The coexistence of diffuse myelomatosis of the skeleton and albumosuria (Bence-Jones) is referred to on p. 474.  Myeloma occurs in the jaws, taking origin in the marrow or from the periosteum of the alveolar process, and is described elsewhere.

#Sarcoma# and #endothelioma# are the commonest tumours of bone, and present wide variations in structure and in clinical features.  Structurally, two main groups may be differentiated:  (1) the soft, rapidly growing cellular tumours, and (2) those containing fully formed fibrous tissue, cartilage, or bone.

(1) The soft cellular tumours are composed mainly of spindle or round cells; they grow from the marrow of the spongy ends or from the periosteum of the long bones, the diploe of the skull, the pelvis, vertebrae, and jaws.  As they grow they may cause little alteration in the contour of the bone, but they eat away its framework and replace it, so that the continuity of the bone is maintained only by tumour tissue, and pathological fracture is a frequent result.  The small round-celled sarcomas are among the most malignant tumours of bone, growing with great rapidity, and at an early stage giving rise to secondary growths.

(2) The second group includes the fibro-, osteo-, and chondro-sarcomas, and combinations of these; in all of them fully formed tissues or attempts at fully formed tissues predominate over the cellular elements.  They grow chiefly from the deeper layer of the periosteum, and at first form a projection on the surface, but later tend to surround the bone (Fig. 150), and to invade its interior, filling up the marrow spaces with a white, bone-like substance; in the flat bones of the skull they may traverse the diploe and erupt on the inner table.  The tumour tissue next the shaft consists of a dense, white, homogeneous material, from which there radiate into the softer parts of the tumour, spicules, needles, and plates, often exhibiting a fan-like arrangement (Fig. 151).  The peripheral portion consists of soft sarcomatous tissue, which invades the overlying soft parts.  The articular cartilage long resists destruction.  The ossifying sarcoma is met with most often in the femur and tibia, less frequently in the humerus, skull, pelvis, and jaws.  In the long bones it may grow from the shaft, while the chondro-sarcoma more often originates at the extremities.  Sometimes they are multiple, several tumours appearing simultaneously or one after another.  Secondary growths are met with chiefly in the lungs, metastasis taking place by way of the veins.

[Illustration:  FIG. 146.—­Periosteal Sarcoma of Femur in a young subject.]

[Illustration:  FIG. 147.—­Periosteal Sarcoma of Humerus, after maceration.

(Anatomical Museum, University of Edinburgh.)]

Clinical Features.—­Sarcoma is usually met with before the age of thirty, and is comparatively common in children.  Males suffer oftener than females, in the proportion of two to one.