656 
BIOLOGY: L. J. HENDERSON 
the dissolved constituents of blood plasma, including water itself, in 
such amounts as to bring the composition of the blood in all respects 
to a hypothetical normal composition; secondly, a certain portion of 
this normal plasma, minus the colloids, must be removed in order to 
regulate the volume. 
Statistically the volume of the urine must vary with the magnitude 
of the volume regulation, although, in particular cases, it need bear 
no relation to this quantity. Now sodium chloride is the principal 
constituent of blood plasma. Hence, statistically the ratio A:NaCl 
(i.e. the ratio of freezing point depression to sodium chloride concen- 
tration in urine) will be small when the volume is large, and large when 
the volume is small. Thus we arrive at a theoretical deduction a 
priori of Koranyi's coefficient.^ This coefficient tells us, therefore, 
nothing about the mode of action of the kidney mechanism. It has 
no bearing on the question of the functions of glomeruli and tubules. 
For the ratio is seen to be, as existing evidence proves, necessarily 
liable to indefinite fluctuations in particular cases. There is involved 
merely a statistical truth, expressing the conditions under which any 
kidney must operate in case sodium chloride is the chief constituent 
of the blood. 
The practical importance of this theoretical discrimination of volume 
regulation from the excretion of the several urinary constituents appears 
to be established by the fact that in pathological conditions the daily 
volume of urine may be constant during variations in amounts of water 
or salt ingested, even when such experiments lead to fluctuations in 
the physico-chemical properties of the urine. ^ For this shows that the 
regulation of volume may be deranged more or less independently of 
the proper excretory functions. 
If the final stage of volume regulation has been neglected, the inter- 
mediate stages have been generally misconceived. Neither the swell- 
ing of colloids nor the pressure which results from osmosis can furnish 
the basis for an analysis of such phenomena. In their stead we must 
turn to the kinetic theory and to Willard Gibbs' thermodynamics. 
But it should be first remarked that osmotic 'pressure' and colloidal 
swelling 'pressure' hardly act as important sources of mechanical tension, 
in the living organism. Even oedema involves very small magnitudes 
of such pressures. Secondly, changes in volume merely consist in the 
passage of material from one phase to another (except in so far as phys- 
ical and chemical changes within a phase may produce very slight 
fluctuations in volume). Finally, apart from the operation of special 
secretory mechanisms which we do not understand, such processes 
