162 PHYSIOLOGY. 



Ludwig also made a second factor in his theory, and that was 

 osmotic changes between the lymph and the blood. 



2. Heidenhain's Theory. He believes filtration of the plasma, 

 due to higher pressure in the capillaries, will not suffice to explain 

 the formation of lymph. When glucose is injected into the blood, 

 there is more glucose after a time in the lymph than in the blood. 

 He calls the agents which cause an increased secretion of lymph, 

 lymphagogues. He makes two classes of these. The first class con- 

 sists of peptone, extract of leeches, and watery extract of crayfish. 

 The second class comprises sugar, sodium chloride, urea, and salts. 



The first class of lymphagogues does not increase blood-pressure 

 or affect the circulation, hence blood-pressure could not be stated 

 as the cause of the increased flow of lymph. He ascribes the action 

 to a stimulation of the endothelial cells of the capillaries. After 

 action by the first class of lymphagogues, the blood-plasma con- 

 tains less organic principles than the lymph. The explanation of 

 the action of the second class of lymphagogues is as follows: The 

 lymph-secretion by these agents is poorer in proteids than normal 

 lymph. Whilst the lymphagogues of the first class do not affect the 

 urinary secretion, those of the second class increase the secretion of 

 both lymph and urine at the same time. If injections of the second 

 class of lymphagogues are made slowly they do not affect blood-pres- 

 sure. Heidenhain explains their action in this way: the crystalloid 

 substances within the circulation are gradually secreted into the 

 lymph-spaces and urinary tubules by the aid of the endothelial cells 

 of the capillaries. Then the crystalloids in the lymph-spaces, by 

 their high osmotic power, attract water from the tissues. 



3. Starling's Theory. In the limbs the flow of the lymph is 

 very scanty, whilst in the liver and the intestinal area it is much 

 more abundant. Starling holds that the capillaries of the liver are 

 most permeable, the capillaries of the intestinal wall are less per- 

 meable, and the capillaries of the extremities the least. Thus, 

 lymph from the extremities contains only 2 to 3 per cent, of pro- 

 teid, from the intestines 4 to G per cent, of proteid, from the liver 

 6 to 8 per cent, of proteid, which is nearly as much as that of blood- 

 plasma. Starling explains the action of the lymphagogues of the 

 first class of Heidenhain (peptone, extract of leeches, and watery 

 extract of crayfish) by a change in the permeability of the capillary 

 wall, as these agents are poisonous and alter the permeability of the 

 endothelial walls of the capillaries, especially those of the liver. To 

 account for the variation in the amount of proteids in the lymph, 



