This form of tumour is composed of mucous tissue, and as this is not one of the physiological tissues of the adult, it will be proper to refer more specially to its characters. The blood-vessels of the umbilical cord are padded and protected from pressure by a gelatinous substance called Wharton's jelly. Under the microscope, this consists of variously shaped cells separated by a clear transparent intercellular substance. The intercellular substance is of gelatinous consistence, and owes this character to the fact that it consists of a watery solution of mucin.

Mucin is nearly allied to albumen, but when present in even small quantity in a solution, it gives the latter a sticky gelatinous character. Its chemical reactions also differ from those of albumen, in respect that, though like it precipitated by alcohol, the precipitate is redissolved by water. Also, acetic acid and other organic acids precipitate mucin, but not usually albumen. The precipitate of mucin by alcohol or acetic acid is more membranous than that of albumen. The reaction with acetic acid can be readily brought out in Wharton's jelly, a microscopic section showing, on adding acetic acid, a reticulated precipitate. These reactions can also be studied in the mucus from any mucous membrane, or in the bile, in which mucin is normally one of the dissolved constituents.

Besides in the umbilical cord, mucous tissue is present in the villi of the chorion of the fœtus, the villi consisting of a covering layer of epithelium with mucous tissue internally. In the fœtus, it is also present in early stages in the subcutaneous tissue where it has the place of the subcutaneous adipose tissue, being, in fact, related to fat very much as the .temporary cartilage is to bone. Some remains of this tissue are met with in the adult, Thus, the vitreous humour of the eye is really composed of soft mucous tissue, and traces of it have been found in places where normally adipose tissue exists, as under the pericardium, at the hilus of the kidney, subcutaneously, and in the medulla of bone. It appears that in these positions there is sometimes a partial recurrence to the foetal condition. The connective substance of the brain, the neuroglia, is allied to mucous tissue, and seems to present a proneness to return to that form.

The hydatid mole, which arises by a great new-formation of mucous tissue in the villi of the chorion, is regarded by some as a form of myxoma. (See further on in Section on Generative Organs).

Mucous-tissue tumours are met with principally in the subcutaneous tissue, where they may be regarded as probably due to a piece of embryonic tissue left over when the mucous tissue was converted into adipose, in this respect resembling the chondromas of bone. They form rounded or oval tumours, generally soft, sometimes almost fluctuant in consistence and hyaline in appearance. These tumours are sometimes described as soft fibromas or fibro-cellular tumours. Examined under the microscope we have a very translucent tissue in the midst of which there are irregularly shaped cells. The tissue is intersected by more or less frequent bands of connective tissue (Fig. 74). The proportion of cells varies considerably in different cases, and even in different parts of the same tumour. •

Myxomas are also of somewhat frequent occurrence in the brain as compared with other tumours of this organ. They arise in connection with the soft membranes of the brain and spinal cord, or in the ventricles, or in the substance of the brain. They sometimes grow to considerable size, and are liable by softening to take the form of cysts filled with a mucous fluid. They are also met with on peripheral nerves. They occur in the mamma, where not infrequently they appear to have the character of a diffuse formation of mucous tissue between the glandular acini, so that the gland as a whole is converted into a tumour. In the salivary glands they occur, but are usually of mixed structure, being partly formed of other kinds of tissue.

Microscopic section of a myxoma of the subcutaneous tissue.

Fig. 74. - Microscopic section of a myxoma of the subcutaneous tissue. Isolated cells and strands of fibres are shown. Between them a clear gelatinous material was present, x 350.

The simple pure hyaline myxoma is not very common, as the tissue is apt to be mixed with other forms. From the relation of mucous and adipose tissues it is not remarkable that a partial conversion of a lipoma into a myxoma, or vice versa, is met with. There is also not infrequently a mixture with fibrous, cartilaginous, or glandular tissue. Then, also, the myxomas vary very greatly in the proportion of cells, the very cellular ones being called medullary myxomas, and frequently graduating into sarcomas. In fact, with pathologists who take the embryonic nature of the tissue as the criterion of sarcomas, the fact that mucous tissue is the embryonic precursor of adipose tissue induces them to class this form of tumour among the sarcomas. Consistently with these facts there are some myxomas which show malignant characters, either local or general. In a case recorded by Virchow there were tumours on the nerves and in the dura mater of the cord and brain.

Literature

Muller, Arch. f. Anat. u. Phys., 1836; Virchow, Arch., xi., 286, and Geschwiilste, i., p. 396.