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396 Etiology.—New-comers to the endemic areas and Europeans are more liable than natives or residents of long standing. Apparently, long residence confers a relative although not an absolute immunity.

All ages and both sexes are susceptible; but, in consequence of their habits and more frequent exposure to the predisposing and immediate causes, men are more subject to siriasis than are women.

Predisposing influences similar to those in heat-syncope and sun-traumatism powerfully influence the liability to siriasis. Amongst these are all physiological depressants; notably intemperance, fatigue, overcrowding, unsuitable clothing, malaria, acute disease, and, also, chronic organic diseases of the important viscera.

Siriasis has generally been attributed to a direct action of atmospheric or solar heat on the body. Many theories of the modus operandi of this assumed cause have been advanced. Among these may be mentioned superheating of the blood by the high temperature of the surrounding atmosphere; paralysis of the thermic centres causing (a) over-production of heat, or (b) retention of body-heat; pressure on the brain by expansion from heating of the cerebro-spinal fluid; vaso-motor paresis; paresis of the heart ganglia excess of carbonic acid in the blood; coagulation of myosin; suppression of sweat; deficient serosity of the blood from excessive sweating, and so forth.

It is well known that neither high atmospheric temperature per se, nor high bodily temperature, unless the latter be associated with some special toxins, gives rise to symptoms at all like those of siriasis. Considering these two facts, and at the same time the peculiar and capricious distribution of the disease, the circumstances that its prevalence curve does not always correspond with the atmospheric temperature curve; that the degree of prevalence varies in the endemic area from year to year that it is not most prevalent in the hottest years, seasons, or places; that it becomes epidemic at times; that it runs a definite course; that it may