236 A Study of the Structural Unit of the Liver 
versa, all vascular channels will disappear in which the rate of the blood- 
stream falls below a certain maximum. If, however, the lumen of the 
vessel bears a fixed relation to the rate of the blood-current, the interval 
between the maximum and minimum cannot be great. From this it ap- 
pears that, after growth is completed, the rate of flow must be fairly uni- 
form in all the capillaries of an organ. 
“The conversion of capillaries into arteries diminishes the resistance 
of the blood-stream, and leads to an increase of pressure in the capil- 
laries. If, then, according to the third histo-mechanical principle, new 
capillaries are formed at all places in the capillary area in which the 
pressure of the blood exceeds a certain limit, these capillaries, again, re- 
duce the pressure by forming new connections between the arteries and 
veins. The third histo-mechanical principle, therefore, implies that, 
during the growth of the organ, new capillaries are being formed every- 
where, and that, after complete growth, the blood-pressure in all capillary 
areas of the same organ is fairly uniform. 
“The width of the lumen of the capillary channel at the close of the 
period of growth must be almost the same in all areas of the same organ, 
since it depends on the rate of flow, and this rate is uniform in all capil- 
laries of the same organ. 
“These conclusions are in perfect harmony with the actual state of 
matters. It appears, however, that in the different organs there are great 
differences in the width of the lumen and in the number of their capil- 
laries, in the rate of flow, and in the quantity of the blood flowing 
through a given area of the vessel in a given time. 
“ Tf these facts be compared with the results which were obtained above, 
according to which the first vascular spaces, the rudimentary capillaries, 
were formed by the secretory activity of the cells forming their wall, we 
are compelled to assume that the metabolic processes and other special 
characteristics of the various organs also exercise a determining influence 
on the peculiarities which distinguish their capillaries. It must be 
imagined that the individual characters of the organ, and its size in rela- 
tion to other organs, decide firstly the number of capillaries in the whole 
organ and in a single part of the organ; further, the special relations 
existing between the rate of flow and the lumen of the capillary channel ; 
and lastly, the height of the blood-pressure which will lead to the forma- 
tion of new capillaries. If, for example, the growth of the capillaries is 
arrested in one organ at a rate of flow a, corresponding to a lumen }, 
in a second organ the growth of the capillary Iumen might perhaps be 
arrested at a rate of flow A corresponding to a lumen B.° Thus, in the 
