40 ELIOT R. CLARK 



The initial stimulus to this study was given by W. Roux ('79), 

 in his Inaugural Dissertation, in which he studied the '"angle of 

 branching" in relation to the relative size of the branch, and 

 the shape of vessels in the neighborhood of a branch. He 

 found that this angle, which lies between a line continuing the 

 axis of the main stem and the axis of the branch, varies with 

 the relative size of the branch — that, in general, the larger (rela- 

 tively) the branch, the smaller the angle, and the smaller the 

 branch, the larger the angle. He also found that the lumen of 

 an artery shows a widening with subsequent narrowing imme- 

 diately after branching, and that the opening of the branch is 

 oval rather than round. By experiments with openings made in 

 vessels and in tubes and with the use of malleable substances 

 such as lard placed in such openings and on the interior of tubes, 

 he found that the direction taken and the shape found is, in the 

 case of the artery, practically the same as the shape and direc- 

 tion of the stream of fluid emerging from openings in vessels 

 and tubes. He concluded that the shape and direction of 

 arteries at the place of branching are determined by the 

 action of hemodynamical factors; that the blood-vessel wall 

 responds by taking the shape which allows a minimum of fric-. 

 tion. The general and important conclusion was that the size 

 and shape of arteries and veins, in the growing and adult 

 animal, are regulated, not by heredity, but by the action of 

 mechanical factors. 



Thoma's conclusions were based mainly on studies made on 

 the extra-embryonic yolk sac vessels of chick embryos. From a 

 series of injections he found that there is formed, first, an indif- 

 ferent plexus of capillaries, interposed between the aorta and 

 the venous end of the heart, and that out of this plexus, those 

 vessels which are so placed as to have the greatest amount of 

 blood flowing through them enlarge to become arteries and veins, 

 while others remain capillaries, or atrophy. 



The results of these and other studies by Thoma ('11) may be 

 briefly summarized. He finds that blood-vessels are regulated 

 in their growth by mechanical factors, which he expresses in the 

 form of 'laws' ('Histomechaniche Principien'), as follows: 



