(6) They do not manifest metastasis.
Benign tumors, though harmless, may, by the accident of their location, indirectly produce death. Mere pressure on the brain substance of an otherwise innocent tumor, compression of the blood supply for vital organs, growth in such manner as to cause obstruction in the alimentary tract or pressure upon nerves, may cause death, or, prior to death, so combine the effects of anemia (deficiency of blood), starvation, and pain, with its consequent restlessness, as to produce a veritable cachexia (condition of general ill health).
On the other hand, a malignant tumor in its primary growth may so implicate a vital organ as to destroy life before metastasis can occur or even before cachexia can develop. Thus, to the untrained observer, environment may so operate as to cause these two classes of new growths to simulate each other. The boundary lines may seem to overlap. It is here that the microscope, as the court of last appeal, adjudicates positively in the diagnosis between these two clearly marked divisions.
It may almost be asserted that a true classification of tumors can not be made until we know more about the cause of them. The arrangement here presented is offered to meet the practical needs of the veterinarian, student, and farmer rather than of the pathologist.
We may roughly divide the tissues of the body into structural and lining tissues. The structural tissues are composed of the tissues of special function and simple connective tissues. The lining or covering tissues, both internal and external, are known as epithelium.
Section A of the table below contains the true tumors or proper neoplasms.
Section B includes the cysts, some of which are true tumors, while others are false ones, but the latter are added because of their gross resemblance to the true and the consequent necessity of considering them at the same time.
| A.—Tumors. | |
| BENIGN. | |
|---|---|
| I.—Tumors composed of tissues resembling those of special function. | |
| 1. Type of muscle tissue | Myoma. |
| 2. Type of nerve tissue | Neuroma. |
| 3. Type of vascular tissue | Angioma. |
| 4. Type of gland tissue | Adenoma. |
| II.—Tumors composed of fully developed connective tissue. | |
| 1. Type of fibrous tissue | Fibroma. |
| 2. Type of adipose, or fat, tissue | Lipoma. |
| 3. Type of cartilage tissue | Chondroma. |
| 4. Type of osseous, or bone, tissue | Osteoma. |
| 5. Type of neuroglia, or nerve, sheath | Glioma. |
| 6. Type of mucoid, or mucous, tissue | Myxoma. |
| MALIGNANT. | |
| III.—Tumors composed of embryonic or immature connective tissues. | |
| 1. Type of immature connective tissue | Sarcoma. |
| 2. Type of endothelial tissue | Endothelioma. |
| IV.—Tumors in which epithelial elements predominate. | |
| 1. Type of various epithelial cells and associated tissues | Carcinoma. |
| B.—Cysts. | |
| I.—Cysts which develop in preexisting cavities. | |
| 1. | Retention cysts. |
| 2. | Proliferation cysts. |
| II.—Cysts which are of congenital origin and are true tumors. | |
| 1. | Dermoid cysts. |
| III.—Cysts which originate independently as the result of pathological changes and are nontumorous. | |
| 1. Cysts formed by the softening and disintegration of lesions | Softening cysts. |
| 2. Cysts formed around parasites | Parasitic cysts. |
| 3. Cysts formed by an outpouring of blood and lymph into the tissue spaces with subsequent encapsulation of the fluid | Extravasation cysts. |