658 THE SECRETION OF URINE. 
An ingenious attempt lias been made by Liebermann x to explain the 
chemical mechanism by which the cells of the tubules effect this change in 
reaction. This author has described a class of bodies which may be extracted 
from the mucous membrane of the stomach or from the kidney, and which 
consist of compounds of lecithin and proteid. These he designates lecith- 
albumins. These substances are acid in nature, and are capable of combining 
with alkalies. Liebermann imagines that, as the alkaline salts of the blood 
plasma pass through the epithelial cells of the kidney, they are split up by these 
acid insoluble lecith-albumins, Avhich combine with a portion of the bases, so 
that the remainder of the fluid which reaches the lumen of the tubule contains 
acid salts or free acid. Of course this process would come to an end as soon as 
the acid affinities of the lecith-albumins in the cells were satisfied, and in this 
way one might explain the speedy appearance of an alkaline reaction when 
large quantities of urine are secreted. Under normal circumstances, however, 
Liebermann assumes that the carbon dioxide, which is the normal product of 
tissue metabolism in the kidney, splits up the compound formed in the cells 
into free lecith-albumin and alkaline carbonates, these latter being then 
removed by the venous blood stream. As supporting evidence for this hypo- 
thesis, Liebermann states that the kidney tissue, like lecith-albumin itself, if 
treated with soda solution, and then washed repeatedly with water to remove 
excess of the latter, becomes strongly alkaline. If now the alkaline tissue be 
suspended in water, through which a stream of CO., is passed, and be then 
again washed thoroughly, it will be found to be strongly acid, having given up 
all its soda to the carbon dioxide. 
Conclusions. — It is evident that the experimental facts at our pre- 
sent disposal do not allow of a definite decision as to the exact manner 
in which the secretion of urine is effected. It will be convenient, 
therefore, to summarise the two modes of interpretation, either of which 
may be applied to the known facts. 
According to the Bowman-Heidenhain hypothesis, the secretion of 
urine is due to the activity of two sets of cells. The flattened epithelial 
cells covering the glomeruli take up from the blood, circulating through 
the glomerular capillaries, water and salts, and transfer these substances 
to the beginning of the urinary tubule. Their activity is chiefly depend- 
ent on the activity of the blood flow through the capillaries. But they 
may be also excited to active secretion by the presence of certain of the 
urinary constituents in the blood, such as w T ater and salts, or possibly by 
diuretics, such as caffein. On the other hand, the rodded cells, lining the 
convoluted tubules and the ascending loop of Henle, secrete specific 
urinary constituents, such as urea and uric acid, together with a certain 
amount of water. They also secrete certain abnormal constituents of the 
blood, such as indigo-carmine. Their activity is chiefly determined by 
the amount of urea or uric acid in the blood. 
If, on the other hand, we accept Ludwig's hypothesis, we must 
introduce into it certain modifications, necessitated by later inquiries, 
and assume that in the secretion of urine, as in so many other of the 
bodily functions, there is a mixture of what we may term physical and 
physiological processes. It seems probable that in the glomeruli the 
process is largely if not exclusively physical ; that is to say, we have here 
a transudation of the watery and crystalloid constituents (including 
urea) of the blood plasma. The extent and nature of this transudation 
are determined — 
1. By the pressure in the glomerular capillaries. 
1 Arch.f. d. ges. Physiol., Bonn, 1894, Bd. liv. S. 585. 
