TUMORS OF DOMESTIC ANIMALS 
CLASSIFICATION BY NATURE (CLINICAL CLASSIFICATION) 
Clinically neoplasms are divided into two classes, benign and 
malignant. 
Benign neoplasms are usually harmless and in themselves do not 
endanger life except by accidental location. They grow slowly from 
the center and mechanically push aside the surrounding tissues and 
remain distinct from them. Their slow growth permits a reaction 
of the surrounding tissues to concentrate around the neoplasm to 
form a capsule which sharply defines the neoplasm from the sur- 
rounding tissue. Even when diffused, benign neoplasms do not in- 
filtrate the surrounding tissues and their growth may be arrested, 
to be renewed later. The paucity of their cells may account for 
their slow growth. Though usually small, they may become rather 
large in certain localities, as, for example, lipomas in the abdominal 
cavity and myomas in the uterus. They do not recur after complete 
removal; neither does metastasis occur. 
Benign neoplasms, though harmless in their nature, may produce 
death indirectly by accidental location if in or contiguous to vital 
organs. A benign neoplasm of the larynx may become lodged 
between the vocal cords and completely obstruct respiration; when 
developed in the brain or the spinal cord it produces paralysis by 
pressure and "eventually may lead to death. Benign neoplasms may 
interfere with the nutrition of vital organs either by pressure, thus 
limiting the blood supply of the latter, or by obstructing the diges- 
tive tract, bringing about a state of ill health commonly referred to 
as cachexia, resulting in emaciation or even starvation. 
Malignant neoplasms are always harmful and usually destroy 
life in whatever situation they arise. Because of their preponder- 
ance in cells and a richer blood supply they grow more rapidly than 
benign neoplasms. They are frequently softer in consistence and 
usually have no capsule and are ill defined from the surrounding 
tissues. They increase by peripheral extension and dissemination of 
the proliferating cells into the surrounding tissues. They give metas- 
tases to internal organs by either the blood vessels, as is the case 
in sarcomas, or the lymph vessels, which is the common mode of 
cancer metastasis. They usually recur after removal. 
The prominent features of malignancy in neoplasms are metastases, 
infiltration of the surrounding tissues, and recurrence. Metastasis 
may take place by direct contact, which is rare but is sometimes ob- 
served in neoplasms of the mucous membranes, or by the transmis- 
sion of particles of the primary growth through the blood or lymph 
vessels, which is the usual mode of metastasis. These particles in the 
blood or lymph stream ultimately lodge in either proximate or distal 
parts of the body, where they grow rapidly in their new environ- 
ment and deprive the tissues of some of the nutrition by appropriat- 
ing blood enough for their own existence. Neoplasms when situated 
near a blood vessel may, during their growth, diminish by pressure 
the blood supply of an organ and interfere with its functional 
activity. The entrance of neoplastic elements into the vascular chan- 
nels is the result of the destructive nature of a malignant neoplasm 
and is comparable to embolism. Anemia of the organ usually results 
and may eventually be followed by general anemia of the body, and 
in the more severe cases by emaciation and cachexia. 
