366 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tions, suggests a different explanation of the action of these lymphagogues. 

 He believes that these substances diminish in some way the osmotic tension of 

 the blood. In consequence of this diminution the diffusion-stream of water 

 from lymph into the blood is lessened, and therefore the filtration-stream in 

 the opposite direction, if it remains unchanged, must cause an increased volume 

 of lymph. This, theory, although supported to some extent by experimental 

 evidence, does not seem to explain the greater concentration of lymph obtained 

 in these cases. So far, however, as the action of the lymphagogues of the first 

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

 sion 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, NaCl, 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 sub- 

 stances cause but little change in arterial blood-pressure, hence Heidenhain 

 thought that the greater flow of lymph could not be explained by an increased 

 filtration. Starling 1 has shown, however, that, although these bodies may not 

 seriously alter general arterial pressure, they may greatly augment intracapil- 

 lary 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 circulating fluid is largely increased. 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 hydraemic plethora is thereby pro- 

 duced, 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 pres- 

 sure 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 

 experiments, and seems to obviate the necessity of assuming a secretory action 

 on the part of the capillary walls. 



5. One of the most interesting facts developed by the experiments of Hei- 

 denhain and his pupils is that after the injection of sugar or neutral salts in 

 the blood the percentage of these substances in the lymph of the thoracic duct 

 may be greater than in the blood itself. It is obviously difficult to explain 

 how this can occur by filtration or diffusion, since it seems to involve the pas- 



1 Op. dt. 



