238 A Study of the Structural Unit of the Liver 
were always equally favored by the circulation, and that many descrip- 
tions and illustrations of the blood-vessel, as, for example, of the villus 
and the glomerules, could not possibly be correct. If in reality the blood- 
vessels of these structures were arranged as is frequently pictured, the 
blood would have to take the capillaries in the course of the least re- 
sistance, while in those of the greatest resistance it would stagnate or 
come to a standstill. 'Thoma’s first law explains how an equal distribu- 
tion which favors no part of an organ is brought about. In development 
the vessels in which the blood stagnates degenerate, and in those in which 
the rapidity is too great the lumen is enlarged. There seems to be a 
tendency to maintain a “normal” fiow of blood through the capillary. 
After capillaries are well dilated they become arteries and veins, and the 
thickness of their walls is now dependent upon their tension, according to 
Thoma’s second law. These two laws are constantly at work, and regu- 
late accurately the diameter and thickness of the walls of the arteries and 
veins. . 
Before considering Thoma’s third histo-mechanical principle, it is 
necessary to discuss his numerous measurements as well as to give data 
which I have accumulated. The whole question hinges upon the cause 
of the new formation of capillaries for which Thoma has not found a 
law, but has merely put forward an hypothesis. 
Thoma made many measurements of arteries and their branches and 
tabulated Bencke’s measurements of the aorta with its branches. These 
measurements show that the area of all of the branches of the aorta equals 
about the area of the ascending aorta, being a little less before the thirti- 
eth year of age and a little greater thereafter. Thoma gives a few 
measurements of small arteries in which the area of the immediate 
branches equals about that of the main stem. 'These measurements, how- 
ever, are not constant in live animals, for if the observations are con- 
tinued, the caliber of the branches increases out of proportion, and ulti- 
mately their area exceeds that of the main stem.” This change Thoma 
ascribes to a change in the vascular tone. In other parts of the same 
work,” as well as elsewhere, he appears to be somewhat uncertain regard- 
ing the equality of the area of a vessel and the area of all of its branches. 
Also in a later publication the arguments seem to accumulate against 
this view.” Thoma states, however, that the exceptional cases are found 
Thoma, Histogenese u. Histomechanik, 66. 
4 Thoma, Ibid., p. 86; Pathology, 275 and 276. 
1 Thoma, Ueber den Verzweigungsmodus der Arterien. Arch. f. Entwick. d. 
Organismen, XII, 1901. 
