72 PRINCIPLES OF VETERINARY SURGERY 



PATHOLOGY AND ETIOLOGY.— The pathology 

 and etiology of oedema are comparatively simple. The re- 

 turn circulation from any given part, when obstructed, either 

 partially or completely, causes an engorgement of the 

 blood carried to it by the arteries. The tension thus pro- 

 duced leads to the straining of the w^atery constituents of 

 the blood through the vessel wall into the areolar spaces 

 of large cavities. 



The intercellular spaces of normal tissues are filled with 

 a clear, watery serum (lymph) that is filtered from the 

 blood serum through the walls of the capillaries (and prob- 

 ably partly secreted by the cells). When the production of 

 this serum exceeds the absorption, the tissues become satu- 

 rated with fluid, the anatomical elements are crowded 

 apart to make room for the accumulation and the result is an 

 oedematous swelling. Both the lymphatics and the • blood 

 vessels are concerned in absorbing lymph and carrying it 

 back to the general circulation. If the former is obstructed 

 the latter will generally perform the function of absorbing 

 the excess of lymph, but if a blood vessel is obstructed, 

 lymphatics seem entirely incapable of offering equal com- 

 pensation. That is to say, if a lymphatic vessel is obstructed 

 the veins will probably absorb the excess of serum in the 

 intercellular spaces and no permanent oedema will result, 

 while, on the other hand, if there is even a trivial interference 

 with the free flow of blood from any given part to the heart 

 more or less oedema is certain to supervene and to remain 

 until the obstruction is removed. 



The accumulation of transudates (oedema) may be 

 favored by pathologic conditions of the blood vessels and of 

 the composition of the blood as in purpura hsemorrhagica, 

 but by far the most common oedemas of animals are those 

 due directly to passive hyperaemia. Veins become obstructed 

 in one way or other and the part drained by them becomes 



