222 THEORY AND PRACTICE 



tirely dissimilar. A tumor grows by cell-proliferation, the same 

 as other parts of the body ; new blood vessels and lymphatics ex- 

 tend into the growth and furnish it nutriment. These nutrient 

 vessels are usually much enlarged. As a rule nerve fibers ex- 

 tend into a tumor. Tumors are subject to all the changes which 

 occur in any other tissue, such as fatty degeneration, suppuration, 

 ulceration, gangrene, pigmentation, calcification, etc. Necrosis 

 sometimes takes place, often to such an extent as to entirely 

 destroy the tumor. 



Tumors are of various shapes and forms : nodular, when re- 

 sembling nodules ; tuberous, when like a tuber ; fungoid, when 

 cauliflower shaped ; polypoid, when like a polypus pear shaped ; 

 papillary, when they are shaped like a papula ; dentritic, when 

 they have roots or branches. Tumors may be single or multi- 

 ple; this is particularly seen in black cancer, such as is seen 

 around the tails of white horses — the melanotic tumors. 



Tumors are either malignant or benign. A malignant tumor 

 has the following characteristics : ( 1 ) It invades the surround- 

 ing tissues by peripheral or eccentric growth ; (2) it has a ten- 

 dency to recur locally after rerrioval ; (3) it will spread to other 

 parts of the body, by metastasis; (4) it has a tendency to inter- 

 fere with the nutrition and well being of the body and results 

 in cachexia. 



The real cause of tumors is not known. Predisposing causes 

 are such as heredity, mechanical injuries, parasitic invasion, — in 

 fact anything that will lower the resistance of the animal. Con- 

 heim developed the theory that tumors are due to embryonal 

 displacement, — either to misplaced cells or to superfluous cells. 

 Tumors do not develop in young animals so much as in old. 

 The melanotic tumors in horses do not develop usually until the 

 age of ten or fifteen years. 



Tumors may be divided into six large classes : 

 1. Connective tissue tumors (histioid). 



a. Fibroma. 



b. Myxoma. 



c. Sarcoma — round, spindle-celled, mixed, and 

 giant-celled. 



d. Endothelioma. 



