Dropsy. 127 



the tissue demands for increased transudation are increased in 

 order to acconiplisli its removal by the lymph ; yet if the blood 

 already contain an excess of the very substance in question the 

 increased transudation only serves to more nearly complete a 

 vicious cycle, and adds to the existing accumulation.] Two 

 reasons may be assumed for the pathological increase of this 

 fluid in the tissues, either an obstruction to the lymph drainage, 

 or an unusual, exaggerated transudation ; both of which factors 

 may be operative at the same time. In_ view of the free anasto- 

 mosis and the great abundance of lymph channels, together with 

 the fact that lymph absorption is also accomplished by the capil- 

 laries and veins, obstruction of isolated lymph channels, it may 

 be said, does not ordinarily occasion lymph stasis. Even in case 

 of unusual formation of lymph there is no essential reason for 

 determination of any abnormal condition, as within certain limits 

 the paths of lymphatic drainage are capable of accommodating 

 and carrying ofl:" large quantities of fluid, provided the tissues 

 retain their ]iroper elasticity (Landerer), and the propulsive 

 forces (muscular movements, etc.) are of normal functional ex- 

 cellence. It is only when the increased transudation becomes ex- 

 treme and the lymphatic flow fails to keep pace, that cedema 

 develops. 



The most common cause for such fault is passive hyperemia. 

 In case of passive congestion the blood pressure in the capillaries 

 becomes raised and the current of blood slowed, as a result of 

 which the endothelium of the walls of the capillaries becomes ab- 

 normally permeable and in conseqtience the fluid element of the 

 blood exudes profusely. This exudate is usually not as rich in 

 albumen as the normal lymph. As examples of oedema of this 

 type may be suggested that caused by compression (as oedema 

 of the lips by application of twitches to a horse's mouth), by 

 venous thrombosis, by cardiac lesions which impair the current 

 of blood in the ven^e cavie, by pulmonary changes which prevent 

 proper progression of blood from the right heart, by structural 

 faults of the liver causing obstruction of the portal capillaries and 

 producing portal congestion (dropsy or cedema from passive 

 congestion). Coincident interference with lymphatic drainage 

 may be determined perhaps by the pressure of fluid accumulations 

 in the thoracic or peritoneal cavities upon the thoracic duct, per- 

 haps by the loss of natural elasticity of the tissues, stretched and 



