THE SECRETION OF URINE 1269 



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 concentration 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 2C., 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 

 concentration 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 concentration 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 distinction 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 capil- 

 laries 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 con- 

 centrated 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 acid phosphates. As to the nature 



