PERIODICITY (in physiology and pathology). The tendency manifested by various phenomena occurring in 'living animals to recur, after equal, or nearly equal intervals of time, is so marked, that Bichat, the great French anatomist and physiologist, described it under the title of the Loi d'Intermittencs. The alternation of sleep and waking, the phenomena of menstruation, and the punctual return of hunger, are some of the most obvious instances of that can be suggested as occurring in the healthy sub ject; while less obvious examples are afforded by the apparently regular variations that have been observed in the excretion of carbonic acid from the lungs, and in the number of the pulsations of the heart at different periods of the 24 hours. As is well known by experience, periodicity may be usefully cultivated and fixed in daily habits. This is well exemplified in the case of sleep, but in a more special degree by the daily relieving of the bowels at a particular hour, a habit in which it is important that all young persons should be carefully instructed with a view to health and convenience.
In certain forms of disease, the law of periodicity or intermission is very distinctly seen. The regular periodic recurrence of the paroxysms of intermittent fever (or ague), is universally known, although the cause of the periodicity has hitherto baffled all inquiry. Among those who have tried to solve this question may be mentioned Willis, Reif, Baillv, Roche, Cullen (who ascribes periodicity to "a diurnal revolution affecting the animal economy"), and more recently, Laycock, who refers it to the diurnal atmos pheric changes in relation to pressure, electricity, etc.
Ague often gives rise to periodic diseases which present no close analogy to that dis ease. Thus it—or, at all events, malaria—is a frequent cause of tic douloureux, 1:Fur ring at regular intervals; eases are recorded in which periodical vomiting, occurring weekly, or, in one case, at an interval of ten days, seemed to be due to it; and Sir.
surgeon to the Middlesex hospital, not long ago published the case of a woman who experienced a periodical inflammatory swelling of the right knee, as a sequence of that disorder. Epilepsy is a disease in which the intervals (especially in women) tend to a regular period. Sir Henry Holland (Medical .2Votes and Reflections, 2d ed., page 341) records a case in which " six attacks occurred, with intervals of 16 or 18 minutes between; so exactly recurring, as noted by the watch, that it was impossible to suppose it a mere casuality;" and another, ",irliere a spasmodic seizure, more of tetanic than epileptic character, occurred twice a (ray for many weeks successively, and almost exactly at the same hours each day." For many other examples of periodic or intermittent mor bid action, the reader is referred to a memoir by Henle, "On the Course and Periodicity of Disease," in his Pathologische Untersnchungtn; and to sir henry Holland's essay (to which we have already referred) in his Medical Notes and Reflections. The most impor tant pra6ical fact in relation to this class of diseases is, that they almost invariably yield to the action of certain medicines, especially bark and arsenic.
Exercising a beneficial or mischievous influence, as the case may be, the habit of peri odicity is to be seduously shunned in every instance likely to prove morally or physically prejudicial. No more marked example of the injudicious cultivation of periodicity could he given than in the evil practice of periodical blood-letting, which once prevailed an over Europe, and was only abandoned in recent times as not only useless, but in all respects injurious.