340 EDEMA 



. 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 increase or 



an intravascular decrease. 



7. Changes in the affinity of the colloids for water. 



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 Lymph=vessels. — ^Be- 

 cause 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 ligation of lymphatic trunks in the 

 limbs or organs of the body, and in pathological conditions this possi- 

 ble 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 lymph- 

 atic channels of an extremity (leg) or part (scrotum) that the an- 

 astomotic channels are thoroughly blocked, with a resulting local 

 edema that in course of time is followed by the production of in- 

 flammatory connective tissue and elephantiasis.-^ Chronic lymph- 

 angitis or plugging of the lymph vessels by cancer cells may also result 

 in lymphatic obstruction to such an extent that chronic edema results. 

 It would seem, from Opie's experiments,-'- that the acute edemas may 

 at times depend upon lymphatic obstruction, for he found that experi- 

 mental edema of the liver, produced by cantharidin, seems to be 

 determined by inflammatory processes which occlude the sinuses of 

 the lymph glands through which the hepatic lymph passes. 



Another way in which edema may be caused or influenced by lympli- 

 atic 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 exudates and swollen 

 tissues in inflamnuition. We see evidence of this in the rapid absorp- 

 tion of exudates that frequently follows 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 inflammatory edema tlie lym])hatic obstruction is probably 

 not great, for Lassar found tliat the amount of lym|)h escaping from 

 an edematous extremity is inncli gi-eatei- than Ironi a normal one; 



siManson, Allhutt's Svstoin, 1S97 (ii), 1082. 

 i:a Jour. Exper. Med., id]2 (16), 8.31. 



