These periods are called the CRITICAL DAYS; carefully marked by Hippocrates and other ancient physicians, as well as by many moderns of the greatest eminence in practice; while at the same time many other moderns, of no inconsiderable authority, deny their taking place in the fevers of these northern regions which we inhabit. :

108. I am of opinion, that the doctrine of the ancients, and particularly that of Hippocrates, on this subject, was well founded; and that it is applicable to the fevers of our climate.

109. I am of this opinion, first, because I observe, that the animal economy, both from its own constitution, and from habits which are easily produced in it, is readily subjected to periodical movements; secondly, because, in the diseases of the human body, I observe periodical movements to take place with great constancy and exactness; as in the case of intermittent fevers, and many other diseases.

110. These considerations render it probable, that exact periodical movements may take place in continued fevers; and I think there is evidence of such movements actually taking place.

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111. The critical days, or those on which we suppose the termination of continued fevers espe

cially to happen, are, the third, fifth, seventh, ninth, eleventh, fourteenth, seventeenth, and twentieth. We mark none beyond this last; because, though fevers are sometimes protracted beyond this period, it is, however, more rarely; so that there are not a sufficient number of observations to ascertain the course of them; and further, because it is probable that, in fevers long protracted, the movements become less exact and regular, and therefore less easily observed.

crates.

112. That the days now mentioned are the critical days, seems to be proved by the particular facts which are found in the writings of HippoFrom these facts, as collected from the several writings of that author by M. De Haen, it appears, that of one hundred and sixty-three instances of the termination of fevers, which happened on one or other of the first twenty days of the disease, there are one hundred and seven, or more than two thirds of the whole number, which happened on one or other of the eight days above mentioned; that none happened on the second or thirteenth day; and upon the eighth, tenth, twelfth, fifteenth, sixteenth, eighteenth, and nineteenth, there are but eighteen instances of termination, or one ninth of the whole.

113. As the terminations which happen on the seven days last mentioned, are, upon the whole,

few; and, upon any one of them, fewer than those which happen on any of our supposed critical days: so there are therefore nine days which may be called NON-CRITICAL; while, on the other hand, the many terminations which happened on the seventh, fourteenth, and twentieth days, afford a proof both of critical days in general, and that these are the chief of them. Hereafter I shall mention an analogy that renders the power of the other critical days sufficiently probable.

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114. It appears farther, that as, of the terminations which were final and salutary, not a tenth part happened on the non-critical days; and of the terminations which were final and fatal, though the greater number happened on the critical days, yet above a third of them happened on the noncritical; so it would appear, that the tendency of the animal economy is to observe the critical days, and that it is by the operation of some violent and irregular cause that the course of things is some. times turned to the non-critical.

115, What has been said, gives sufficient ground for presuming, that it is the general tendency of the animal economy to determine the periodical movements in fevers to be chiefly on the critical days. At the same time, we must acknowledge it to be a general tendency only; and that, in par ticular cases, many circumstances may occur to

disturb the regular course of it. Thus, though the chief and more remarkable exacerbations in continued fevers happen on the critical days, there are truly exacerbations happening every day; and these, from certain causes, may become considerable and critical. Further, though intermittent fevers are certainly very strongly determined to observe a tertian or quartan period, we know there are circumstances which prevent them from observing these periods exactly, and which render them either anticipating or postponing so much, that the days of paroxysms come to be quite changed; and it is allowable to suppose, that the like may happen with respect to the exacerbations of continued fevers, so as thereby to disturb the regular appearance of critical days.

A particular instance of this occurs with respect to the sixth day of fevers. In the writings of Hippocrates, there are many instances of terminations happening on the sixth day; but it is not therefore reckoned among the critical days; for, of the terminations happening on that day, there is not one which proves finally of a salutary kind; greater number are fatal; and all the rest are imperfect, and followed with a relapse. All this shews, that some violent cause had, in these cases, produced a deviation from the ordinary course of nature; that the terminations on the sixth day are nothing more than anticipations of the seventh, and therefore a proof of the power of this last.

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116. The doctrine of critical days has been much embarrassed by some dissonant accounts of it, which appear in the writings imputed to Hippocrates. But this may be justly accounted for from these writings being truly the works of different persons, and from the most genuine of them having suffered many corruptions; so that, in short, every thing which is inconsistent with the facts above laid down may be ascribed to one or other of these causes.

117. This, further, has especially disturbed the doctrine of critical days, that Hippocrates himself attempted, perhaps too hastily, to establish general rules, and to bring the doctrine to a general theory, drawn from Pythagorian opinions concerning the power of numbers. It is this which seems to have produced the idea of odd days, and of a quaternary and septenary period; doctrines which appear so often in the writings of Hippocrates. These, however, are inconsistent with the facts above laid down; and indeed, as Asclepiades and Celsus had observed, are inconsistent with one another.

118. Upon the whole, therefore, it is apprehended, that the critical days above assigned, are truly the critical days of Hippocrates, and may be consistently explained in the following man

ner.