THE VASCULAR MECHANISM. 241 



From these experiments it may be concluded that the walls of the 

 capillaries form a membrane through which lymph can be filtered off, 

 and that the amount of fluid which passes through the membrane in a 

 given time depends directly upon the capillary pressure. 



Another factor in the formation of lymph is the variable readiness 

 with which filtration takes place through the capillaries in different 

 parts of the body under the same pressure. The least permeable 

 capillaries are those of the limbs, the most permeable being those of 

 the liver ; and almost all the lymph flowing from the thoracic duct of a 

 resting animal is formed in the liver and digestive tract. The perme- 

 ability of the capillaries can be increased by the injection of various 

 substances called lymphagogues, including peptone and leech extract. 

 The injection of one or other of these substances into the blood stream 

 leads to an increased formation of lymph, although the capillary 

 pressure, after a short time, is almost unaltered. The lymph is derived 

 almost entirely from the liver, the capillaries of which become more 

 permeable, as is shown by the fact that, if the lymphatic vessels of the 

 liver are ligatured, the subsequent injection of peptone does not increase 

 the formation of lymph. 



The permeability of the capillaries is also increased when their nutri- 

 tion is impaired, e.g. by lack of oxygen, and this may give rise to dropsy. 



The formation of lymph is also dependent upon the metabolism 

 of the tissues themselves. The injection into the blood of bile salts, 

 for example, leads to the secretion of bile by the liver, and the flow of 

 lymph from the thoracic duct is increased. This is not due to raised 

 capillary pressure or to changes in the permeability of the capillaries, 

 but is brought about in the following manner. In normal circum- 

 stances the osmotic pressure of the tissue cells, the lymph, and the 

 blood is almost the same. When the metabolism of the liver is 

 increased, metabolic products are formed in the liver cells and diffuse 

 into the lymph, raising the osmotic pressure of the lymph and liver cells 

 as compared with that of the blood. Consequently water passes from 

 the blood into the lymph, and this fluid is increased in amount and 

 gives rise to a larger flow from the thoracic duct. Similar results 

 have been observed in other organs, and probably increased functional 

 activity of any tissue in the body leads to increased formation of 

 lymph. We may conclude, therefore, that lymph formation is not 

 a secretory process, but is brought about by purely physical processes, 

 namely, filtration and osmosis ; and the factors concerned in its pro- 

 duction are (1) the capillary pressure, (2) the degree of permeability of 

 the capillary walls, and (3) the metabolic activity of the tissues. There 

 is no reason to suppose that in health the permeability of the 



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