58 



ELIOT R. CLARK 



and absorption are approximately equal, as in the liver. It 

 seems almost inconceivable that the great richness of capillaries 

 in each of these cases can be due to the presence of an unusual 

 amount of tropic substances. 



The only common factor that can be discovered would seem to 

 be the total amount of passage of substances through the endo- 

 thelial wall, whether the direction be to or from the lumen of 

 the capillary. That it is the quantity of substances passing 

 through, and not some specific chemical body is strongly indi- 

 cated by the fact that, in different organs, the substances which 

 pass through the capillary wall are, in many cases, of a widely 

 different nature. 



Fig. 9 Diagram to represent the modification in amount of tissue sup- 

 plied by a capillary as the result of the addition of a new capillary. The dotted 

 area indicates the area supplied by the portion of capillary DBF; the lined area 

 that supplied by the new capillary DEF. 



This, then, would seem to me to be the most likely hypothesis 

 as to the nature of the stimulus which is chiefly responsible for 

 the formation of new sprouts — that it is the total quantity of pas- 

 sage of substances through the endothelial wall. This conclusion 

 is in agreement with that of Roux ('95) which has previously 

 been fully quoted. According to this hypothesis, when the 

 amount of fluid passing through any part of the endothelial wall 

 exceeds a certain point, the endothelium reacts by sending out 

 a sprout, which eventually becomes a new capillary, thereby 

 increasing the endothelial surface and diminishing the relative 

 amount of interchange through any part of the wall. Figure 9 

 has been constructed as a diagram to show how such a law, re- 



