The Round-Celled Sarcoma (Fig. 97) is also called the granulation sarcoma and the encephaloid sarcoma. It is composed of round or slightly oval cells generally about the size of white blood-corpuscles, but sometimes much larger. It is an exceedingly soft tumour, often half diffluent, and has usually a grey medullary appearance. Its bloodvessels are in the form mainly of large capillaries, whose walls are embryonic in structure and often present varicose or aneurysmal dilatations. The vessels are liable to rupture, and so these tumours often present interstitial haemorrhage. Betweenthe cells there may be some intercellular substance. This is sometimes homogeneous and becomes opaque with acetic acid (Myxo-sarcoma), or it may be somewhat fibrous or reticulated.

This form of sarcoma is met with in the skin, where it may originate in a congenital soft wart or mole; in the subcutaneous tissue; in the bones - forming the majority of the so-called medullary cancers of bone; in the muscles; in the glands - especially the mamma and testicle; in the brain and elsewhere. Being a soft tumour with delicate vessels it more readily produces secondary tumours by metastasis than other sarcomas. It is also usually a tumour of rapid growth, and commonly imperfectly delimited from the surrounding tissue.

The Lympho-sarcoma deserves special mention as a variety of the round-celled sarcoma. It originates from lymphatic glands, its commonest seat of origin being the glands at the root of the lungs in the mediastinum, but it also arises in the glands of the mesentery. It presents very typically the characters of local malignancy, involving and moulding itself on neighbouring structures, such as the pericardium, heart-wall, veins, and bronchi in the case of mediastinal tumours, and the intestine in the case of mesenteric ones.