SECRETION OE WATER. 
641 
depending only on the blood pressure in the glomerular capillaries 
and the perineal >il it y of the filtering membrane. 
2. That this dilute urine is concentrated in the tubules by giving up 
its water to the surrounding lymph, in consequence of differences of 
concentration between the glomerular filtrate and the lymph. 
.*!. That all the urinary constituents are turned out of the blood 
through the glomeruli (i.e. with the water) in dilute solution. 
In discussing the experimental data bearing on these propositions, 
we shall find that only in the first part of the theory are the 
experimental facts consonant with Ludwig's hypothesis, and that it is 
impossible to explain the formation of normal urine without assuming 
the active intervention (i.e. the performance of work) by certain of the 
living elements of the kidney in the process. In this case, as in so 
many others in physiology, the " how " of the cellular activity is 
at present absolutely unknown to us, although we may confidently 
expect, with the advance of the science, to be a hie to trace the 
manner in which the cell utilises the energy of its food for this special 
purpose. 
Secretion of water. — One of the strongest facts in favour of Ludwig's 
hypothesis is the indubitable 
connection which exists be- 
tween the circulation through 
the kidney and the amount 
of urine, i.e. of water, secreted. 
It is evident that a mechanical 
filtration or separation of the 
watery and crystalloid consti- 
tuents of the blood in the 
glomeruli must depend on two 
factors — 
1. The difference of pres- 
sure between the blood in the 
glomerular capillaries and the 
urine in Bowman's capsule. 
Since under normal circum- 
stances there is a free outflow 
of urine from the capsule by 
means of the tubules, we may 
regard its ^pressure as practi- 
cally nil, so that the only 
■changeable factor in the pro- 
cess will be the blood pressure 
in the capillaries. 
2. The rapidity of the blood flow through the glomeruli must also 
have some influence on the filtration, as this process will go on the 
more readily the more often the fluid presented to the filter is renewed. 
As a rule, the changes that raise the pressure in the capillaries also 
increase the velocity of the blood through them, so that it becomes 
difficult to dissociate the part played by each factor in influencing the 
urinary secretion. 
If we consider the manner in which changes in the glomerular 
circulation are brought about, we see that it may be affected by changes 
either in the general blood pressure or in the calibre of the smaller 
vol. 1. — 41 

\a 
Fig. 59. — Roy's oncometer. (For explanation of 
lettering, see next figure.) 
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iU~ 
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