A sarcoma is a tumor made up of cells that resemble physically those found in embryonal connective tissues. As a rule the greater the departure from the adult cell, the greater is the malignancy. They are characterized by the preponderance of the cells over the intercellular substance, which may be granular, fibrillary, or reticular. The sarcoma cells are not truly embryonal, as they never continue to a complete development.

They arise from the mesoblastic layer and often retain the characteristics of the tissue from which they arise, periosteal sarcomata sometimes containing bone.

The sarcomata are essentially malignant; that is, they infiltrate the surrounding tissues, give metastases, cause cachexia, and return after excision. It is only occasionally that they are encapsulated.

The blood-vessels are generally few in number and imperfectly formed, the single layer of endothelium being supported by a very few connective-tissue fibers. In many cases the blood-channels are simply spaces whose walls are formed by the tumor cells. The imperfect vessel wall explains why hemorrhage in these tumors is so common and why metastasis takes place by means of the blood.

Fig. 39. - Small Round-cell Sarcoma of the Lower Jaw. Oc. 3; ob.

D. D. (McFarland).

Sometimes the blood-spaces may be very large and numerous, thus forming the angiosarcoma.

As a rule, no lymphatics are present.

Sarcomata may occur in any part of the body; as a rule, they are seldom primary within organs.

They generally occur before the age of thirty. Are frequently rounded in shape, somewhat tabulated, and to a certain degree circumscribed. Are hard or soft according to the amount of intercellular substance present, or to the variety of the tissue of which they are composed.

Their color is generally pink or grayish; this, however, depends to a great extent upon the condition and number of the blood-vessels.

Fig. 40. - Spindle-cell Sarcoma from the Brain. X 300 (Durck). 1. Spindle cells cut longitudinally; 2, spindle cells in transverse section.

On account of the poor blood supply, degenerations, particularly myxomatous, frequently take place. Interstitial hemorrhages as a result of the degeneration are not infrequent.

If there is pigment present, either melanin or hemosiderin, the tumor is called a pigmented one.

These tumors vary greatly in their malignancy, the small round-cell type, especially if melanotic, being rapidly fatal. The greater the amount of cellular elements, the greater is the malignancy.

The varieties of the tumors depend upon the kind of cell that predominates.