The life-history of an untreated gumma varies with its environment.  When protected from injury and irritation in the substance of an internal organ such as the liver, it may become encapsulated by fibrous tissue, and persist in this condition for an indefinite period, or it may be absorbed and leave in its place a fibrous cicatrix.  In the interior of a long bone it may replace the rigid framework of the shaft to such an extent as to lead to pathological fracture.  If it is near the surface of the body—­as, for example, in the subcutaneous or submucous cellular tissue, or in the periosteum of a superficial bone, such as the palate, the skull, or the tibia—­the tissue of which it is composed is apt to undergo necrosis, in which the overlying skin or mucous membrane frequently participates, the result being an ulcer—­the tertiary syphilitic ulcer (Figs. 40 and 41).

Tertiary Lesions of the Skin and Subcutaneous Cellular Tissue.—­The clinical features of a subcutaneous gumma are those of an indolent, painless, elastic swelling, varying in size from a pea to an almond or walnut.  After a variable period it usually softens in the centre, the skin over it becomes livid and dusky, and finally separates as a slough, exposing the tissue of the gumma, which sometimes appears as a mucoid, yellowish, honey-like substance, more frequently as a sodden, caseated tissue resembling wash-leather.  The caseated tissue of a gumma differs from that of a tuberculous lesion in being tough and firm, of a buff colour like wash-leather, or whitish, like boiled fish.  The degenerated tissue separates slowly and gradually, and in untreated cases may be visible for weeks in the floor of the ulcer.

[Illustration:  FIG. 40.—­Ulcerating Gumma of Lips.

(From a photograph lent by Dr. Stopford Taylor and Dr. R. W. Mackenna.)]

The tertiary ulcer may be situated anywhere, but is most frequently met with on the leg, especially in the region of the knee (Fig. 42) and over the calf.  There may be one or more ulcers, and also scars of antecedent ulcers.  The edges are sharply cut, as if punched out; the margins are rounded in outline, firm, and congested; the base is occupied by gummatous tissue, or, if this has already separated and sloughed out, by unhealthy granulations and a thick purulent discharge.  When the ulcer has healed it leaves a scar which is depressed, and if over a bone, is adherent to it.  The features of the tertiary ulcer, however, are not always so characteristic as the above description would imply.  It is to be diagnosed from the “leg ulcer,” which occurs almost exclusively on the lower third of the leg; from Bazin’s disease (p. 74); from the ulcers that result from certain forms of malignant disease, such as rodent cancer, and from those met with in chronic glanders.