278 EDEMA 



under equal pressure, which is the pressure of the blood ; there- 

 fore there is the same pressure both on the inside and on the 

 outside of the capillary walls. Nevertheless, the filtration 

 theory held for many years, not only as an explanation of lymph 

 formation, but also as an explanation of urinary secretion and 

 of the secretion by other organs. It was only within a compar- 

 atively short time that it became clear that filtration alone could 

 not account for all the phenomena of secretion. For example, 

 in many lower forms with undeveloped circulatory systems, and 

 almost no blood pressure, secretion goes on vigorously ; the 

 pressure of glandular secretions may be much higher than the 

 blood pressure within the capillaries ; the activity of secretion 

 is by no means in proportion to blood pressure, etc. If in 

 glandular secretion, therefore, fluids are removed from the blood 

 and transferred into an excretory duct through the action of 

 some force other than that of the blood pressure, it is probable 

 that lymph formation is equally independent of blood pressure. 

 On this basis Heidenhain advanced his 



Secretory theory of lymph formation, in which he sug- 

 gested that lymph is the product of an active secretion by the 

 endothelial cells of the capillaries, just as saliva is the product 

 of the activity of the glandular cells. He showed that certain 

 chemical substances may stimulate lymph flow, independent of 

 blood pressure, just as pilocarpine and other drugs may stimu- 

 late the secretion of saliva. These lymph-stimulating sub- 

 stances, which he named lymphagogues, fall into two distinct 

 classes. One, which includes such substances as peptone, leech 

 extract, strawberry juice, extracts of crayfish, mussel or oysters, 

 and numerous other tissue extracts, are characterized by causing 

 the secretion of a lymph which is rich in proteids, even richer 

 in proteids than the blood plasma ; and, furthermore, there is 

 no simultaneous increase in urinary secretion. Heidenhain con- 

 sidered that these substances caused lymph secretion by stimulat- 

 ing the capillary endothelium in a specific manner ; as they 

 caused no appreciable rise in blood pressure the increased lymph 

 secretion certainly could not be attributed to filtration. This 

 independence of the lymph flow on blood pressure is most con- 

 clusively shown by postmortem lymph secretion; for example, 

 Mendel and Hooker l observed lymph flow for four hours after 

 death, in a dog that had received an injection of peptone eight 

 minutes before being killed. 



The second class of lymphagogues includes crystalloidal sub- 

 stances, such as sugar, urea, and salts. Lymph secreted under 

 1 Amer. Jour, of PhysioL, 1902 (7), 380. 



