A common species of tumour is that which is attached to a mucous surface, and is called POLYPUS. Polypi vary in structure and disposition; some are simple and benign, others are most malignant. The simple mucous polypus has a shining appearance, being invested by an extremely delicate membrane, in some degree resembling the mucous, and moistened by a fluid similar to the mucous secretion; it is of soft consistence and homogeneous structure. They are generally light brown, sometimes greyish, and in some degree diaphanous. They are connected to the mucous membrane by a narrow pedicle; generally occur in clusters, and are of pyriform shape; one or more are often suspended from one narrow base, and they seldom attain a large size. They possess but little vascularity, though occasionally minute vessels are seen ramifying pretty freely on their surface, and may be considered as almost devoid of sensibility. The malignant polypus, on the contrary, is always attached to the mucous membrane, and also to the subjacent parts, by means of a broad base; and its form and structure vary according to the particular action which it has assumed. Most frequently it is encephaloid, of an irregular form, and often presenting a cauliflower appearance, its surface being studded with numerous excrescences of medullary consistence and colour. Such tumours will be afterwards treated of, as occurring in different situations. OF ENCYSTED TUMOURS. Along with these have been classed the enlargements of bursæ, sheaths of tendons, &c., but with equal propriety might we include hydrocele and other collections in natural cavities. Encysted tumours are almost always situated superficially. The skin is distended, seldom inflamed, and often contains enlarged bloodvessels, which give it a streaked appearance. They consist of an external cyst, which is in some instances extremely thin and delicate, in others dense, of considerable thickness, and composed of fibrous looking substance, occasionally it is almost cartilaginous; the internal structure may be said to be almost always more or less fluid, but varies much in consistence. The tumour is surrounded with condensed cellular substance, which is of greater or less thickness and strength according to the size of the tumour. Some of these tumours are supposed, and on good grounds, to be mere enlargements of the natural mucous follicles, in consequence of obstruction in their ducts, by hardened and vitiated secretion; the cyst, therefore, will be at first thin and delicate; its contents will resemble the natural secretion of the follicle, and in many cases may be readily squeezed out. There is a black point on the most prominent part, marking the obstructed orifice of the follicle, and the sac is found at this point to adhere firmly to the skin. Even after all other marks of its original formation have disappeared, the situation of the orifice is sometimes indicated by small dark spots, by depression, or by a minute ulcer.