SECRETION OF THE URINE. 397 



Such a theory, however, cannot adequately explain the forma- 

 tion of urine, because the urine differs so materially from the 

 fluid one could obtain as a filtrate from the blood. In health it 

 contains no albumin, a substance in which the blood is very rich ; 

 and it has enormously more urea and salts than the blood. There 

 is, therefore, both a quantitative and qualitative difference, which 

 implies a distinct process of selection, and although filtration 

 cannot be altogether excluded from the process, it must be com- 

 pletely modified by other forces. 



Moreover, in the general description of the organ, we have just 

 seen that, in a great part of the tubules, both the epithelial and 

 vascular supply give the idea of actively secreting gland tubes. 

 From the mere construction of the different portions of the gland 

 it has been concluded that there are two distinct departments, 

 each of which plays a different part in the production of the 

 urine. One is a simple filtering mechanism, and the other a 

 definitely secreting glandular tubule. 



It is not surprising that, with such a complex arrangement as 

 the tubules above mentioned, there should exist different views 

 as to the exact mode in which the urine is secreted. As these are 

 more or less at variance in their explanation of the method of 

 secretion, and as it is difficult to put any of them aside as quite 

 erroneous, it becomes necessary to enumerate each somewhat in 

 detail. 



Feeling convinced of the filter-like function of the glomerulus, 

 and recognizing the fact that some other agency was also at work 

 in the formation of urine, Bowman explained the process thus : 

 From the glomerulus the watery parts of the fluid are filtered, 

 while the glandular epithelium selects the important solid con- 

 stituents which it is necessary to remove from the blood. 



Ludwig takes a different view. He believes that the watery 

 part of the plasma, bearing with it the salts, etc., is filtered from 

 the glomerulus. As this fluid passes through the tortuous urinary 

 tubules, a large portion of the water is reabsorbed into the capil- 

 lary networks surrounding them. This reabsorption is assisted 

 by the high specific gravity of the blood and the low pressure in 

 these capillaries as compared with the glomeruli, where the fil- 



