Is a chronic inflammatory process due to the presence of a pathogenic fungus, formed of sporangia and mycelia, the sporo-trichium. These organisms are strictly aerobic; grow on the ordinary culture-media at a temperature of 200 to 280 C. They do not stain well, taking the ordinary anilin dyes faintly; if not decolorized too vigorously they stain by Gram's method.

The lesion quite commonly starts at the site of some trifling injury, particularly of the hand. The infected area may or may not become sluggishly inflamed, and there develops a small dermic, sometimes subcutaneous, nodule which may break down and become soft. This may develop into a sluggishly inflamed, discharging sore.

Sooner or later a subcutaneous nodule will be felt at the lower end of the forearm. This gradually enlarges to the size of a cherry, may spread laterally, and finally softens. The overlying skin becomes thinned and of a purplish color, then breaks through, the discharge being of a viscid, gelatinous, seropurulent character. This formation is successively followed by several such nodules higher up the arm along the lymphatic vessels which can be felt usually as hard cords.

Microscopically may conform more or less closely to one of three types, or there may be an admixture of all varieties: 1. Proliferation of the connective tissue with lymphoid cells, syphiloid. 2. Epithelioid proliferation with giant cells, tuberculoid. 3. Polynuclear infiltration resembling suppuration.

It may be difficult to find the organism by staining the discharges, but the fungus grows readily on ordinary culture-media.