extensors.  The depth of the track being so disproportionate to its superficial area, the condition closely simulates a tuberculous sinus, for which it is liable to be mistaken.  The raw surface is absolutely insensitive, so that the probe can be freely employed without the patient even being aware of it or suffering the least discomfort—­a significant fact in diagnosis.  The cavity is filled with effete and decomposing epidermis, which has a most offensive odour.  The chronic and intractable character of the ulcer is due to interference with the trophic nerve-supply of the parts, and to the fact that the epithelium of the skin grows in and lines the track leading down to the deepest part of the ulcer and so prevents closure.  While they are commonest on the sole of the foot and other parts subjected to pressure, perforating ulcers are met with on the sides and dorsum of the foot and toes, on the hands, and on other parts where no pressure has been exerted.

The tuberculous ulcer, so often seen in the neck, in the vicinity of joints, or over the ribs and sternum, usually results from the bursting through the skin of a tuberculous abscess.  The base is soft, pale, and covered with feeble granulations and grey shreddy sloughs.  The edges are of a dull blue or purple colour, and gradually thin out towards their free margins, and in addition are characteristically undermined, so that a probe can be passed for some distance between the floor of the ulcer and the thinned-out edges.  Thin, devitalised tags of skin often stretch from side to side of the ulcer.  The outline is irregular; small perforations often occur through the skin, and a thin, watery discharge, containing grey shreds of tuberculous debris, escapes.

Bazin’s Disease.—­This term is applied to an affection of the skin and subcutaneous tissue which bears certain resemblances to tuberculosis.  It is met with almost exclusively between the knee and the ankle, and it usually affects both legs.  It is commonest in girls of delicate constitution, in whose family history there is evidence of a tuberculous taint.  The patient often presents other lesions of a tuberculous character, notably enlarged cervical glands, and phlyctenular ophthalmia.  The tubercle bacillus has rarely been found, but we have always observed characteristic epithelioid cells and giant cells in sections made from the edge or floor of the ulcer.

[Illustration:  FIG. 16.—­Bazin’s Disease in a girl aet. 16.]

The condition begins by the formation in the skin and subcutaneous tissue of dusky or livid nodules of induration, which soften and ulcerate, forming small open sores with ragged and undermined edges, not unlike those resulting from the breaking down of superficial syphilitic gummata (Fig. 16).  Fresh crops of nodules appear in the neighbourhood of the ulcers, and in turn break down.  While in the nodular stage the affection is sometimes painful, but with the formation of the ulcer the pain subsides.