THE CAUSES OF EDEMA 285 



1. Obstruction to outflow through the lymph- vessels. 



2. Increased blood pressure. 



3. Decreased extravascular pressure. 



4. Increased permeability of the capillary walls. 



5. Increased filterability of the blood plasma. 



6. Osmotic pressure changes either an extravascular in- 



crease or an intravascular decrease. 



These may be taken up one by one, and considered in relation 

 to their bearing upon the general problem of edema. 



1. Obstruction to Outflow through the I/ymph- 

 vessels. Because of the very abundant anastomosis of the 

 lymphatic vessels it is extremely difficult or impossible to cause 

 any appreciable obstruction to the lymphatic circulation by liga- 

 tion of lymphatic trunks in the limbs or organs of the body, 

 and in pathological conditions this possible cause of edema is 

 seldom actually observed. The chief instance of edema from 

 lymphatic obstruction is observed after occlusion of the thoracic 

 duct by tumors, tuberculous processes, animal parasites, or 

 thrombosis ; such occlusion is usually followed by rupture of 

 the duct or its tributaries, with the production of chylous ascites 

 or chylothorax, and chyluria. Filaria or their ova may occupy 

 so many of the lymphatic channels of an extremity (leg) or part 

 (scrotum) that the anastomotic channels are thoroughly blocked, 

 with a resulting local edema that in course of time is followed 

 by the production of inflammatory connective tissue and ele- 

 phantiasis. 1 Chronic lymphangitis may also result in lymphatic 

 obstruction to such an extent that chronic edema results. 



Another way in which edema may be caused or influenced by 

 lymphatic obstruction is generally overlooked, but it is possibly 

 of great importance ; namely, from pressure upon the lymph 

 channels by dilated vessels in hyperemia, or by cellular exu- 

 dates and swollen tissues in inflammation. We see evidence 

 of this in the rapid absorption of exudates that frequently fol- 

 lows the removal of but a part of the fluid in a chest cavity ; 

 apparently the decrease in pressure frees the paths of absorption 

 and permits them to take up the remaining fluid. In inflam- 

 matory edema the lymphatic obstruction is probably not great, 

 for Lassar found that the amount of lymph escaping from an 

 edematous extremity is much greater than from a normal one ; 

 but in the case of strangulated hernias or other conditions in 

 which edema results from circular constriction, obstruction of 

 the lymphatic vessels may be a factor of no mean importance. 



There is no difficulty in understanding edema from the above 

 1 Hanson, Allbutt's System, 1897 (ii), 1082. 



