time upon culture-media, especially agar-agar, the virulence was rapidly lost and the bacillus eventually died. On the other hand, when constantly inoculated from animal to animal the virulence of the bacillus is much increased.

The bacillus probably attenuates readily. Kitasato found that it did not seem able to withstand desiccation longer than four days; and Yersin found that although it could be secured from the soil beneath an infected house at a depth of 4-5 c.cm., the virulence of such bacilli was lost.

Kitasato found that the bacillus was killed by two hours' exposure to 0.5 per cent. carbolic acid, and also by exposure to a temperature of 80° C.

It seems possible to make a diagnosis of the disease in doubtful cases by examining the blood, but it is admitted that a good deal of bacteriologic practice is necessary for the purpose.

Kitasato's experiments have shown that it is possible to bring about immunity to the disease, though nothing definite in the way of experiment has as yet been recorded.

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CHAPTER VI.

TETRAGENUS.

THERE can sometimes be found in the normal saliva, more commonly in tuberculous sputum, and still more commonly in the cavities of tuberculosis pulmonalis, a large micrococcus grouped in fours and known as the Micrococcus tetragenus (Fig. 103). It was discovered by

FIG. 103. Micrococcus tetragenus in pus from a white mouse; x 615 (Heim).

Gaffky, and subsequently carefully studied by Koch and Gaffky. It sometimes occurs in the pus of acute abscesses, and may be of importance in connection with the pulmonary abscesses which so often complicate tuberculosis.

The cocci are rather large, measuring about I μ in diameter. In cultures they show no particular arrangement among themselves, but in the blood and tissues of animals they commonly appear arranged in groups of four surrounded by a transparent gelatinous capsule. The organism stains well by ordinary methods, and

most beautifully by Gram's method, by which it can be best demonstrated in tissues.

Upon gelatin plates small white colonies are produced in from twenty-four to forty-eight hours. Under the microscope they are found to be spherical or elongate (lemon-shaped), finely granular, and lobulated like a raspberry or a mulberry. When superficial they form white, elevated, rather thick masses 1-2 mm. in diameter (Fig. 104).

In gelatin punctures a large white surface-growth

takes place, but very scant development occurs in the puncture, where the small spherical colonies generally remain isolated.

Upon the surface of agar-agar spherical white colonies are produced. They may remain isolated or may become confluent.

Upon potato a luxuriant thick, white growth occurs.

The growth upon blood-serum is also abundant, especially at the temperature of the incubator. It has no distinctive peculiarities.

The introduction of tuberculous sputum or of a most. minute quantity of a pure culture of this coccus into white mice generally causes a fatal septicemia.

The organisms are found in small numbers in the heart's blood, but are numerous in the spleen, lungs, liver, and kidneys.

House-mice and field-mice are comparatively immune; dogs and rabbits are also highly resistant. Guinea-pigs sometimes die from general infection, though sometimes local abscesses may be the only result of subcutaneous inoculation.

The tetragenococci are of no special importance in human pathology, but probably hasten the tissue-necrosis in tuberculosis pulmonalis, and may aid in the formation of abscesses of the lung and contribute to the production of the hectic fever.

CHAPTER VII.

CHICKEN-CHOLERA.

THE barnyards of Europe, and sometimes of America, are occasionally visited by an epidemic disease which affects pigeons, turkeys, chickens, ducks, and geese, and causes almost as much destruction among them as the occasional epidemics of cholera and small-pox produce among men. Rabbit-warrens are also at times seriously affected by the epidemic. When fowls are ill with the disease, they fall into a condition of weakness and apathy which causes them to remain quiet, seemingly almost paralyzed, and ruffle up the feathers. The eyes are closed shortly after the illness begins, and the birds gradually fall into a stupor from which they do not awaken. The disease leads to a fatal termination in twenty-four to forty-eight hours. During its course there is profuse diarrhea, the very frequent fluid, slimy, grayish-white discharges containing numerous microorganisms.

The bacilli which are responsible for this disease were first observed by Perroncito in 1878, and afterward thoroughly studied by Pasteur. They are short, broad bacilli with rounded ends, sometimes united to each other, with the production of moderately long chains (Fig. 105). Pasteur at first regarded them as cocci, because when stained with a penetrating anilin dye the poles stain intensely, but a narrow space between them remains almost uncolored. This peculiarity is very marked, and sharp observation is required to observe the outline of the intermediate substance. The bacillus does not form spores, and does not stain by Gram's method. When examined in the living condition it is found to be motile.