Preventive Influence. Upon the same principles as those on which periodical diseases may be cured, they may also be prevented by sulphate of quinia. There is no prophylactic measure against the miasmatic fevers at all comparable in efficacy to the use of this medicine. It seems reasonable to suppose that an impression on the system, such as prevents the return of the paroxysms, would prevent the occurrence of the first. Experience has established the correctness of this inference. All that is necessary is to give, twice every week, in divided doses, a quantity equal to that required for the interruption of the disease when formed. From ten to fifteen grains, in doses of two grains every two hours, will probably answer the purpose. I am not sure that the same amount taken weekly will not be sufficient, considering the tendency to septenary periods which characterizes the relapses of the disease.

In the irregular intermittent diseases, such as hectic fever, for example, sulphate of quinia, given in the same manner as in regular intermit-tents, will often check the paroxysms; but cannot be relied on even for this purpose, and is generally quite inadequate to the cure. The cause of these affections is generally continuous in its action, and is always, therefore, lying in wait to renew its assaults, though sometimes temporarily restrained. In hectic fever, the nervous centres, through which the sources of irritation operate in producing the paroxysm, may be rendered insensible for a time to their influence; but, as soon as the protective force is withdrawn, the cause again operates; and, if the quinia be given constantly, the system at length becomes insensible to its effects, and its remedial power ceases.

Nature of the Antiperiodic Action. Various theories have been broached to explain the antiperiodic effect of Peruvian bark. Most of them are scarcely deserving of notice. One of the most plausible, in reference to the miasmatic intermittents and remittents, is that the alkaloids have the property of neutralizing the poison in the system. But this is not tenable; as quinia cures the irritative intermittents even more readily than the miasmatic; and, if there be any disposed to deny the existence of the former affection, the argument will still hold; for intermittent neuralgia, which often occurs where there can be no possible suspicion of the influence of marsh miasms, even in the midst of cities the air of which serves as a protection against these miasms, will yet almost invariably yield to the same remedy.

I know no better explanation of the antiperiodic property, than that which supposes it to depend upon the powerful influence exercised by the remedy upon the nervous centres, through which probably the paroxysms are produced. Every consideration, in connection with the peculiarities of regular intermittent diseases, leads to the conclusion, that the paroxysms are caused by an influence acting through the cerebral centres, without which the result would not take place. Now, if these cerebral centres can be preoccupied by a strong impression from some other source, they may be rendered insensible to the morbid influence, and the paroxysm, therefore, is set aside. Quinia is characterized by its disposition to act energetically upon certain nervous centres, which are probably the same as those through which the cause of the disease operates. Quinia, therefore, interrupts the succession of the paroxysms; and, as they are probably sustained, in part at least, either by habit, or by some chain of morbid action passing insensibly from one paroxysm to the succeeding, the interruption is either permanent, or continuous until the original cause may reassume, in some mysterious way, its original activity, and produce a relapse in the now unguarded system. It is obvious that this explanation of the antiperiodie power of remedies implies an identity with that next to be considered; namely, the property of supersession; but, as the explanation is only conjectural, it is deemed best to treat of these two therapeutic agencies distinctly.

3. As a Supersedent

What has been stated above explains the meaning attached to this term. It simply implies that the medicine so named has the power, by insinuating an action of its own in any part or organ, to displace disease previously existing in that part or organ, or to exclude it if not already established. It is known that quinia acts powerfully on the cerebral centres, especially those of the organic functions, and produces in those centres an impression of considerable permanence. The probable influence of such an impression in preventing the return of regular periodical paroxysms has been referred to. It is evinced also though much less certainly and strikingly, in the prevention of irregular paroxysms of various kinds, and even in the relief of existing disease occupying especially the nervous centres, or acting through them, though not necessarily paroxysmal. Upon this principle, we may explain the occasional efficiency of sulphate of quinia in irregular neuralgia, when given very freely. In chorea, functional epilepsy, hysteria, spasmodic asthma, and the advanced stage of pertussis, it has sometimes been used advantageously; and it is said to have proved efficacious, in large doses, even in tetanus. Associated with infusion or oil of valerian, I have been much in the habit of using it in certain forms of nervous headache, imitating an old practice taught me by the late Dr. Jos. Parrish, in which Peruvian bark was used for the same purpose, with the same addition. Much efficacy in the cure of nervous or sick headache is claimed, by MM. Debout and Serre, for a combination of sulphate of quinia and digitalis; forty-five grains of the former and twenty-two of the. latter, being made with syrup into thirty pills, of which one is to be taken every night for at least three months. (Bullet de Therap., lviii. p. 311).