_The Plague_ (_Pestis Bacillus_).--The Bombay plague of 1897-98 will ever be remembered as one of the most appalling visitations ever known. The number of deaths will never be accurately determined, as the native population, among whom the disease chiefly prevailed and became so fatal, concealed their dead or carried them away by night. The outbreak from the first proved to be most infectious, its incubation lasting from a few hours to a week only. It prevailed in all the over-crowded native quarters of the city. The rats and mice that infested the dwellings of the poor were found to be equally susceptible with human beings, and these vermin also died by hundreds. Those that survived left their holes and made off, in this way helping to spread the infective virus. On examining the bodies of dead rats, they were found to have swollen legs, the blood being filled by bacilli and curious monads, with whip-like appendages. The bacillus of plague was discovered by Kitasato in 1894; it is characterised by short rods with rounded ends, and a clear space in the middle. The bacilli stain readily with aniline dyes, and when cultivated on agar, white transparent colonies are formed which present an iridescent appearance when examined by reflected light. In addition to the bubonic swellings, the neighbouring lymphatic glands were also swollen and blocked by bacilli. [Illustration: Fig. 271.--Monads in Rat’s Blood, 1,200 ×. (Crookshank.) _a_. Monad threading its way among the blood-corpuscles; _b_. Another with pendulum movement attached to a corpuscle; _c_. Angular forms; _d_. Encysted forms; _e_ and _f_. The same seen edgeways.] My illustration (Fig. 270) is from a micro-photograph taken in 1897, when the death rate stood very high. The general distribution of the bacilli, together with phagocytes and the contents of swollen lymphatic glands, magnified 1,200 ×, is from a preparation made in hospital. The monads from the rat’s blood, 1200 ×, seen threading their way among the blood corpuscles of a rat, and represented in Fig. 271, are somewhat larger than those found in the Bombay rats, but the flagella in the latter were quite as marked, while the encysted forms were wholly absent and the blood corpuscles less crenated. The white bodies (Fig. 270) were in some preparations, together with the lymphatic bodies, more numerous and more swollen. With regard to the conditions of life of the bacteria, they may be divided broadly into two classes. When the organisms draw their nourishment from some living body or “host,” they are known as “parasites.” These are further termed “obligate” parasites if they exclusively live on their “host.” If the bacteria draw their nourishment from dead organic matter, they are called “saprophytes.” These are also divided into “obligate” and “facultative” saprophytes. Thus it will be apparent that a parasite under certain circumstances may readily become a saprophyte.