KATABOLISM. 429 



cles. In fact, the kidney is little else than a mass of 

 such tubes, lined with epithelium, webbed together with 

 connective tissue, and invested with peritoneal mem- 

 brane, and then the whole liberally supplied with nerves 

 and blood vessels. The epithelial surface thus produced 

 is enormously great. 



Function. — The function of the kidneys is apparent- 

 ly twofold — the one. physical, the other vital. The one is 

 a purely physical and therefore nonselective filtration, the 

 other is a selective excretion of a particular product of 

 katabolism — viz., urea. The one is probably through the 

 capillary tufts of the terminal vesicles, and apparently 

 not by the agency of the ordinary epithelial cells (cells 

 here are squamous epithelium) ; the other, as in the case 

 of all the secretions and excretions, is by the agency of 

 the living nucleated cells lining the tubes. These are 

 glandular epithelium. By the former are eliminated from 

 the blood superabundant water and salts of all kinds, 

 normal or accidental. By the latter is removed a char- 

 acteristic product of katabolism — urea. This is the spe- 

 cial characteristic function of the kidneys. 



Composition of Urine. — The secretion of the kid- 

 neys is a solution of urea (sometimes partly replaced by 

 uric acid) and of various salts — viz., sulphates, phos- 

 phates, chlorides and carbonates of potash, soda, and 

 lime. The most important parts are the urea (or its 

 equivalent, uric acid), together with the sulphur and 

 phosphorus of the sulphates and phosphates. The chlo- 

 rides and carbonates are constantly present in the brood, 

 and their excess eliminated by the kidneys. Urea and 

 to some extent also the sulphates and phosphates are 

 products of katabolism. The essential function is the 

 elimination of N and a little S and P. Of the elements of 

 albuminoids — viz., C, H, O, N, and S and P, as already 

 explained — the larger part of the C, H, and O are re- 



