692 THE SECRETION OF URINE 



resemblance to other glands ; not only is the glomerulus a struc- 

 ture without analogy elsewhere in the body, except perhaps in the 

 choroid plexus of the brain, but the extreme length and com- 

 plexity of the tubule is a feature peculiar amongst glands to the 

 kidney. 



It has become an axiom of physiology that difference of 

 function and structure go hand in hand. This idea was clearly 

 appreciated by Bowman, who, in 1842, published his anatomical 

 researches on the structure of the kidney, and at the same time 

 a theory of urinary secretion founded on those results alone. He 

 pointed out the striking structural peculiarities of the kidney as 

 compared with other glands, and suggested that in the glomerulus 

 the watery portion of the urine was separated off from the blood, 

 and that in the tubule urea, uric acid, and other solid con- 

 stituents of the urine were secreted. 



Two years later Ludwig published a mechanical theory of 

 urinary formation founded on experimental work. He supposed 

 that the capillary blood pressure in the glomerulus filtered off a 

 dilute fluid which was concentrated in its passage down the tubule. 

 This concentration of the glomerular filtrate in the tubules was 

 due to the passage of water by diffusion to the more concentrated 

 lymph on the other side of the epithelial cells. Heidenhain has 

 pointed out that Ludwig's view contains three propositions : 

 (a) That the secretion of water in the glomerulus is a mechanical 

 filtration depending on blood pressure ; (6) that all the solid 

 constituents of the urine are passed out through the glomerulus 

 with the water in dilute solution ; (c) that this dilute urine is 

 concentrated in the tubules. 



Both of these views, which are in the main the two held to-day, 

 have undergone some modification from the form in which they 

 were originally stated. Ludwig's view had to be altered when 

 it was demonstrated that the osmotic pressure of urine was 

 generally greater than that of the blood. For, it was impossible 

 to explain how by a process of diffusion the urine might finally 

 be turned out from the tubule with a concentration four or more 

 times greater than that of the blood. It was necessary to believe 

 that the concentration of the glomerular filtrate was brought about 

 by the active intervention of the tubule cells. The view as 

 amended has, therefore, ceased to be purely mechanical, and 

 consists now of mechanical filtration and physiological absorption. 



