694 THE SECRETION OF URINE 



filter or do the cells of Bowman's capsule determine the kind of 

 material and rate at which it shall pass through them ? (2) Do 

 the cells of the tubule secrete material from a glomerular filtrate 

 back into the blood, or do they excrete material from the blood 

 and add it to the glomerular secretion, or do they both absorb 

 and excrete ? It is theoretically impossible to discuss these two 

 points apart, because we cannot, as a rule, experiment separately 

 upon the glomeruli and tubules, nor are we able to obtain the 

 glomerular fluid until it has passed down the tubules ; but, as 

 far as possible, the two points will be considered apart. 



A large number of experiments have been performed on excised 

 kidneys by various observers, and especially by Sollmann. He 

 admits at once that the normal vitality of the kidney begins to 

 disappear soon after excision, and that the " urine " obtained by 

 the perfusion of such kidneys can be looked upon only as a physical 

 filtrate and not as a vital secretion. Nevertheless, the experiments 

 are of great value, for they do show which of the experimental 

 results obtained on living kidneys are capable of a purely 

 mechanical explanation ; but of course they leave untouched the 

 question whether in the living body the process is mechanical 

 or vital. This being so, we shall refer to experiments on excised 

 kidneys only incidentally. 



Most of the experiments which have been performed on the 

 secretion of urine have given results which are considered to be 

 in favour either of Lad wig's or Heidenhain's view, and in order 

 that the bearing of the various experiments to be described may 

 be more readily understood, it will be well to point out in greater 

 detail what these two views really imply. According to Ludwig's 

 view a filtering force, derived ultimately from the heart, drives 

 through the filtering membrane a fluid exactly similar to blood 

 plasma, except that it contains no proteid. The glomerular filtrate 

 will therefore be practically isotonic with the plasma, and the cells 

 of Bowman's capsule will have done no work in separating the 

 filtrate. Filtration may be assisted by an increased rate of blood- 

 flow past the filter, but will depend fundamentally on the magni- 

 tude of the filtering force. The filtering membrane is impermeable 

 to the blood proteids, but must be equally permeable to alt other 

 molecules, and any alteration in the permeability of the filter must 

 affect equally all filterable substances. The work of the kidney 

 will be done entirely by the cells of the tubule during the process 



