All cutaneous lesions caused by the tubercle bacillus must be placed in this class. It is only recently that the full importance of tuberculosis in all forms has been appreciated by the profession and the lay public. At the present time well-directed efforts are being made to relieve and prevent the development and spread of tuberculosis. It is owing to the importance of these tuberculids as a part of the tubercular process that so much space is given to their consideration in this work.
The tubercle bacillus is only feebly infectious and the wide distribution of tuberculosis is probably due to its general and constant prevalence. While many conditions may create a predisposition to tuberculosis, there is every reason to believe that the actual disease is always acquired and that no child is born with tuberculosis. Individual immunity from the disease or susceptibility to one of its various forms may be traced to a host of predisposing and exciting causes. In general it may be said that poor sanitation, improper or scanty food, early life with its lessened resistance, and accidental traumatisms or inflammations affecting the skin or mucous membranes, present an open invitation to tubercular infection. Cutaneous tuberculosis is not doubt derived from those affected with the pulmonary type but it is possible that visceral infection may be secondary to the skin manifestations.
The tuberculids may be conveniently studied in four divisions: (1) tuberculosis cuts orificialis; (2) tuberculosis verrucosa; (3) lupus vulgaris; (4) scrofuloderma. A number of other conditions which probably are not directly due to the tubercle bacillus but to its toxins such as lichen scrofulosus, erythema induratum, acne varioliformis, lupus erythematosus and possibly others are referred to separately under their respective titles.