lium of the pia. The very sharp line of demarcation between the epithelium and the normal pia mater made the idea of the origin of the tumor from separated epithelial cells seem very probable. This idea has a strong support in the case described by Bonorden, in which the tumor contained glands and hair-follicles, structures belonging only to the external skin. Beneke has shown that the meningeal steatomata are produced by proliferation of the endothelial cells of the pia. He bases his opinion upon the fact that silver staining yields the outlines of endothelial cells, which would not be the case with epithelial cells.

Cholesteatoma is found, besides, in the meninges and the ventricles of the brain, in bones, especially the petrous portion of the temporal bone, and in the mastoid process, in the testicle, and in the ovary. In the meninges of the brain cholesteatoma probably starts in the perivascular lymph-spaces. Rindfleisch very strongly maintains that these tumors in the meninges of the brain are of endothelial origin. Wendt believes that in the petrous portion of the temporal bone cholesteatoma is produced by inflammation of the middle ear resulting in desquamation and accumulation of epithelial cells, but he has described also genuine cholesteatoma of endothelial origin in the drum of the

ear. In tumors of the pia mater belonging to this category this membrane surrounds the tumor mass, but the space is not lined by endothelial cells. Ziegler found hair in some of these tumors, in which case we must assume for some of them an epithelial origin from a displaced tumor-matrix; but these cases must be exceedingly rare. Eberth found that in cholesteatoma of the pia mater the first change that is seen in the formation of the tumor is the appearance of protoplasmic masses which surround the vessels like a sheath. In the sheath irregular nuclei are seen, besides giant-cells. Virchow and Eberth claimed that these cells were epithelial cells produced by heteroplastic proliferation of the connective tissue. This view is, of course, no longer tenable, as we have shown repeatedly that epithelial cells are never produced from connective tissue. Cholesteatoma never gives rise to metastasis, and it manifests no tendency to invade surrounding tissues to any extent, resembling in these respects psammoma, with which it is histologically and clinically so closely allied.

XVIII. LIPOMA.

Definition.-A lipoma is a circumscribed or diffuse tumor composed of fatty tissue produced from a matrix of lipoblasts. The subcutaneous fatty tissue is the favorite seat of lipoma. Toldt ascertained that in the embryo the panniculus adiposus is formed by cell-islets, the socalled "fat-organs," each of which has a separate and independent blood-supply. These islets are separated from one another by connective tissue. Young fat-cells are called "lipoblasts." Their number and activity, as well as the assimilation of fat from the blood or the food, determine the amount of fat. Each fat-lobule has its own artery and capillary circulation, terminating in a common vein. The lobule therefore represents an organized unity, like an acinus in the liver. According to Virchow, the lipoblasts develop from fetal myxomatous tissue into which the mature fat-cells can revert. If the cells of any of these fat-forming centres should become arrested in their development and remain in a quiescent state, it is easy to see how at any time, by their resuming active tissue-proliferation, they could give rise to a fatty tumor. Having become emancipated, as it were, from the organ

FIG. 281.-Fat-cells imbedded in subcutaneous areolar tissue (after Schiefferdecker): f, fat-cells; n, nucleus; c, connective-tissue corpuscles; w, migratory cells; e, elastic fibres; b, capillary blood-vessels.

ism, their growth, development, and reproduction would no longer be controlled by the laws which regulate normal nutrition. It would be difficult to explain localized hyperproduction of fatty tissue in any other way.

Histology. The fat-cells in a lipoma, as in normal adipose tissue, represent connective-tissue cells with oily contents. The cells are

round or oval sacs; the transparent contents are limited by a delicate envelope composed of cell-membrane and of an extremely thin layer of protoplasm. The nucleus is located on one side of the sac (Fig. 281).

There is nothing to distinguish a fat-cell in a lipoma from a fat-cell in normal adipose tissue. The cells occur in groups supported and held together by areolar tissue through which ramifies a rich vascular network. The amount of stroma varies: in the soft lipomata it is very scanty, so that under the microscope it is difficult to recognize it, it being almost completely overshadowed by the fat-cells. In the hard lipoma the fibrous structure of the tumor is well developed and the fat-cells are crowded into the large areolæ of the stroma. Some lipomata are exceedingly vascular, and we then speak of a lipoma telangiectodes. In other instances the stroma contains venous channels of large size, when the tumor is called lipoma cavernosum. The writer has met with such vascular lipomata most frequently in congenital lipoma.

Regressive Metamorphoses.-The stroma of a lipoma is more prone to undergo retrogressive metamorphosis than is the parenchyma of the tumor. The most frequent degenerative change observed is myxomatous degeneration of the stroma. The connective-tissue fibres are separated by the myxomatous material, and the stroma presents the appearance of juvenile connective tissue. The tumor or part of a tumor undergoing this process becomes softer. Calcification of the stroma arrests the growth of the tumor affected by this change, the parenchyma-cells degenerate, and the tumor becomes eventually completely petrified. Burow found cholesterin in a large lipoma of the axilla. The lime-salts found in a calcified lipoma are carbonate and phosphate of lime. Fürstenberg found in the fat-cells lime-salts in combination with fatty acids. Ossification of parts of the stroma occurs in rare cases. Oil-cysts have been found in the interior of fatty tumors, and are supposed to have been formed by atrophy of the cell-envelopes and accumulation of their contents in the stroma.

Anatomical Varieties.-All lipomata are encapsulated. The capsule is perfect in the circumscribed variety; in the diffuse form the tumor sends out into the surrounding loose connective tissue prolongations which sometimes are not discovered in the removal of the tumor, and lead to a recurrence of the growth. The diffuse form frequently occupies a large territory, as, for instance, the anterior surface of the neck. The lipoma arborescens or racemosum described by J. Müller is a branching fatty tumor (Fig. 282). It is found most frequently in the knee-joint, where it starts beneath the synovial mem

brane, and, pushing this before it, sends branching lobes into the joint. Lipoma arborescens is also found quite frequently as a diffuse tumor under the peritoneum and the pleura.

Symptoms and Diagnosis.-Lipoma frequently occurs as a congenital tumor. Sometimes it is found as a symmetrical affection-for instance, the simultaneous occurrence of a lipoma in each axillary space. The writer has observed such a case in a woman fifty years of

age. Billroth, in a paper published shortly before his death, called attention to the occurrence of symmetrical lipoma. As a post-natal tumor it commences most frequently after puberty. Its growth is always slow. Sometimes it remains stationary for a certain length of time, when, without any apparent provocation, it resumes its growth. It attains occasionally an immense size. which the tumor weighed sixty pounds. ten pounds, however, are very rare.

Rhodius recorded a case in Tumors weighing more than If the tumor is subcutaneous, the

skin over it, from tension, atrophies, and ulceration from impaired nutri