THE KIDNEYS AND SKIN. 435 



Kreatinin (C 4 H 7 N,0) is closely allied tokreatin (C 4 H 9 N 3 2 ), 

 of which it is a simple dehydration product. Kreatin is a 

 normal constituent of muscle (0.2-0.3$), being, indeed, most 

 conveniently prepared from Liebig's extract; it is also known 

 that kreatin introduced into the Body is converted into 

 kreatinin; for if given in the food it causes an equivalent 

 increase of the kreatinin excreted in the urine. Kreatin 

 formed in the muscles has accordingly been supposed to be a 

 source of the kreatinin of the urine, but this does not appear 

 to be the case, as all kreatinin disappears from the urine 

 during starvation. The kreatinin of normal urine probably 

 has its source in the kreatin of flesh eaten as food. 



The Urinary Pigments are still very imperfectly known, 

 but appear in part to be derived from uro-bilin, which, as we 

 have seen (Chap. XXIV), is itself probably a derivative of 

 haemoglobin. 



Of the inorganic salts sodium chloride is by far the most 

 abundant, but the phosphates deserve notice because the 

 acidity of normal fresh urine is dependent on the presence of 

 acid sodium phosphate. 



In various diseases abnormal substances are found in the 

 urine: the more important are albumens in albuminuria or 

 Bright's disease; grape sugar or glucose in diabetes; bile 

 salts ; bile pigments. 



The Secretory Actions of Different Parts of an Urinif- 

 erous Tubule. The microscopic structure of the kidneys is 

 such as to suggest that in those organs we have to do with 

 two essentially distinct secretory apparatuses : one represented 

 by the glomeruli, with their capillaries separated only by a 

 single layer of flat epithelial cells from the cavity of the 

 capsule and especially adapted for filtration and dialysis; the 

 other represented by the contorted portions of the tubules, 

 with their large granular cells, which clearly have some more 

 active part to play than that of a mere passive transudation 

 membrane. And we find in the urine substances which like 

 the water and mineral salts may easily be accounted for by 

 mere physical processes, and others, urea especially, which are 

 present in such proportion as must be due to some active 

 physiological work of the kidney, whether a merely selective 

 activity of its cells or a constructive one. More direct evi- 

 dence does, in fact, justify us in saying that in general the 

 glomeruli are transudation organs, the contorted portions of 



