teristic fashion. They attach themselves to the floating drops of butter forming little islands of growth, from which they grow down in the medium in the form of long threads producing the appearance of stalactites. If the culture be shaken the stalactites fall to the bottom, but they are reformed on subsequent inoculation.

The disease can be communicated to most laboratory animals, but rats and mice are particularly susceptible. As is well known, the epidemic is frequently preceded

and accompanied by the sudden death of many of these animals. It is stated that the nomadic tribes on the northern slopes of the Himalayas, even to-day, when they observe an extraordinary number of dead rats, are so certain of an approaching epidemic that they immediately vacate their quarters and shift elsewhere.

Monkeys can be easily inoculated with the plague-a single puncture with a needle dipped in a culture of B. pestis being sufficient. The importance of this observation in relation to infection in man is self-evident.

Plague bacilli have been detected in the dead bodies of fleas, but the majority of observers agree that suctorial insects play no part in the transmission of the disease to man.

Birds and bovines are immune. It is probable that vultures feeding on the corpses of the plague-stricken suffer no ill effects, but they may spread the disease by means of their excreta.

Like other spore free organisms the bacilli are readily killed by desiccation, heat, and ordinary antiseptics. A solution of sulphuric acid (1 in 250) is especially recommended by Hankin as a cheap and efficient germicide. The bacilli are also destroyed by direct sunlight in three or four hours. But it is doubtful if any of these measures can be absolutely relied upon during an epidemic.

On account of their slight resistance and absence of spores infection usually occurs by close contact. The skin is the most frequent path, a slight wound or abrasion being sufficient for inoculation. In a few instances the disease may be produced by the entrance of bacilli into the respiratory passages, the organisms being transmitted not in the form of dust, but in small drops of moisture diffused by plague patients (Flügge's drop infection, see p. 86).

It is improbable that in human subjects infection ever takes place by means of ingesta, although experiments in animals have yielded positive results.

In regard to the spread of the disease from infected cases, it may be stated that the bubonic variety is relatively unimportant, for in this case the bacilli are locked up in the buboes, and when these break down into pus most of the bacilli likewise perish. In septicæmic cases, on the other hand, the bacilli escape from the circulation and pass out of the body by means of sputum, urine,

stools, etc. It is for this reason that the pneumonic form of plague so frequently leads to the spread of infection.

Prophylactic Measures. In addition to isolation, disinfection, and other sanitary measures, an important point is the destruction of rats and mice. Whether the infection is carried to man directly through external wounds, or indirectly by means of fleas infesting rats, is not clear. But whatever be the precise mode of human infection there can be little doubt as to the immense danger of harbouring infected rats in a locality. Rats can be exterminated by the usual methods, or by soaking bread with "Danysz rat virus," which is the culture of a virulent variety of an organism allied to the B. coli communis.

As regards specific prophylaxis, Haffkine's vaccine may be recommended. For, although absolute protection is not afforded by the inoculation, both the incidence and case mortality are considerably less in the inoculated than in the uninoculated. The duration of protection is said to be about six months.

The vaccine is essentially a sterilised culture of the B. pestis in broth (prepared from goat's flesh), the microbe being grown in it for four to six weeks. The flasks are shaken every few days so as to break the stalactite growths and induce fresh crops. The culture is then sterilised, and a small quantity of antiseptic added.

Haffkine's prophylactic fluid thus contains not only the dead organisms, but also the broth in which they have been cultivated. In other words, it is a mixture of intra- and extracellular toxins, although it is doubtful if the organism forms any soluble poisons. The fluid, when introduced into the body, is assumed to produce

antagonistic substances which neutralise the infective agent should it attack the individual.

Various curative sera (Yersin's, Lustig's) have been tried, but without much success.

The antitoxic treat

ment of plague; therefore, cannot be regarded as a success.

CHAPTER XXIII.

TETANUS.

THE bacillus tetani, which was first isolated by Kitasato, is a slender organism usually growing in the form of long threads. The threads are motile, but after growing for some time at 37° C. the motility ceases and the spores are formed.

These are rounded in form and are situated

FIG. 23. Tetanus Bacilli and Spores.

[From Curtis's Essentials of Practical Bacteriology.]

at one end of the bacillus, giving it the appearance of a "drumstick ".

This organism differs from those we have studied previously in the fact that it only grows in the absence of oxygen (obligatory anaerobe). It may be cultivated in