TUMORS OF DOMESTIC ANIMALS 11 
supporting connective tissue is coarser and contains fewer blood 
vessels. 
Combinations. — Fibromatous change in lipomas is the most com- 
mon. It is considered by many authorities to be more of a hyper- 
. trophy of the supporting fibrous connective tissue than a true com- 
bination with fibroma. Myxofibroma is not infrequent and may also 
be regarded as a fibrolipoma undergoing myxomatous change. 
Degeneration. — Ulceration and necrosis in areas of isolated lobules 
may take place at times on the periphery of large lipomas. More 
rarely calcification is present, and then only in limited areas. 
Nature. — Clinically, lipomas are benign neoplasms which give no 
metastases to internal organs. 
XANTHOMA 
In connection with lipomas there may be mentioned the xanthoma, 
a peculiar form of fatty neoplasm, which is a small, flat elevation, 
yellowish, found in the skin about the eye and eyelids, and more 
rarely about the internal organs in man. It is composed of modified 
fatty tissue resembling embryonal adipose tissue, with large vacu- 
oles containing cholesterin esters (cholesterin and fatty acid clumps) 
and is supplemented by numerous round cells such as are found in 
cell infiltrations. Xanthoma has not been described from domestic 
animals. 
MYXOMA 
Definition. — Myxoma is a neoplasm composed of mucous tissue 
which is an embryonic connective tissue similar to that of the umbili- 
cal cord, or the jelly of Wharton, or the vitreous humor of the eye. 
As myxoma originates from a type of connective tissue from which 
fat develops in the embryo, the relation of myxoma to lipoma and 
fibroma is very intimate. These growths vary in size from that of 
a pea to the size of two fists. 
Appearance. — Pure myxomas are soft, jellylike, translucent, en- 
capsulated, grayish neoplasms, which are rather rare in their pure 
state. When their consistence is lacking in softness and gelatinous 
composition it would be more proper to consider such neoplasms as 
the myxomatous degeneration of a fibroma, lipoma, or sarcoma, in- 
stead of a myxoma. The most characteristic form of myxoma is the 
soft, grayish-colored, polyp-shaped mass commonly spoken of as na- 
sal polypus. Myxoma may also appear as a hemispherical elevation 
projecting from a surface, or it may be lobulated, and occasional^ it 
presents a diffuse mass without any definite limits, having no capsule. 
Seats. — The mucous membrane of the nasal passages and the uterus 
of cattle are some of the common seats, also the serous membrane of 
the heart. Less common are the myxomas found in the mammary 
gland and those found in the course of the nerve trunks. Rarely 
myxomas are found in the marrow of bones and in the periosteum, 
also in the brain and spinal cord. 
Structure. — On microscopic inspection myxoma consists of loosely 
scattered cells, some of which are spindle shaped, but most are star 
shaped with long processes that frequently anastomose with one 
another. These cells have a fair amount of cj^toplasm and large, 
oval nuclei. Numerous fine, loose fibers, which are often gelatinous 
