THE SECRETION OF URINE 1135 



in turn is determined by the molecular concentration of the two fluids. It 

 is easy to estimate the molecular concentration of any sample of serum or 

 urine. The method which is most convenient is to determine the depression 

 of freezing-point in the two fluids. Whereas serum ordinarily freezes at 

 0-56 C. to 0*59 C., the freezing-point of urine is generally lower and 

 may vary from this figure to as much as - 4*5 C. For the production 

 therefore of urine from blood-plasma, a certain amount of work has to be 

 done, and the seat of this work we can locate only in the cells of the kidney. 

 We may determine the minimum work necessary to form a certain amount of 

 urine of a given concentration by measuring the amount of heat that must 

 be imparted to the blood-plasma in order to reduce it to the same concentra- 

 tion and volume, or we can calculate it if we know the freezing-points of the 

 two fluids. A depression of freezing-point A = 1 C. corresponds to an 

 osmotic pressure of 122*7 metres of water. To concentrate 100 c.c. of a 

 saline fluid such as urine so as to halve its bulk and double its depression of 

 freezing-point, e.g. from - 1 C. to 2 C., would therefore require the 

 expenditure of work equivalent to that which would be required to compress 

 100 c.c. of a gas at a pressure of 122-7 metres of water to half its bulk. 



The abandonment of Ludwig's view as to the mechanism of the concentra- 

 tion does not, however, place his theory out of court. The question will 

 still have to be discussed whether the chief object of the tubules is the con- 

 centration of the fluid produced in the glomeruli, or whether they add to this 

 fluid by a further secretory process, or whether they may not possibly 

 possess both functions and in their various parts alter the fluid flowing 

 through them either by addition or by withdrawal of water or dissolved 

 constituents. The common point in the two theories is the sharp distinc- 

 tion which is drawn between the nature of the glomerular activity and the 

 nature of the activity of the tubules. The questions which we have to 

 decide by experiment are : 



(1) The nature of the glomerular activity and the conditions which 

 determine the amount of fluid formed by the glomeruli, and especially 

 whether the energy required for the formation of the glomerular fluid is 

 furnished by the heart through the blood -pressure within the capillaries or 

 by the endothelium covering these capillaries. 



(2) The function of the tubules, whether they secrete or absorb, and 

 what part is played in these processes by the various segments of the tubules 

 which differ so widely in their histological characters. 



THE SECRETION OF WATER AND SALTS. FUNCTIONS 



OF THE GLOMERULI 



It is generally assumed, as the best explanation of known facts with 

 regard to the secretion of urine, that a watery exudation free from protein is 

 formed in the glomeruli, and that this becomes concentrated on its way 

 through the tubules either by the absorption of water and certain salts or by 

 the secretion and addition of urea, uric acid, &c., as well as such salts as 



