340 ■ EDEMA 



that the thoracic lymph has a higher osmotic pressure than the blood 

 of the same animal (Luckhardt),^^ so that the lymph which enters 

 the duct must do so against the osmotic pressure. 



THE PATHOGENESIS OF EDEMA 



With the facts and h3''potheses mentioned in the preceding para- 

 graphs in mind, we may consider their bearing on the production of 

 abnormally large accumulations of lymph in the tissues, that is, edema. 

 We can imagine any one of the following factors as causing or helping 

 to cause such a pathological accumulation: 



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 occlusion 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 occup.y so many of the lymph- 

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

 motic channels are thoroughly blocked, with a resulting local edema 

 that in course of time is followed by the production of inflammatory 

 connective tissue and elephantiasis.-^ Chronic lymphangitis or plug- 

 ging 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 exi)erimental 

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

 by inflammatory processes which occlude the sinuses of the l^-mph 

 glands through which the hepatic l3'niph passes. 



=" Anicr. .Jour. Physiol., 1910 (25), 345. 



''' Miiiison, Allbutt's System, 1897 (ii), 10S2. 



" Jour. Expcr. Med., 1912 (16), 831. 



