70 ELIOT R. CLARK 



sufficiently vascularized and how supernumerary capillaries are 

 disposed of — in fact, how any equilibrium is established between 

 the extent of metabolism of the various organs and the rich- 

 ness of their blood capillary supply. 



The precise nature of the stimulus may well be conceived as a 

 physical one — the frictioii produced by the passing through the 

 endothelium of fluid substances, just as the other regulating 

 factor — the blood-flow over the interior of the endothelium — is 

 a physical character. With a • certain (undetermined) amount 

 of interchange, the endothelial cell remains unchanged; with in- 

 creased interchange beyond a certain (undetermined) point, the 

 endothelial cell sends out a sprout; with diminished interchange 

 below a certain (undetermined) point the endothelial cells con- 

 strict the lumen and are eventually withdrawn into the active 

 capillaries, leaving no trace of the capillary which had fallen into 

 disuse. It is probable, if this hypothesis is correct, that the re- 

 traction of capillaries through which circulation has ceased as 

 the result of equalized pressures at the extremities or of plugging 

 of the vessel, is due to the operation of this factor of endothelial 

 response to diminished interchange. 



These two factors, then, amount of interchange through and 

 amount of flow over the inner surface of the endothelial wall of 

 capillaries appear to be the chief ones concerned in the regula- 

 tion of the new growth of capillaries, their maintenance, increase 

 in diameter to form arterioles and venules, or decrease in diame- 

 ter with e^'entual solidification and retraction. Of these two 

 factors, the amount of interchange is primary and the amount of 

 flow secondary, since increased or diminished blood-flow depends 

 in the main upon changes incident to the formation of new 

 capillaries. 



Endothelium subjected to either of these factors will survive, 

 grow, or retract according to the intensity of the stimulus. If 

 the blood-flow is increased, there is increase in the diameter of 

 the vessel. If the rate of interchange is increased there results 

 sprout formation. Diminution of blood-flow causes diminution 

 in diameter, and diminution of interchange, narrowing, solidifica- 

 tion and retraction of endothelium. 



