THE CIRCULATION 



307 



The pressure of the blood within the capillaries is sufficient to force 

 some of the same through their walls. The capillary blood-pressure is 

 said to average about 15 mm. of mercury (more at the arterial end of 

 the capillary and surely less at the venous end), and if no other forces 

 aided these there would doubtless be a slow and continual soakage 

 outward into the tissues wherever a thoroughfare could be established. 

 Between the endothelial plates, through them, or through the small rents 

 which must continually occur in them some plasma would escape under 

 this mechanical influence alone. This process would be filtration, at 

 least in part. The nature of osmosis has been described, and nowhere 

 does it occur more actively or more importantly than here. It is one of 

 the most powerful factors of the lymph-flow, but whether more so than 

 filtration cannot be determined. Its nature makes of it under certain 

 conditions a force of enormous power, and it may exert a powerful influ- 



FIG. 168 



FIG. 169 



Network of 

 terminal bars 



Top-plate 



Intercellular 

 substance 



The lymphatics of the outer skin, in- Diagram of some columnar epithelial cells to show 



jected; black. 5 %. (Rauber.) how the intercellular spaces are shut off from the 



free surface of the gland by the "terminal bars." 

 (Stohr.) 



ence on the plasma and lymph. It is dependent not on differences of 

 hydraulic pressure, but on molecular conditions of solution still open both 

 to study and to doubt. (See our brief consideration of osmosis on page 

 221 and those following.) It is to maintain the normal osmotic pressure 

 in the capillaries, perhaps, as well as in the cells and elsewhere, that the 

 salines of our food have a chief usefulness. 



Having passed through the capillary-wall, the plasma, now termed 

 lymph, finds less resistance beyond than behind, and so soaks between the 

 cells and out again. It then osmoses into freer channels and into the 

 lymphatics. The osmosis and the filtration are continually going on, 

 and they constitute the continual "force from behind" which crowds 

 the lymph onward through the tissue-chinks. Perhaps this alone would 

 be sufficient to continue the flow even into the subclavian veins, but 

 more likely not, at least in that regularity, certainty, and promptness the 

 metabolic processes require. 



