The Lesions of tertiary syphilis have usually their seat of origin in the periosteum, although the subjacent bone may be simultaneously involved. The condition may be briefly described by stating that gummata are produced, while inflammation occurs in the neighbourhood.

Gummata of the usual structure are produced in the internal layers of the periosteum, and there is thickening of the periosteum around.

Fig. 284. - Necrosis of parietal bone in syphilis.

This membrane being tightly stretched over the tumour, the swelling is hard to the feeling, but often with a certain elastic resistance. The gumma undergoes caseous necrosis in its central parts, but advances peripherally, and as it advances against the bone it causes erosion of it. The advance is in the first instance along the vessels, and as erosion occurs around them a worm-eaten appearance is produced in the bone. Beneath the gumma, which has most frequently its seat on the bones of the skull, especially the frontal bone, there may be thus considerable loss of substance, so that even the entire thickness of the skull may be penetrated (Fig. 284). The caseous material often undergoes softening, and suppuration ensues; there may be very obstinate and even progressive ulcers produced. Without ulceration the gumma may undergo resolution, and a cicatricial depression result, or after ulceration healing may occur.

Along with these processes immediately related to the gumma, there are, in the neighbourhood, conditions referable to inflammation. The bone around is condensed by new-formation filling up the medullary spaces, and there is sometimes a thickening of the bone by subperiosteal inflammation. This is much less common in the skull than in the long bones, especially those of the legs, where it sometimes leads to a very striking hypertrophy of the bone, whose surface is exceedingly rough from the loss of substance in some parts, and the irregular new-formation in others. The bone around the syphilitic defect or ulcer is dense bone, and in this respect contrasts very markedly with that around the ulceration in tuberculosis.

Congenital syphilis leads to important changes in the bones, which, according to Wegner, occur in a large proportion of syphilitic foetuses. The lesions affect the structures concerned in the processes of ossification, and in this respect resemble those of rickets, but they partake more of the inflammatory character, thus agreeing with the lesions of hereditary syphilis generally (see p. 302).

In the Cranial bones the periosteum is thickened by inflammation, and this leads to softening of the bone beneath. Craniotabes is a frequent result, the bone being lost in parts exposed to pressure when the child is lying, and the brain being so far covered only by the soft parts.

In the Long bones the ossifying border is the part affected. The cartilage cells undergo excessive proliferation, as in rickets, and there is an irregular calcification of the matrix. Beneath this the proper ossifying zone frequently shows a new-formation of granulation tissue or pus, so as to produce a soft layer almost interrupting the continuity of the bone. There may be a partial necrosis of the calcified cartilage. .The whole process occurs very irregularly, the ossifying part of the bone being swollen and occupied by calcified and proliferating cartilage and inflammatory tissue, while true ossification is delayed and occurs irregularly. A not infrequent consequence is a partial fracture of the bone or a Separation of the epiphysis.

The disease affects many bones, but may present different degrees. According to Wegner the usual seats, in order of frequency, are, lower end of femur, lower ends of bones of leg and forearm, and upper end of tibia.