This form of tumour consists of adipose tissue exactly like that of the body, as for instance the subcutaneous adipose tissue. Adipose tissue contains bands of fibrous connective tissue which carry the vessels and nerves, and so do lipomas, but in different tumours this is variously abundant. If there is little connective tissue the tumour is soft, and may even feel fluctuant. If there is much it is hard, and we may have an approach to the fibrous tumour, the fibro-lipoma. The fibrous character may be increased by irritation, as where a tumour-is exposed to friction, producing a kind of indurative inflammation in the tumour.
Fatty tumours are mostly surrounded by a distinct capsule, but sometimes they are continuous with the surrounding fat. Thus the fat around the mamma or the kidney may undergo such an enlargement as to warrant the name of tumour (Lipoma capsulare). Billroth mentions a lipoma which had grown in between the muscles of the* thigh in such a way as that it could not be removed completely. Lipomas which are not definitely circumscribed may be called Diffuse lipomas.
Fig. 71. - Pedunculated submucous lipoma of colon. Half natural size.
Lipomas are of common occurrence. They are sometimes congenital, especially in connection with spina bifida. Their most frequent seat is under the skin, especially of the trunk. They are rare under mucous membranes, as that of the stomach (Fig. 72), where there is normally a small quantity of fat corresponding with the subcutaneous fat. Figure 71 represents an unusual form of submucous lipoma. They are occasionally met with in connection with serous and synovial membranes, in the appendices epiploic of the intestines, or the synovial fringes. They are rarely heterologous, occurring where fat is not present normally, as in the substance of the kidneys, brain, etc.
They are of rare occurrence on the surface of bones, constituting the Parosteal lipoma. The author met with a case in which an elongated piece of fat lay on the upper surface of the corpus callosum. Lipomas are for the most part single, but in some cases they are multiple, although rarely symmetrical. When multiple they do not grow simultaneously, and may appear in succession for months or years.
Fig. 72. - Lipoma in the wall of the stomach. It was the size of a hazel-nut, situated near the pylorus in the submucous tissue. Natural size. (Virohow).
Fig. 73. - Pendulous lipoma of appendix epiploica of colon. A pedunculated tumour (a) projects from the general mass of subserous adipose tissue (b). It is twisted twice on its axis, and the peduncle is very thin. (Virchow).
Fatty tumours sometimes become pendulous. Those in the appendices epiploicae (Fig. 73) are so from the first, and the neck of the polypoid tumour may get severed, the tumour becoming a loose body in the peritoneum; it is similar with the lipomas of the synovial fringes. Subcutaneous lipomas may also become pendulous and polypoid, and may grow to great dimensions in this form. There is apt to be ulceration of the surface of such tumours, and even haemorrhage. The tumours of the subcutaneous tissue may, by their weight, gradually slide downwards, leaving their old attachments and acquiring new ones. Besides these changes we have the occasional induration already mentioned, and sometimes calcareous infiltration follows. Softening, with the formation of a cyst inside the tumour, is a rare occurrence.