THE SECRETION OF THE URINE 503 



simple or equally abstruse. For a cell which is able to take up 

 ' Locke's fluid ' from a deproteinized plasma is able in that very act 

 to reject any constituent which does not belong to 'Locke's fluid,' 

 as well as an excess of any constituent which does belong to it. What is 

 rejected and its amount are quite as important, if the final product is 

 to be constant, as what is accepted and its amount. If we are willing 

 to attribute a power of this kind to the free end of a tubule cell we 

 need not shrink on the ground of added complexity from investing 

 the attached end of the cell with the power of refusing passage to 

 ' Locke's fluid ' from the plasma or the lymph while accepting crys- 

 talloid constituents like urea which do not belong to it, as well as an 

 excess of any constituent which does belong to it. There is nothing 

 more ' occult ' about a cell which bars out sodium chloride till it 

 exceeds 0-9 per cent, in the fluid offered to it, and then lets the surplus 

 through, than there is about a cell which takes up sodium chloride 

 from the fluid offered to it till it has amassed a concentration of 0-9 

 per cent., and then bars out the surplus.* In like manner, the financial 

 organization required by a Government to take from a citizen in taxes 

 the surplus of his income above one hundred pounds would be of the 

 same general nature as that required to take from him his whole income, 

 returning him one hundred pounds to live on. If a machine could 

 manage the one operation, a mandarin would not be needed for the other. 

 It is impossible in this place to go further into the discussion of the 

 reabsorption theory so ably presented by Cushny. In the absence of 

 definite evidence of reabsorption on the great scale required by the 

 theory, it remains simply a working hypothesis, which is all that can 

 be claimed at present for any theory of urine formation. One of the 

 objections always urged against filtration (with reabsorption) theories 

 has been the enormous amount of liquid which must under certain con- 

 ditions be poured into the renal tubules. Heidenhain calculated that 

 in a man no less than 70 litres of liquid per day must be filtered through 

 the glomeruli in order that the urea found in the urine may be obtained 

 from a glomerular filtrate containing 0-05 per cent, of urea. This is 

 probably a moderate estimate, and considerably larger amounts of 

 nitrate in proportion to body and kidney weight have been deduced 

 from data obtained in animals. It has been argued by advocates of 

 the reabsorption theory that equally great quantities of lymph con- 

 taining urea in the small concentration in which it exists in the plasma 

 must be poured out around the attached ends of the tubule cells on 

 the hypothesis of direct secretion. This argument, however, is based 

 upon an erroneous conception of the manner in which exchange between 

 the blood and the tissues proceeds. There is no reason to suppose" 



* The reabsorption theory is perhaps inferior to the ' direct secretion ' 

 theory in one point. If a solution of constant composition is always absorbed 

 from the glomerular nitrate, the smaller the concentration in the filtrate of 

 any substance capable of being taken up by the cells, the greater will be the 

 proportion of it absorbed; and the greater its concentration in the filtrate, 

 the smaller will be the proportion of it absorbed. On the direct secretion 

 theory, the greater the concentration of a substance in the blood or lymph 

 in contact with the cells, the more of it will pass into the cells and be ex- 

 creted by them. The ' modern ' theory which sets out by saying to the 

 Bowman-Heidenhain theory, ' Stand by thyself, come not near to me, for I 

 am more physical than thou,' thus abandons a physical process, diffusion, 

 which its rival utilizes. The fact is that the simplification attained by postu- 

 lating filtration as the first stage in urine formation has to be paid for in 

 the reabsorption. The boat having shot the rapids in the glomeruli with 

 next to no physiological expense has straightway to be more or less painfully 

 locked a certain distance upstream. 



