216 TEXT-BOOK OF PHYSIOLOGY 



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 nitration 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 nitration. 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 Heidenhain, 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 secretion 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 substances 

 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 the injection of 

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

 etc., is followed by an increase in the amount of lymph discharged from 

 the thoracic duct; the lymph also possesses a high degree of concentration, 

 being rich not only in inorganic but also in organic constituents. The 

 cause of this increase in both the quantity and quality of the lymph is 

 believed to be an increased activity in the secreting power of the endothelial 

 cells. These substances Heidenhain regarded as true lymphagogues. 



It has also 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 the increased percentage 

 of salts and a higher osmotic pressure. The increased volume of blood 

 raises the intra-capillary blood pressure which in turn gives rise to a greater 

 flow of lymph. 



The more recent experiments of Starling indicate that in addition to the 

 difference of pressure between the blood in the capillaries and the lymph in 

 the tissue spaces, a new factor must be considered and that is, the permea- 

 bility of the capillary wall. This he finds to vary considerably in different 

 parts of the vascular apparatus, being greatest in the capillaries of the 



