444 BLOOD AND LYMPH. 



far, therefore, as the action of the lymphagogues of the first class is 

 concerned, it may be said that the advocates of the filtration-and- 

 diffusion theory have suggested a plausible explanation in accord 

 with their theory. The facts emphasized by Heidenhain with 

 regard to this class of substances do not compel us to assume a 

 secretory function for the endothelial cells. 



4. Injection of certain crystalline substances such as sugar, 

 sodium chlorid, and other neutral salts causes a marked increase 

 in the flow of lymph from the thoracic duct. The lymph in these 

 cases is more dilute than normal, and the blood-plasma also becomes 

 more watery, thus indicating that the increase in water comes from 

 the tissues themselves. Heidenhain designated these bodies as 

 "lymphagogues of the second class." His explanation of their 

 action is that the crystalloid materials introduced into the blood are 

 eliminated by the secretory activity of the endothelial cells, and that 

 they then attract water from the tissue elements, thus augmenting 

 the flow of lymph. These substances cause but little change in 

 arterial blood-pressure; hence Heidenhain thought that the greater 

 flow of lymph can not be explained by an increased filtration. 

 Starling * has shown, however, that, although these bodies may not 

 seriously alter general arterial pressure, they may greatly augment 

 intracapillary pressure, particularly in the abdominal organs. His 

 explanation of the greater flow of lymph in these cases is as follows : 

 " On their injection into the blood the osmotic pressure of the circu- 

 lating fluid is largely increase^. In consequence of this increase 

 water is attracted from lymph and tissues into the blood by a process 

 of osmosis, until the osmotic pressure of the circulating fluid is 

 restored to normal. A condition of hydremic plethora is thereby 

 produced, attended with a rise of pressure in the capillaries generally, 

 especially in those of the abdominal viscera. This rise of pressure 

 will be proportional to the increase in the volume of the blood, and 

 therefore to the osmotic pressure of the solutions injected. The 

 rise of capillary pressure causes great increase in the transudation 

 of fluid from the capillaries, and therefore in the lymph-flow from 

 the thoracic duct." This explanation is well supported by experi- 

 ments, and seems to obviate the necessity of assuming a secretory 

 action on the part of the capillary walls. 



5. Numerous other experiments have been devised by Heidenhain 

 and his followers to show that the physical laws of filtration, diffu- 

 sion, and osmosis do not suffice to explain the production of lymph ; 

 but in all cases possible explanations have been suggested in terms 

 of the physical laws, so that it may be said that the facts do not 

 compel us to assume a secretory activity on the part of the endo- 

 thelial cells of the capillaries. In recent years Asherf and his 



* Loc. cit. t "Zeitschrift f. Biologie," vols. xxxvi-xl, 1897 to 1900. 



