232 A Study of the Structural Unit of the Liver 
which relates to the capillaries in particular is to be found in his brilliant 
It is not 
10 
study of the blood-vessels of the area vasculosa of the chick. 
possible to discuss in detail the many observations and arguments in this 
model research without extending this paper far beyond the space of this 
Journal. However, an excellent summary of Thoma’s work is given in 
his Pathology, from which I will quote several pages from the English 
translation.” Thoma’s work is summed up in three laws or histomechan- 
ical principles (page 265), as follows: 
(1) “The increase in the size of the lumen of the vessel, or what ts 
the same thing, the increase in the surface of the vessel wall, depends 
upon the rate of the blood-current. The surface of a vessel wall ceases 
to grow when the blood-current acquires a definite rate. The vessel in- 
creases in size when this rate is exceeded, becomes smaller when the 
blood-stream is slowed, and disappears when it is finally arrested. 
“This law which brings the growth of the surface of the vessel wall 
into dependence upon the rate of the flow of the blood is, I consider, 
the first and most important histo-mechanical principle which determines 
the state of the lumen of the vessel under physiological and pathological 
conditions. It will be further proved, however, in many places in the 
general, as well as in the special parts of this book. 
“A second histo-mechanical principle may be added to this, viz., the 
growth in thickness of the vessel wall is dependent wpon its tension. 
Further the tension of the wall is dependent upon the diameter of the 
lumen of the vessel and upon the blood-pressure. 
“The proof of this law is to be sought, in the first place, in the varying 
strength of the wall of the larger and smaller arteries, veins, and capil- 
laries. In certain diseases of the vessels (arteriosclerosis, aneurism) 
there are apparent exceptions which will be discussed in their proper 
place. 
“The third histo-mechanical principle has not hitherto been so com- 
pletely demonstrated as the first two. It will, therefore, be put forward 
merely as an hypothesis, which runs as follows: increase of blood-pres- 
sure in the capillary areas leads to new formation of capillaries. 
“The three histo-mechanical principles were, in the first place, em- 
ployed to explain the developmental processes in the area vasculosa of 
the chick. In this flat extended area a capillary network is found at an 
” Thoma, Untersuchungen ueber die Histogenese und Histomechanik des 
Gefasssystems. Stuttgart, 1893. 
4 Thoma, Text-book of General Pathology and Pathological Anatomy. Trans- 
lated by Bruce. London, 1896. 
