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XVII] about the beginning of the second week, and persist in fatal cases. Respiration is rapid. Slight catarrh of the respiratory tract is present throughout; the pulse loses in volume as it increases in frequency. There is but little diminution in the blood count, and only a feeble leucocytosis— 12,000 to 13,000; the hæmoglobin is slightly diminished.

Pathology.— Post-mortem, in addition to the foregoing skin lesions,, there is marked hypostatic congestion of the lungs, subserous petechiæ, softened myocardium, enlarged and softened spleen, fatty degeneration of the hepatic cells, and congestion of the cortex of the kidneys.

Etiology.— The evidence is now conclusive that the germ of Rocky Mountain fever is introduced by the bites of D. venustus* It is only the adult that attacks man. The- Rocky Mountain goat and the domestic sheep also serve as hosts to the adult forms, but the larval and nymph stages develop principally on the ground squirrel, Citellus columbianus, and the woodchuck, Marmota flaviventer. The tick takes two years to develop, hibernating during the winter, feeding but languidly after the beginning of August, and reviving during the first warm days in April. It is possible that the Rocky Mountain goat serves as a natural reservoir of the disease virus.

According to Ricketts, the unrecognised germ is intimately attached to the blood corpuscles, is easily inoculated into man, and can be passed through an indefinite series of monkeys and guineapigs, giving rise in them to the characteristic symptoms, and producing immunitv. The larvæ, nymph, and adult male and female tick are all of them efficient intermediaries Ricketts suggests, seeing that in one place— Montana —the case mortality in man is as high as 90 percent., whereas in another place— Idaho— it is only 5 per cent., that there are two species of tick, in the former D. venustus, in the latter D. maturatus,