394 MANUAL OF PHYSIOLOGY. 



constantly greater than that of the second capillary network 

 around the convoluted tubules. 



The general arrangement of these vessels, and the high pressure 

 in the glomerulus, give the impression that it is simply a filtering 

 apparatus by means of which the fluid parts of the blood pass 

 into the urinary tubules. This view seems supported by the fact 

 that the quantity of urine secreted bears a direct proportion to 

 the blood-pressure in the minute renal vessels, whether the change 

 in pressure depends on local vascular mechanisms or on changes 

 in the general blood -pressure. 



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 con- 

 tains 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: 



