62 ELIOT R. CLARK 



and yet the rate of circulation in veins is less than in arteries, 

 while the amount of blood flowing through the two sets of vessels 

 is the same. 



It is clear, then, that the size of the lumen is not solely de- 

 pendent upon either the rate or the amount of blood-flow. The 

 chief difference in the condition existing in arteries as compared 

 with veins, aside from the rate of blood flow, is the difference in 

 blood-pressure. It would, therefore, seem 'that, with the same 

 amount of blood to be propelled, the size of the lumen varies 

 inversely as the pressure, providing the resistance is such that, 

 with the greater pressure, there is a higher rate of blood-flow. 



The diminution in caliber of blood-capillaries, after the early 

 stage, is probably to be explained in this way. At the early stage 

 the strength of the heart-beat is relatively small, as is shown by 

 the slow rate of the circulation. Later the heart-beat evidently 

 becomes stronger, for the rate of circulation increases markedly 

 in all vessels. With this increase in rate there is diminution in 

 caliber of all vessels. 



If, however, we consider the changes which take place in a 

 number of vessels which are subjected to approximately the 

 same pressure conditions and rate of circulation, we find that 

 the lumen varies with the amount of blood flowing through. 

 Given a sufficient amount of blood to fill all vessels, and a suf- 

 ficient strength of heart-beat to keep the capillary circulation 

 up to the necessary standard, and it is found that the size of the 

 vessel varies with the amount of blood flowing through. Thus, 

 of two capillaries near one another, the one so placed that it 

 forms a pathway for the supply or drainage of an enlarged capil- 

 lary area has an increased circulation and increases in caliber, 

 while the one not so situated remains the same size or becomes 

 smaller. 



The objection might be raised that the movement of the blood 

 is not a formative factor — that the vessel merely fits the stream, 

 and that its size is solely the result of the mechanical distention. 

 That this is not valid is shown by the fact that vessels in which 

 the circulation ceases altogether, or in which the circulation never 

 starts, grow smaller and smaller until their lumen is entirely 



