I have taken this third paper of the series to call attention that if you have in the x-ray a definite bone shell, and the bone destruction is all within that shell and there is no evidence, in the x-ray and on palpation, of any tumor formation outside the bone shell, and, at the exploratory incision, you find within the bone shell a mass of fibrous tissue difficult to distinguish from sarcoma in the gross and frozen section, you will be quite safe in concluding that you are dealing with osteitis fibrosa. Conservative measures are in order.

If there are many minute cysts, or one large cyst, this is evidence still more positive of the benign bone cyst (osteitis fibrosa).

If the patient is under twenty years of age, this is additional evidence that the lesion is benign.

As more adults are x-rayed for sprains and contusions, for slight pain or limp as well as for fracture, we may expect to see more and more frequently evidence of the single, healed or latent bone cyst, and after definite injuries these quiescent lesions may grow and produce conditions seen in Cases 1, 3 and 4, illustrated in this paper.

The predominant central lesions of bone are osteitis fibrosa and the giant-cell tumor. Next in order of frequency comes the metastatic tumor. Chondroma, myxoma and sarcoma, developing in one or the other or both, are not frequent tumors. The multiple myeloma without evidence of other bone involvement is a rare disease. It is time now for all of us to learn to recognize osteitis fibrosa and the giant-cell tumor in all their clinical, gross, microscopic and x-ray variations.

I trust the discussion of these four cases with their illustrations will be helpful, and I wish to repeat that up to the present time there is no positive evidence of a central fibroma or fibrosarcoma of bone, such as occurs in the lower jaw. If Case 2 had been seen with an intact bone shell and no tumor tissue outside the cortical bone, we would have definite evidence of a central sarcoma of the osteogenic type. But at present I have no evidence of such a tumor. The only type of central sarcoma is chondromyxosarcoma, which should be easily distinguished from the giant-cell tumor and osteitis fibrosa, and the very rare bone aneurysm which can be differentiated from the hemorrhagic giant-cell tumor in the frozen section.