OPEEATIVE SUEGERY. 299 



used before this stage is reached. Insert the knife in the most 

 dependent part of the abscess, making a wide opening to permit 

 free drainage, after the contents have been evacuated the injec- 

 tion of an antiseptic solution will be all that is necessary. 



■Cumors. 



Any unnatural swelling or protuberance of the tissues of 

 the animal body is termed a tumor. Tumors are virtually hyper- 

 trophies or overgrowths of tissue. A tumor may be composed 

 of tissue similar to that surrounding it, in which event it is 

 termed a homologous tumor; or it may be entirely different 

 from the contiguous tissue, and is then termed .heterologous. 

 Tumors grow in size, either by central or peripheral 

 growth; both, however, depend on cell proliferation, the 

 growth being formed in the same manner as normal tissue. 

 The blood-vessels leading to the tumor usually become enlarged^ 

 to supply nutrition to the growth, new blood vessels form among 

 the meshes of the tumor cells. Consequently, as a rule, we find 

 overgrowths of tissue to be very vascular, and bleed extensively 

 when excised. Tumors are said to be malignant when they tend 

 to destroy the tissues in which they are situated, and have a 

 tendency to recur after removal. A benign tumor is that which 

 does not diifer materially in structure from the tissue surround- 

 ing it, and does not show a tenderxy to degeneration and ulcer- 

 ation, as do the malignant variety, neither does it as a rule recur 

 after removal. 



Tumors are classified generally according to the structure 

 composing them. We shall not here, however, enter into this 

 subject, as the sheep rarely, with the exception perhaps of goitre, 

 is affected with overgrowths of tissue, and malignant cancerous 

 swellings in this species of animal are exceedingly rare. 



The etiology or cause of tumor formation has never been 

 verv satisfactorily demonstrated. Bevond a doubt a blow or 



