70

ELEMENTS OF MATEIIIA MEDICA.

rium, or a total deficiency of these faculties, or coma, or continualvoluntary actions or paralysis) are presumed to act specifically on thecerebral, or sentient and voluntary system. Opium, alcohol, and aconite,may be mentioned as examples of agents acting on this part of thenervous system.

On the other hand, those cerebro-spinants which occasion convulsionsor spasms affect the true spinal or excitomotory system of Dr. Hall.Thus strychnia, hydrocyanic acid, belladonna, and most of the metalliccerebro-spinants, act on this portion of the nervous system.

The precise pathological condition of the brain or spinal marrowproduced by cerebro-spinants has not been satisfactorily ascertained.Some of them (as opium) give rise to a congested state of the cerebralvessels, but this may be a secondary effect.

Active principles. —The active principles of each of the cerebro-spinants will be examined separately in a subsequent part of this work ;but as several of the vegetables of this group owe their activity toalkaloids, it will be useful to point out here the general properties ofthese bodies.

The vegetable or organic alkalies, or the alkaloids, have only been recog-nised during the present century. They are salifiable and inflammablecompounds of carbon, hydrogen, nitrogen, and oxygen. Most of themare solid, inodorous, and crystallizable, but conia is odorous and liquidat ordinary temperatures. They are usually fixed; but some of them,as cinchonia and daturia, are volatile at elevated temperatures. Theyreact on vegetable colours as alkalies, and unite with acids, to formsalts; but their saturating power is very low, that is, their atomicweights are very high. Each atom contains one equivalent of nitrogen.Those alkaloids which are best known are only slightly soluble in water;but, in general, they readily dissolve in hot alcohol, and frequentlyseparate in a crystalline state from this liquid, as it cools. Their tasteis bitter or acrid.

Tannic acid unites with them to form tannates, which usually arevery slightly soluble only in water. Hence the infusion of galls (whichcontains this acid) is employed for detecting the alkaloids, and as anantidote in poisoning by them. Iodic acid, in excess, precipitatesseveral of them ; but is decomposed by morphia, iodine being set free.Concentrated nitric acid reddens morphia, strychnia, and brucia, andgives a yellow tinge to narcotine; but a green one to aricina. Bichlo-ruret of mercury precipitates the hydrochlorates of some of thesealkaloids, forming with them double salts. The sulphates, nitrates,hydrochlorates, and acetates of the alkaloids, are generally soluble inwater. Ammonia and magnesia decompose these solutions, and precipi-tate the alkaloid.

Tiie usual method of obtaining the vegetable alkalies is to digest andboil the substances yielding them in water, acidulated with hydrochloricacid. To the filtered liquor add ammonia, lime, or magnesia, andsubsequently purify (by repeated solutions in alcohol) the precipitatedalkaloid.

Rasp ail ( Nouveau Systeme de Chimie Organique, p. 488) maintainsthat the alkaloids are artificial combinations of a vegetable acid(benzoic ?) and excess of ammonia, with perhaps a resinoid substance.But there are no just grounds for such a conclusion. It is, however,