534 TRANSUDATION. [BOOK n. 



into the blood, a copious transudation leading to oedema takes place 

 in the leg, the femoral vein of which has been tied. This substance 

 like the others mentioned above has a marked effect on the clotting 

 of blood, due apparently, in part at least, to the substance exerting 

 some influence on the walls of the blood vessels. That influence 

 has, we may infer, as one of its effects an increase in the transudation 

 of lymph. Pointing in the same direction is the experience that 

 the plugging of a vein by clotting, by a thrombus as it is called, 

 more readily produces oedema than does the ligature of the vein. 

 The pressure in the two cases is the same, but in the former the 

 blood is altered, and the altered blood affects the vascular walls in 

 such a way as to promote transudation. Again, the increased 

 transudation leading to oedema which occurs in certain diseases 

 of the kidney in certain forms of " Bright's disease," may probably, 

 though the matter is one greatly debated, be considered as being 

 due to changes in the blood so acting upon the vascular walls 

 as to facilitate the passage of lymph and not to any increase 

 of capillary pressure. 



These and other facts which we might bring forward shew that 

 certainly an increase of transudation may be due to changes in the 

 vascular wall, independent of changes of pressure. And this leads 

 us to reexamine the influence of venous pressure as contrasted with 

 arterial pressure in promoting transudation. No doubt a higher 

 capillary pressure is as we have said more easily reached by 

 working on the venous side than on the arterial side ; but it has 

 not been shewn that the greater potency of venous pressure in 

 thus promoting transudation is simply dependent on the actual 

 pressure exerted being greater. On the other hand increase of 

 capillary pressure due to venous obstacles and that due to dilation 

 of small arteries or to a general increase of arterial pressure, differ 

 from each other in this, that the latter is accompanied by a rapid 

 flow of blood through the capillaries, and the former by a slower 

 flow, or, in extreme cases, by stagnation. Now it is open for us to 

 suppose that the latter conditions so act upon the capillary wall, 

 induce such changes in the capillary wall, that transudation is 

 rendered more easy; that in other words the effect of venous 

 pressure is not one of pressure merely but also of the action of 

 the venous blood, or of the blood rapidly becoming venous, on 

 the properties of the capillary wall. And this view is supported 

 by the experience that when, as in cases of heart disease, such 

 venous pressure is long continued or repeated, the oedema, as time 

 goes on, is more easily excited and with greater difficulty removed ; 

 the vascular walls become increasingly changed by the long-con- 

 tinued action of the venous blood. 



Further, in dealing with the nature of the process of transu- 

 dation two important facts must be borne in mind. In the first 

 place the lymph differs in composition in different regions of the 

 body; that for instance which transudes into the lymphatics 



