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shall now endeavour to apply the doctrine already delivered, towards explaining the diversity of fevers.

CHAP. III.

OF THE DIFFERENCE OF FEVERS AND ITS CAUSES.

53. To ascertain the difference of fevers, I think it necessary to observe, in the first place, that every fever of more than one day's duration, consists of repeated, and in some measure separate, paroxysms; and that the difference of fevers taken notice of above, (from 25 to 30), appears to consist in the different state of paroxysms, and in the different circumstances of their repetition.

54. That fevers generally consist of distinct, and, in some measure, separately repeated, paroxysms, I have alleged above to be a matter of fact : but I shall here endeavour to confirm it, by assigning the cause.

55. In every fever, in which we can distinctly observe any number of separate paroxysms, we constantly find that each paroxysm is finished in less than twenty-four hours; but as I cannot per

ceive any thing in the cause of fevers determining to this, I must presume it to depend on some general law of the animal economy. Such a law seems to be that which subjects the economy, in many respects, to a diurnal revolution. Whether this depends upon the original conformation of the body, or upon certain powers constantly applied to it, and inducing a habit, I cannot positively determine : but the returns of sleep and watching, of appetites and excretions, and the changes which regularly occur in the state of the pulse, shew sufficiently, that in the human body a diurnal revolution takes place.

56. It is this diurnal revolution, which, I suppose, determines the duration of the paroxysms of fevers; and the constant and universal limitation of these paroxysms, (as observed in 55), while no other cause of it can be assigned, renders it sufficiently probable that their duration de pends upon, and is determined by, the revolution mentioned. And that these paroxysms are connected with that diurnal revolution, appears further from this, that though the intervals of paroxysms, are different in different cases, yet the times of the accession of paroxysms are generally fixed to one time of the day; so that quotidians come on in the morning, tertians at noon, and quartans in the afternoon.

57. It remains to be remarked, that as quartans and tertians are apt to become quotidians, these ta pass into the state of remittents, and these last to become continued; and that even in the continued form, daily exacerbations and remissions are generally to be observed : so all this shews so much the power of diurnal revolution, that when, in cer. tain cases, the daily exacerbations and remissions, are with difficulty distinguished, we may still presume, that the general tendency of the economy prevails, that the disease still consists of repeated paroxysms, and, upon the whole, that there is no such disease as that which the schools have called a continued fever. I expect that this doctrine will be confirmed by what I shall say hereafter con. cerning the periodical movements observed in continued fevers.

58. It being thus proved, that every fever, of more than one day's duration, consists of repeated paroxysms ; we, in the next place, remark, that the repetition of paroxysms depends upon the cir. cumstances of the paroxysms which have already taken place. From what was observed in 30 and 31, it appears, that the longer paroxysms are protracted they are the sooner repeated; and, therefore, that the cause of the frequent repetition is to be sought for in the cause of the protraction of paroxysms.

59. Agreeably to what is laid down in 46, and

to the opinion of most part of physicians, I suppose, that, in every fever, there is a power applied to the body, which has a tendency to hurt and destroy it, and produces in it certain motions which deviate from the natural state ; and, at the same time, in every fever which has its full course, I suppose, that in consequence of the constitution of the animal economy, there are certain motions excited which have a tendency to obviate the effects of the noxious power, or to correct and remove them. Both these kinds of motion are considered as constituting the disease.

But the former is perhaps strictly the morbid state, while the latter is to be considered as the operation of the vis medicatrix naturce, of salutary tendency, and which I shall hereafter call the RE. ACTION of the system,

60. Upon the supposition that these two states take place in every paroxysm of fever, it will appear to be chiefly in the time of the hot stage that the reaction operates in removing the morbid state; and therefore, as this operation succeeds more or less quickly, the hot stage of paroxysmis will be shorter or longer. But as the length of paroxysm depends chiefly upon the duration of the hot stage, so the longer duration of this and of paroxysms, must be owing either to the obstinacy of resistance in the morbid state, or to the weakness of the şalutary reaction; and it is probable, that some

times the one, and sometimes the other of these circumstances, takes place.

61. It seems to be only by the state of the spasm, that we can judge of the resistance of the morbid state of fever : and with respect to this spasm, I observe, that either the cause exciting it may be different in different cases; or, though the cause should be the same in different persons, the different degree of irritability in each may give occasion to a greater or lesser degree of spasm ; and therefore, the reaction in fever being given, the continuance of the hot stage, and of the whole paroxysm, may be longer or shorter, according to the degree of spasm that has been formed.

62. One cause of the obstinacy of spasm in fe. vers, may be clearly perceived. In inflammatory diseases there is a diathesis phlogistica prevailing in the body, and this diathesis we suppose to consist in an increased tone of the whole arterial system. When, therefore, this diathesis accompanies fever, as it sometimes does, it may be supposed to give occasion to the febrile spasms being formed more strongly, and thereby to produce more protracted paroxysms. Accordingly we find, that all inflammatory fevers are of the continued kind; and that all the causes of the diathesis phlogistica have a tendency to change intermittent into continued fevers. Continued fevers, then, being often at.

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