286 EDEMA 



causes it is simply a passive congestion of the lymphatic cir- 

 culation, 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 filtration theory 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 pressure, 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 results. 

 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 (Ranvier, 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 branches, 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 frequently observed clinically. Welch, 1 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 generally, increased blood 

 pressure would seem to be an important factor, and there is no 

 doubt that with an increased pressure of the degree observed in 

 such conditions some increase in the lymph flow would result ; 

 but from the evidence at hand it is improbable that the amount 

 of lymph so secreted would ever be more than the lymph-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 satisfactorily account for the edema of cardiac 

 incompetence. Not to go into details here, it may be stated that 

 1 Virchow's Arch., 1878 (72), 375; see also Meltzer (loc. cit.}. 



