Descriptions of the pathological varieties of leprosy have been very diverse. The classification now generally adopted includes three forms: the _tuberculated_, the _anæsthetic_ (or maculo-anæsthetic), and the _mixed_. _Lepra tuberculosa_ is that form of the disease affecting chiefly the skin, and resulting in nodular tuberculated growth or a diffuse infiltration. It causes great disfigurement. The anæsthetic form causes a destruction of the nerve fibres, and so produces anæsthesia, paralysis, and what are called "trophic" changes. Not infrequently patches occur on the skin, which appear like parchment, owing to this trophic change. Bullæ may arise. When the tissue change is radical or far advanced, considerable distortion may result. The mixed variety of leprosy, as its name implies, is a mixture of the two other forms. The _Bacillus lepræ_ was discovered by Hansen in 1874. He found it in the lepra cells in the skin, lymph glands, liver, spleen, and thickened parts of the nerves. It is common in the discharges from the wounds of lepers. It is conveyed in the body by the lymph stream, and has rarely been isolated from the blood (Köbner). [Illustration: BACILLUS OF PLAGUE (B. PESTIS BUBONICÆ) (From liver of rat) × 1000 _By permission of the Scientific Press, Limited_] [Illustration: BACILLUS OF LEPROSY (HANSEN) (From the tissues of a leper) × 1000] [Illustration: STREPTOTHRIX ACTINOMYCES (RAY FUNGUS) × 700] [Illustration: BACILLUS MALLEI (GLANDERS) × 1000] The bacillus is present in enormous numbers in the skin and tissues, and has a form very similar indeed to _Bacillus tuberculosis_. It is a straight rod, and showing with some staining methods marked beading, but with others no beading at all. It measures 4 µ long and 1 µ broad. Young leprosy bacilli are said to be motile, but old ones are not. Neisser has maintained that the bacillus possesses a capsule and spores. The latter have not been seen, but Neisser holds that this is the form in which the bacillus gains entrance to the body. There is a characteristic which fortunately aids us in the diagnosis of this disease in the tissues, and that is the arrangement of the bacilli, which are rarely scattered or isolated, but gathered together in clumps and colonies. Bordoni-Uffreduzzi and Campania claim to have isolated the bacillus and grown it on artificial media, the former aërobically on peptone-glycerine-blood-serum, at 37° C., the latter anaërobically. But no other worker has been able to do this. Hence we are not able to study the bacteriology of leprosy at all completely, nor have inoculation experiments proved successful. Nevertheless there is little doubt that leprosy is a bacterial disease produced by the bacillus of Hansen. Bordoni-Uffreduzzi maintains that the parasitic existence of the _Bacillus lepræ_ may alternate with a saprophytic stage. This may be of importance in the spread of the disease. There is evidence in support of the non-communicability of the disease by