The term sarcoma is applied to any connective-tissue tumour which exhibits malignant characters.  The essential structural feature is the predominance of the cellular elements over the intercellular substance or stroma, in which respect a sarcoma resembles the connective tissue of the embryo.  The typical sarcoma consists chiefly of immature or embryonic connective tissue.  It most frequently originates from fascia, intermuscular connective tissue, periosteum, bone-marrow, and skin, and forms a rounded or nodulated tumour which appears to be encapsulated, but the capsule merely consists of the condensed surrounding tissues, and usually contains sarcomatous elements.  The consistence of the tumour depends on the nature and amount of the stroma, and on the presence of degenerative changes.  The softer medullary forms are composed almost exclusively of cells; while the harder forms—­such as the fibro-, chondro-, and osteo-sarcoma—­are provided with an abundant stroma and are relatively poor in cells.  Degenerative changes may produce areas of softening or liquefaction which result in the formation of cystic cavities in the interior of the tumour.  The colour depends on the amount of blood in the tumour, and on the presence of the products of degeneration.

The blood vessels are usually represented by mere chinks or spaces between the cells.  This peculiarity accounts for the facility with which haemorrhage takes place into the substance of the tumour, the persistence of the bleeding when it is incised or ulcerates through the skin, and the readiness with which the sarcomatous cells are carried off and infect distant parts through the blood-stream.  Sarcomas are devoid of lymphatics, and unless originating in lymphatic structures—­for example, in the tonsil—­they rarely infect the lymph glands.  Minute portions of the tumour grow into the small veins, and, becoming detached, are transported by the blood-current to distant organs, where they are arrested in the capillaries and give rise to secondary growths.  These are most frequently situated in the lungs, except when the primary growth lies within the territory of the portal circulation, in which case they occur in the liver.  The secondary growths closely resemble the parent tumour.  Sarcoma may invade an adjacent vein on such a scale that if the invading portion becomes detached it may constitute a dangerous embolus.  This may be observed in sarcoma of the kidney, the growth taking place along the renal vein until it projects into the vena cava.

[Illustration:  FIG. 55.—­Recurrent Sarcoma of Sciatic Nerve in a woman aet. 27.  Recurrence twenty months after removal of primary growth.]