EXCRETION 271 



The skin is commonly spoken of as an organ of excre- 

 tion, but, as we shall see later, it is very doubtful whether 

 it is right to regard it as such in any but an accidental 

 way. 



Excretion by the Kidney. 



It is admitted by physiologists that we are still very 

 much in the dark as to the mechanism by which the 

 kidney produces the urine, but we know it to be an organ 

 of excretion in the strict physiological sense, for, with 

 the exception of hippuric acid, all the ingredients of the 

 urine are already present in the blood. Hence, in 

 disease of the kidneys the urinary constituents may 

 accumulate in the body. 



Opinion has long been divided between the relative 

 merits of the ' vital ' theory of urine production pro- 

 pounded by Bowman and the so-called 'mechanical' 

 theory of Ludwig, nor can the controversy be regarded 

 as even yet finally closed. Perhaps the view which finds 

 most favour at the present day is that which attributes 

 to the glomeruli the function of separating out water 

 and mineral constituents (except phosphates), and per- 

 haps some foreign ingredients, such as sugar, whilst 

 regarding the cells of the convoluted tubules as respon- 

 sible for picking out of the blood the specific organic 

 components of the urine, such as urea. Whether any 

 reabsorption of water takes place during the passage of 

 the urine along the tubules is still disputed. The strictly 

 mechanical theory of filtration through the glomeruli, 

 however, has been generally abandoned since evidence 

 has accumulated to show that the cells covering the 



