238 THE LYMPH 



mechanical way by the filtration of the blood plasma through the capil- 

 lary wall. The second theory, propounded by Heidenhain and his 

 pupils, maintains that the formation of lymph is not effected by 

 filtration alone, but is also dependent upon osmosis and diffusion, and 

 in addition upon a certain vital activity of the cells. To the latter 

 the term of vitalismus has been applied. It must be clearly under- 

 stood, however, that this designation does not allot to these cells meta- 

 physical qualities, but only intends to imply that the formation of 

 lymph is associated with certain microchemical and microphysical 

 changes which at this time are still beyond scientific analysis. 



In accordance with the pure filtration theory, it is held that a por- 

 tion of the blood plasma is forced through the passive cells of the 

 capillary wall as through a filter, i.e., from a place of high pressure to 

 a place of low pressure. The driving force is furnished in this case 

 by the pressure prevailing in the blood-capillaries, while the area of 

 low pressure is formed by the intercellular lymphatic spaces. As no 

 other chemicophysical force is brought to bear upon this process, 

 the quantity of the lymph formed must be directly proportional to 

 the capillary pressure. This theory finds support in the composition 

 of the lymph, because its inorganic constituents are practically the 

 same as those of the blood plasma, while its content in protein is 

 considerably less. 



It also finds some support in certain observations to which atten- 

 tion has been called by Starling. Thus, it has been found that while 

 the quantity of lymph is usually directly proportional to the height 

 of the capillary pressure, it does not preserve this relationship at all 

 times. This fact tends to show that a relatively low blood pressure 

 may be associated with a very copious secretion, and vice versa. At all 

 events, these results are not in accordance with the conditions neces- 

 sary for filtration. This author also claims that the permeability of 

 the capillary endothelium is not the same in all parts of the body. 

 Thus, it is stated that the walls of the capillaries traversing connective 

 tissue, offer a much greater resistance to the escaping lymph than 

 those of the intestine and liver. The latter, in fact, are surprisingly 

 permeable. In harmony with these structural and functional differ- 

 ences, it has been found that the lymph derived from the former is 

 usually very dilute, while that from the aforesaid abdominal organs 

 contains a very appreciable amount of proteins. The greatest pos- 

 sible permeability is shown by the capillaries of the liver, in which 

 organ a copious transudation of concentrated lymph is frequently 

 associated with very low pressures. In this connection it is of interest 

 to note that the lining of the hepatic capillaries is in many places very 

 incomplete, so that the blood is enabled to come into much closer 

 contact with the liver-cells. For this reason, it cannot surprise us to 

 find that a fluid which is injected into the arterial supply channels of 

 this organ, frequently escapes into the spaces between the hepatic 

 cells. 



