THE BONES AND JOINTS. 747 



frequently does, become converted, in some parts of the tumor, into true 

 bone. On the other hand, combinations with sarcomatous tissue are of 

 frequent occurrence (see below). 



Sarcoma is especially common in the bones. It grows from the inner 

 layers of the periosteum or from the medulla, so that we may distinguish 

 a periosteal and a myelogenic sarcoma. Sometimes the tumor involves the 

 bone itself so early that it is impossible to say whether the tumor began 

 in the periosteum or in the medulla. There is also a variety which grows 

 close to the outside of the periosteum and becomes connected with it 

 parosteal sarcoma. 



The periosteal sarcomata usually belong to the varieties fibro-, myxo-, 

 chondro-, and osteo- sarcoma, more rarely to the medullary variety. 

 They commence in the inner layers of the periosteum, pushing this mem- 

 brane outward. After a time the periosteum is involved and the tumor 



FIG. 493. SARCOMA OF THE BONE PERIOSTEAL. 



The growth has invaded the shaft of the bone, which is fractured. Spicula of bone, new formed in the 

 tumor, may be seen below, passing outward from the periosteum. 



invades the surrounding soft parts. The bone beneath may remain nor- 

 mal, or may be eroded and gradually disappear until the tumor is con- 

 tinuous with the medulla. Portions of the tumor may be calcified, or a 

 growth of new bone may accompany its growth. The new bone usually 

 takes the form of plates, or spicula, radiating outward (Fig. 493). The 

 minute structure of these tumors is very variable. The simplest the 

 fibre-sarcomata are composed of fusiform, round, stellate, and some- 

 times giant cells (myeloplaxes), in varying proportions, packed closely 

 in a fibrous stroma. In the medullary form the stroma is diminished 

 to a minimum and the round cells are most numerous. In the chondro- 

 and myxo -sarcoma the basement substance may be hyaline or mucous, 

 and the cells follow the type of cartilage and mucous tissue more or less 

 closely. There is a mixed form of tumor, called osteoid sarcoma, which 

 is very apt to spread and to form metastases. The growth consists in 

 part of tissue corresponding to fibro-sarcoma and round-celled sarcoma, 

 [n addition to this there occurs, in greater or less quantity, immature 



