84 ELIOT R. CLARK 



The new formation, enlargement, maintenance and atrophy of 

 capillaries is dependent upon two factors (1) the amount of blood 

 flow and (2) the amount of interchange of substances through 

 the wall. 



1. A capillary or part of a capillary so located that an in- 

 creased amount of blood passes through it, increases in diameter 

 until it may form part of an arteriole or venule; one so located 

 that there is no especial increase or decrease in the amount of 

 blood passing through remains a capillary; while to decrease or 

 absence of blood-flow the capillary reacts by a diminution in 

 lumen to final solidification and complete retraction. This, in a 

 general way, agrees with Thoma's first histomechanical law, ac- 

 cording to which the diameter of the vessel responds to the 

 action of the moving blood stream. A difference, however, lies 

 in this, that, while Thoma assigns chief importance to the rate 

 of the blood stream, I find, in capillaries, that it is rather the 

 amount of blood flow. 



The changes in the amount of flow through different capillaries 

 are brought about in various ways. The addition of new capil- 

 laries beyond, may cause an increase in a capillary so placed as 

 to help supply or drain the new area. Again, the opening up 

 of a new capillary may place an older capillary in such a position 

 that it forms merely a cross connection between two parallel 

 vessels, in which the pressure is equal, bringing about a slowing 

 or stoppage of circulation in the older capillary. In certain 

 cases of stoppage of circulation the cause is more puzzling — 

 that is, in case of branches of capillaries which enlarge to form 

 arterioles. It is suggested that the stoppage, here, is due to the 

 constriction about the beginning of the branch, resulting from 

 the increased thickness and elasticity of the arteriole. Some 

 support is lent to this explanation by the fact that blood cells 

 are often seen to plug the entrance to such branches often for 

 long periods. 



It is possible, however, that the narrowing of such vessels, as 

 well as the slowing of circulation in the case of other capillaries, 

 is due to the second factor mentioned. 



2. The amount of interchange through the wall. This is pro- 



