THE CAUSES^ OF EDEMA 341 



but in the case of strangulated hernias or other conditions in wliich 

 edema results from circular constriction, obstruction of the lymphatic 

 vessels may be a factor of no mean importance. In general stasis the 

 increased pressure in the veins of the neck may interfere with the 

 passage of the fluid out of the thoracic duct into the blood. 



There is no difficulty in understanding edema from the above causes 

 — it is simply a passive congestion of the lymphatic circulation, and no 

 chemical factors are involved. The nature of the fluid found in such 

 forms of edema will be discussed later. 



2. Increased Blood Pressure. — This takes us back to the filtra- 

 tion theoiy of lymph formation, and as it is generally conceded that 

 more or less fluid escapes from the vessels by this mechanical process, 

 the questions to be decided are : Can and does increased blood pres- 

 sure, alone and without other aiding factors, cause edema? If not, 

 does it play an auxiliary part in producing edema, and how important 

 a part may this be? Many experiments have been performed with the 

 object of answering these questions, with more or less conflicting re- 

 sults. Cohnheim demonstrated that vasodilation (active hyperemia) 

 alone will never bring on an edema ; and many observers state that 

 ligation of the femoral or other large veins will not cause edema in 

 animals. However, when the vein is occluded, and the arteries are 

 dilated by cutting their vasoconstrictor nerves, then edema may result 

 (Ran\aer, Cohnheim) ; but whenever venous outflow is impeded, we 

 have other factors than simply increased pressure to consider, for the 

 nourishment of the parts is decidedly impaired, and, as we shall see 

 later, this, may be of much greater importance than is the associated 

 rise in blood pressure. To produce edema in the lungs by mechanical 

 forces it is necessary to ligate the aorta and its branche^s, or the pul- 

 monary veins (Welch). As such high pressures do not occur in any 

 pathological conditions, it is safe to assume that increased pressure 

 alone is not capable of causing by itself the pulmonary edema so fre- 

 quently' observed clinically. AVelch,'^ however, has supported the 

 hypothesis that ..a disproportion between the working power of the 

 left ventricle and of the right' ventricle may lead to pulmonary edema 

 through pulmonary hyperemia. In the edema of-passive congestion, 

 increased blood pr-eJ*5ure'^woul{i seem to be an impoj'tant factor, and 

 there is ^o (^ohbt tha,t with ^n increased pressiia;^ of the degree' 

 observed in such conditions s(5me increase- in the\ljniiph floVv would 

 result ; but from the evidence at hand it is •improbarrle -that the amount^- 

 of lymph so secreted woulft.gner be more than the lyniph-vessels could^ 

 carry away. Even the added obstruction to lymphatic flow due to 

 pressure upon the lymph capillaries by congested blood-vessels, and 

 the resistance to the lymph escaping from the thoracic duct offered 

 by the increased pressure in the subclavian vein, would not satisfac- 

 torily account for the edema of cardiac incompetence. Not to go into 



zsVirchow's Arch., 1878 (72), 375; see also Meltzer {loc. cit.) . 



