TUMORS OF DOMESTIC ANIMALS 31 
a foreign body, and as such attract leucocytes by their positive 
ehemotactic property. The leucocytes are deposited around this area 
of multiplying epithelium. The epithelial multiplication continues 
to the extent of forming buds or roots which penetrate the newly 
formed hyperplasia constituting the stroma. The penetration takes 
place in the direction of the least resistance, which is alon^ the course 
of the lymph vessels, and these therefore become the principal ave- 
nue for the extension of the cancer roots as well as the leading chan- 
nels for the transmission of secondary cancer deposits. 
The lymph glands that are interposed in the course of the lymph 
vessels act as temporary detention places or sieves for the retention 
of any particles which may be carried in the lymphatic stream. This 
is the reason that secondary carcinomatous deposits are transmitted 
by the lymph vessels and are generally multiple and numerous. 
Appearance. — Carcinomas differ considerably in various parts of 
the body and may grow on free surfaces, where they are known as 
epitheliomas, or in the interior of organs, where when soft they are 
known as medullary cancers. Epitheliomas that start from the 
Malpighian layer of the skin and are composed of flat epithelial cells 
are known as squamous epitheliomas. The epitheliomas starting 
from mucous membranes lined by cylindrical cells are called cylindri- 
cal or columnar epitheliomas. The medullary cancers usually have 
an abundance of parenchyma and a scanty amount of stroma ; when 
they are soft they are known as encephaloid or soft cancers. A pre- 
ponderance of the supporting stroma conveys rigidity or hardness 
to the touch, and the carcinoma is known as scirrhous or hard cancer. 
SQUAMOUS EPITHELIOMA 
Definition, — Squamous epitheliomas are malignant epithelial 
carcinomas occurring in the skin and mucous membranes lined with 
stratified squamous epithelium. Epitheliomas are common in man 
and are even more frequent in domestic animals. The malignancy 
of epitheliomas is expressed by proliferation of the epithelial cells, 
the subsequent infiltration and destruction of the affected tissue lead- 
ing to inflammation, suppuration, and ulceration. 
Appearance. — Squamous epitheliomas differ somewhat in appear- 
ance, depending on the location and the structure of the affected part. 
"When in the skin they appear as nodular elevations wnieh are very 
prone to ulcerate. In the beginning the elevation hardly rises above 
the surface, but in the later stages epithelioma often assumes a 
dendritic shape. It then resembles a papilloma, differing., however, 
from the latter by its growth, which is only outward in the papilloma 
but both inward and outward in the squamous epithelioma. 
Seats. — Squamous epithelioma is often found at the junction of the 
skin with a mucous membrane. This is the vulnerable point, beyond 
which the cornification of the skin does not extend into the mucous 
membrane, as at the conjunctival margin of the external auditory 
meatus, the external nares. lips, and muzzle. Less frequently it is 
found in the larynx, tongue, esophagus, cervix uteri, vagina (which 
is very often affected in women but seldom in animals), the penis in 
horses and dogs and also around the anus. Squamous epithelioma is 
rarely found in the bladder, scrotum, and pelvis of the kidney. 
