CH. XXIII.] LYMPHAGOGUES 321 



such substances as sugar and salt (Heidenhain's second class of 

 lymphagogues) into the blood. These attract water from the tissues 

 into the blood, and thus increase the volume of the circulating fluid 

 and raise the intracapillary pressure. 



2. By increasing the permeability of the capillary wall by injuring 

 its vitality. This may be done locally by scalding a part; or 

 generally, by injecting certain poisonous substances, such as peptone, 

 leech extract, decoction of mussels, etc. (Heidenhain's first class of 

 lymphagogues). These act chiefly on the liver capillaries; curare 

 acts chiefly on the limb capillaries. There is no doubt that in 

 pathological conditions which lead to the production of a great 

 increase of lymph (dropsy) this second factor is the more important 

 of the two ; the increased permeability of the capillaries may be the 

 result of malnutrition, or due to the action of poisons produced 

 by the disease. 



In reference to the action of the endothelial cells, it is necessary 

 to recognise that they are alive, and are therefore capable of exerting 

 a selective action which may mask or counteract or assist purely 

 physical processes. If the action of poisons was simply to injure the 

 vessel wall and increase its permeability, the amount of lymph 

 should be proportional to the intensity of the injury; but this 

 is not always found to be the case. Lymph formation is doubtless 

 mainly influenced by the physical conditions present, for the 

 action of such thin cells as those of the capillary wall cannot be 

 sufficiently great to entirely counteract these conditions; at the 

 same time it is impossible to deny that there is some such action 

 as may be described by the terms " selective " or " secretory." The 

 question is closely related to that of absorption from the alimen- 

 tary canal, and we shall find in studying that subject that there 

 has been a similar difference of opinion, but that recent research has 

 confirmed the theory of selective activity of the absorptive epithelium. 



Dr Martin H. Fischer has advanced a new theory of dropsy or 

 oedema within the last few years. He believes that circulatory 

 conditions are of minor importance, but that the main factor leading 

 to transudation of water into the tissues is to be found in the tissues 

 themselves. He finds that colloids imbibe more water from an acid 

 solution than under other conditions. He therefore believes that it 

 is the accumulation of acid products (such as lactic acid) in the 

 tissues that determines their increased affinity for water, and thus 

 they attract it out of the blood. The theory has met with much 

 adverse criticism, and it is too early at present to state whether 

 Fischer or his critics are correct. 



