This form of microbe was first observed by Hansen in Norway, and the observation was confirmed by Neisser. It is a rod which closely resembles the tubercle bacillus, and, like it, is motionless. It presents clear spots, which remain un-ooloured when the bacillus is stained, but it is not known whether these are spores or not. It is stained by the same process as the tubercle bacillus, but does not resist decolorization so strongly, and it is stained by concentrated watery solutions of the aniline dyes, especially fuchsine and methyl-violet, somewhat more readily than the tubercle bacillus.

The bacillus is found in all cases of leprosy in the lesions in the skin, nerves, and elsewhere. It occurs in very large numbers, especially in the tubercular form, so that when stained by Gram's method a section will have a decided blue colour from the stained bacilli alone. The bacilli are in cells, and so present themselves in little rounded clumps (see Fig. 134, p. 319).

Numerous attempts have been made to cultivate the bacillus, but hitherto without success. Melcher and Ortmann have succeeded in inoculating the anterior chamber of the eye in rabbits with pieces from a freshly excised leprosy nodule. Characteristic new-formations appeared in almost all the internal organs, especially the caecum, lymphatic glands, spleen, and lungs. In these the bacilli were abundantly present.

In man there is now sufficient evidence of direct communication of the disease from one person to another, but the period of inoculation is very long and the mode of communication often difficult to trace (see p. 318).