212 TEXT-BOOK OF PHYSIOLOGY. 



ena of diffusion, osmosis, and filtration the reader is referred to paragraphs 

 at the end of this chapter. It is assumed that the capillary wall, being an 

 animal membrane, is freely permeable to water and crystalloid bodies gener- 

 ally; less so, however, to colloid bodies, such as the proteins of the blood- 

 plasma; moreover, it is further assumed that the physiologic conditions of 

 the capillary walls are such as not only to permit of the passage of the con- 

 stituents of the blood into the tissue spaces, but also the passage of the con- 

 stituents of the intercellular lymph into the blood, according to laws similar 

 at least to those determining the passage of substances through animal 

 membranes as determined experimentally. The force giving rise to filtra- 

 tion is the difference of pressure between that exerted by the blood within 

 the capillary vessels and that exerted by the fluid in the tissue spaces; hence 

 any increase or decrease of this difference of pressure is attended by an 

 increase or decrease in the production of lymph. Thus compression of the 

 veins of a part which interferes with the outflow of blood from the capillaries, 

 or a dilatation of the arterioles which increases the inflow of blood to them 

 will increase the capillary pressure and therefore the production of lymph. 

 The reverse conditions will, of course, diminish the intracapillary pressure 

 and lymph production. Hemorrhages which lower the general blood-pres- 

 sure may so lower the capillary pressure as not only to stop the flow of 

 lymph to the tissues, but may give rise to a filtration current from the tissues 

 into the blood. 



The quantitative composition of the lymph compared with that of the 

 blood indicates that it is produced by diffusion, osmosis, and filtration. In 

 the lymph the concentration of the inorganic salts is practically the same as 

 in the blood; the concentration of the proteins, however, is somewhat less. 

 These facts are in accordance with what is known regarding the diffusibility 

 of both crystalloids and colloids through animal membranes. 



According to other investigators, the production of lymph is not so 

 much due to intracapillary pressure as it is to the specialized activities 

 of the endothelial cells, activities which indicate that lymph is a secre- 

 tion the composition of which varies in different situations by virtue of a 

 difference in the molecular structure of the endothelial cells. As is the case 

 with many of the secreting cells of the body, the injection of various sub- 

 stances into the blood apparently increases the activity of the endothelial 

 cells, as shown by an increased lymph production without any appreciable 

 increase of intracapillary pressure. Thus it has been shown that after the 

 injection into the blood of sugar, sodium chlorid, sodium sulphate, urea, etc., 

 there is an increase in the flow of lymph from the thoracic duct. The lymph, 

 however, under these circumstances is richer in water than is normally the 

 case. As the blood at the same time increases its percentage of water, it is 

 assumed that the water is extracted from the tissues, by reason of an increased 

 percentage of salts in the tissue spaces due to increased activity of the endo- 

 thelial cells. A higher percentage of salts in the lymph than in the blood is 

 difficult to account for on the diffusion-filtration theory. The injection of 

 peptones, albumin, the extract of the muscles of the leech, crab, mussel, etc., 

 is also followed by an increase in the amount of lymph discharged from the 

 thoracic duct; but in this instance the lymph possesses a higher degree of 

 concentration, being richer not only in inorganic but also in organic con- 



