28 BULLETIN 1449, U. S. DEPARTMENT OF AGRICULTURE 
structure and cell grouping they may resemble carcinoma very 
closely. Endotheliomas are rich in cells which are arranged in clus- 
ters or nests that suggest the acini of glands ; hence these neoplasms 
are sometimes spoken of as endothelial cancers. This arrangement 
bears also close resemblance to alveolar sarcoma. The endothelial 
cells in the acini are often cylindrical in shape, forming cords or 
hollow tubes. The cylindrical cords may undergo hyaline degenera- 
tion and appear as homogeneous or hyaline masses, when the neo- 
plasms are called cylindroma. Some endotheliomas originating in 
the dura mater or the pia mater may have a scanty stroma and 
numerous cell nests, in the interior of some of which globular clus- 
ters of lime salts are deposited. These deposits when found in the 
brain are known as psammoma or brain sand. Endotheliomas 
originating from the choroid plexus always contain a considerable 
amount of cholesterin in deposits and are therefore called " chole- 
steatomas." 
Endotheliomas are rather slow-growing neoplasms and are only 
slightly malignant, hardly ever giving metastasis. They are not 
common in man or in animals, though several cases of endothelioma 
and psammoma from the spinal dura mater, the choroid plexus, and 
the anterior part of the brain have been reported in cattle. Chole- 
steatoma in the ventricles of the brain in horses has been described 
by several pathologists. Endotheliomas on serous surfaces are by no 
means rare in domestic animals. 
EPITHELIAL NEOPLASMS 
A brief consideration of the normal relation or grouping of epi- 
thelium in the adjacent structures aids in a proper understanding of 
epithelial neoplasms. Epithelium in the animal economy depends 
on connective tissue for support, as in the skin and in all mucous 
membranes, or for the grouping, as in the formation of glands 
(secretory) or in the compound glandular organs (liver, kidney, 
etc. ) . In other words, wherever epithelium is present it is intimately 
related with connective tissue, which participates during the growth 
of the neoplasms. 
PAPILLOMA 
Definition. — Papillomas are benign fibroepithelial neoplasms re- 
sulting as outgrowths from surfaces covered by epithelium. The 
neoplasm originating from a surface covered by stratified squamous 
epithelium is called hard papilloma, whereas the neoplasm starting 
from a mucous membrane lined by columnar epithelium is known as 
soft papilloma. 
In its simplest form papilloma constitutes the common wart, which 
is simply a conical or rounded elevation of the derm covered by the 
epiderm, a thick, stratified layer of epithelium. Warts usually occur 
singly but may be multiple. Usually they are not painful, but 
when irritated they may ulcerate and bleed. They have been known 
to appear suddenly and sometimes disappear spontaneously. When 
persistent they may become large and may be readily mistaken 
for malignant growths. The connective-tissue elevation or core may 
become expanded at the apex and have secondary plications be- 
