rectly
excited by the spirochaetes.

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