TUMORS OF DOMESTIC ANIMALS 27 
animals, especially in gray and light-haired horses. The most fre- 
quent places to find melanotic sarcomas are the skin and the choroid 
coat of the eye, where pigment normally exists. The skin at the 
root of the tail and the external genitals in horses are especially 
common seats, and the neoplasms occur less frequently in the adrenal 
glands and in the meninges of the brain. As secondary metastatic 
deposits melanotic sarcomas may be found in every organ, especially 
in the liver. 
The dark color of melanotic sarcoma is due to the presence of 
melanin, a brown or black pigment elaborated by the connective cells 
as the result of metabolism. It was formerly supposed that melanin 
was of hematogenous origin, but chemical analysis has proved the 
absence of iron in melanin, whereas hematogenous pigments contain 
iron. Melanin consists of brownish-black granules found in the 
cells as well as between the cells. There may be present also a cer- 
tain amount of diffuse melanin. The granules, which are variable 
in size, may be sparsely or densely distributed not only in different 
neoplasms but also in different parts of the same neoplasms. Any 
variety of sarcoma may become pigmented, especially the alveolar 
sarcomas. 
The consistence varies with the shape of the cells and the amount 
of vascularity. The spindle-cell type is usually harder to the touch 
and generally less pigmented than the round-cell variety, which is 
not only softer but may be very vascular and almost black in color. 
Melanotic sarcoma originating in the choroid coat of the eye is 
common in man and is very malignant, but is seldom found in do- 
mestic animals, and when found in them the liver is always involved. 
COMBINATIONS OF SARCOMAS 
The combinations, intermediate or mixed types, of sarcoma com- 
prise all the atypical neoplasms in which sarcoma combines with 
benign connective tissue or with epithelial growths and converts 
them into malignant neoplasms. The most common forms of these 
mixed types are fibrosarcoma, chondrosarcoma, osteosarcoma, osteo- 
chondrosarcoma, myxosarcoma, liposarcoma, neurosarcoma, angio- 
sarcoma, lymphangiosarcoma, gliosarcoma, rhabdomyosarcoma, and 
adenosarcoma. In all these neoplasms the original tissue indicates 
so plainly the type of the tissue that a variable amount of sarcoma 
cells does not prevent recognition by microscopic examination. A 
detailed description of these neoplasms would be only a repetition 
of the previously described structures. 
ENDOTHELIOMA 
Definition. — Endotheliomas are neoplasms which are composed of 
connective-tissue cells that have originated from the endothelial sur- 
face or lining of blood vessels or lymph vessels. When the jiew 
growth arises from serous membranes, such as dura mater, pia 
mater, peritoneum, pleura, or tunica vaginalis the name of meso- 
thelioma is used by some writers. When arising from the endothe- 
lium of the perivascular lymphatics and the adventitia of blood ves- 
sels, it is known as perithelioma. 
Nature and structure. — In origin and malignancy endotheliomas 
resemble sarcomas, but are less malignant and not so metastatic. In 
