesystem are termed general stimulants; while others, which influence oneor two organs only, are called special stimulants. Those which excite theparts to which they are applied are frequently denominated local stimu-lants, or irritants; though the term local is used by Murray ( System ofMateria Medico) to indicate the substances which I have here termedspecial stimulants.

The vital or vivifying stimuli (a certain degree of external heat,atmospheric air, water, and nutriment) are to be distinguished from theagents used in medicine under the name of stimulants. The former areessential to vitality : they renovate the tissues, by entering, in a mannerindispensable to life, into their composition ; and, lastly, their continuedaction does not give rise to exhaustion. The latter, on the other hand,are not necessary to life : they have no renovating action; but, bycausing reaction, give rise to exhaustion. Moreover, the so-calledstimulants do not merely excite; most of them act as alteratives, andmany of them, by long-continued use, or by employment in too largequantities, destroy life.

Stimulants, for the most part, produce their effects by the agency ofthe nervous system (i. e. the true spinal and ganglionic systems), andprobably in a considerable number of instances by a reflex action.Many of them become absorbed, and have been recognised in the bloodand secretions.

Stimulants are closely related to some other classes, especially tocerebro-spinants, tonics, and some of the evacuants. Thus, alcohol andether are at the same time stimulant and narcotic ; myrrh, cascarilla, andthe ferruginous compounds, possess both stimulant and tonic qualities;lastly, several of the stimulants are sudorific, diuretic, emmenagogue, &c.

Most stimulants are odorous,—many of them indeed powerfully so.Their taste is warm, acrid, and pungent. Swallowed in moderatequantities, they give rise to a sensation of warmth in the stomach, expelgaseous matters, and assist digestion. In larger quantities, they excitethirst, and often give rise to nausea or vomiting. Many of them increasethe force and frequency of the heart’s action, and promote the warmth ofthe surface of the body.

They may be arranged in groups, founded in part on then - chemicalcomposition, and in part also on their effects.

a. The first group is one which was termed by the late Dr. Duncan(, Supplement to the Edinburgh Dispensatory, p. 229), volatile pungentstimuli. It includes the officinal substances belonging to the orderCrucifera (such as mustard and horse-radish) and certain bodies ofLiliacece (garlic, the onion, and the leek). These substances contain a