THE SECRETION OF URINE 



1199 



as normal constituents of the serum, such as sodium chloride, which could 

 not be made good at the expense of the food. It is for this reason that an 

 absorptive mechanism sensitive to and reflecting the nutritive condition 

 of the whole body, especially as concerns water and salts, should be present 

 in the tubules. 



According to Cushny, the whole of the changes by which the glome- 

 rular transudate is transformed into urine may be ascribed to processes 

 of absorption occurring in the tubules, there being no need to assume the 

 possession of any secretory functions by this part of the kidney. He would 

 indeed deny any fine discrimination to the kidney, since the fluid absorbed 

 is always the same whatever the needs of the organism at the moment. In 

 the following Table are given the changes which must be effected in the 

 glomerular transudate in order to transform it into urine. 



It will be seen that, while there is no absorption of urea and of sulphate, 

 the whole of the dextrose is absorbed, a portion of the uric acid and the 

 greater part of the sodium, potassium and chloride. The absorbed fluid thus 

 resembles strongly Locke's fluid. According to this view the constituents 

 of the glomerular transudate, i. e. the diffusible constituents of the blood 

 plasma, may be divided into two classes, ' threshold substances ' and 

 ' no-threshold substances,' the former being only excreted in the urine so 

 far as they exceed a certain threshold value, while the others are excreted 

 in proportion to their absolute amount in the plasma. Thus, of the threshold 

 substances, the dextrose of the plasma is normally below the threshold, 

 and is therefore not present in normal urine. The sodium chloride also 

 comes within the threshold class, but its threshold is more frequently 

 exceeded in normal conditions, and the excess is then eliminated. When 

 the sugar of the plasma rises, as in diabetes, to 0-3 per cent., it appears in 

 the urine and then undergoes concentration just as urea does. Thus, so far 

 as concerns the cells of the. tubules, the no-threshold substances are not 

 absorbed and must all escape by the ureter, whereas the threshold bodies- 



