426 THE URINARY APPARATUS. 



though nerve-fibrils are described as ramifying among the epithe- 

 lium of the tubules. 



Function of the Kidneys. The function of the kidneys is to 

 form urine. This is sometimes spoken of as a secretion, but inas- 

 much as its constituents pre-exist in the blood when that fluid 

 comes to the kidneys, and these organs simply remove these sub- 

 stances from the blood, urine is more properly an excretion. The 

 ingredients composing the urine may, from a physiologic stand- 

 point, be divided into two groups : (1) Water and some of the 

 salts ; and (2) urea, uric acid, and allied substances. 



The first group, consisting of water and salts, is eliminated 

 from the blood while that fluid is passing through the glomerulus 

 within Bowman's capsule, the beginning of the uriniferous tubules ; 

 while the urea group is excreted while the blood is passing through 

 the venous plexus surrounding the convoluted portions of the 

 tubule. 



Excretion of Water and Salts. While all authorities agree that 

 it is at the glomeruli that water and some of the inorganic salts 

 are eliminated from the blood, there is a diversity of opinion as to 

 the factors engaged in this process ; some regarding it as a simple 

 filtration in which the glomerular epithelium plays a passive part, 

 while others attribute to these cells a very important part in the 

 process. Whenever the renal blood-supply is increased, the 

 quantity of urine is also increased, and it has therefore been 

 assumed that this increase was due to the heightened blood-press- 

 ure within the glomerulus, and this is the basis of the filtration 

 theory ; but it has been pointed out that under these circumstances 

 there is an increased blood-flow through the organ, and it is to 

 this that Heidenhain attributes the increased secretion, rather than 

 to the simple increase of pressure within the glomeruli. This 

 authority is one of the principal exponents of the theory that the 

 glomerular epithelium is the efficient agent in the elimination 

 which takes place within Bowman's capsule. 



Bowman, in 1842, advanced the opinion that "the Malpighian 

 bodies might be an apparatus destined to separate from the blood 

 the watery portion " of the urine. On the other hand, he held 

 that " the tubes and their plexus of capillaries were probably the 

 parts concerned in the secretion of that portion of the urine to 

 which its characteristic properties are due." 



In 1884 Ludwig expressed the opinion that all the constituents 

 of the urine escape from the blood while passing through the 

 glomeruli, and that this separation is due to the high pressure 

 under which the blood is within these structures. 



Heidenhain combated the view of Ludwig on various grounds : 

 Among others, that a rise of arterial pressure elsewhere in the 

 body, as in the salivary glands, does not cause increased transuda- 

 tion through the walls of the blood-vessels ; that the epithelium 



