Quina may be taken as the type of the whole order. It is the most powerful; and the others all more or less resemble it. Perhaps Cusparia bark comes nearest to the Cinchona alkaloids in its anti-periodic action. It is worthy of remark, that some common bitters which owe their efficacy, not to alkaloids, but to neutral principles, as Quassia, Calumba, and Gentian, possess the least control over periodic affections. There can be no doubt that these active principles are all absorbed, and pass into the blood. They may easily be dissolved out of the vegetable which contains them, if not by simple water, at all events by such an acid liquid as we find in the stomach. We possess positive proof of absorption in the case of Quina, which has been detected by Tiedemann and Gmelin in the blood of a patient to whom it was administered. Having them now in the blood, we arrive at the first question. Do Quina and other vegetable Tonics act on the blood or on the nerves? This has been answered at once by many writers, as if it were a thing of great simplicity. But it does not seem to be so. Tonics, as we know, have the power of communicating health and strength in debility produced by various causes, and also of arresting the progress of intermittent fever. Do they effect these things by bettering the condition of the blood, and, from it, that of the system at large, or do they at once, and in the first place, influence the nervous system? This is an important question, and it has been variously answered. Dr. Pereira, in his classification of medicines, ranks them among _Cinetics_ ([Greek: kineô], to move,) which are defined to be medicines exerting a power over the motor system of nerves, and through them on the muscles. But in a subsequent account of Quina, he states that its action is quite inexplicable, and that its use in Ague must be ranked with some other special and ill-understood actions, as that of Mercury in Syphilis, or of Arsenic in Lepra. Dr. Neligan, (in his work _On Medicines,_) lays it down that Tonics act as stimulants when given to a healthy man. Dr. Pereira, on the contrary, states what is more consonant with general experience, _i.e._ that a moderate dose of a tonic has little or no effect on a man in perfect health. Dr. Neligan admits that their action is permanent, and produced slowly; he also declines any explanation of the action of Quina and others in Ague, calling them specifics. Some other authors have been still more decided in classing Tonics with nerve-medicines. Dr. Guy, (in his edition of _Dr. Hooper's Physician's Vade Mecum_,) considers that Stimulants and Tonics should rightly be classed together, for that Stimulants act as Tonics to the weak, and Tonics as Stimulants to the strong.