EXCRETION AND THE EXCRETORY ORGANS 471 



of creatinin is constant, regardless of the extent to which the 

 muscles are used, unless they are used to excess, it is believed that 

 muscle-cells, and perhaps other cells as well, live at a rate which 

 varies scarcely at all from day to day, and is independent of their 

 functional activity. The interesting observation that the amount 

 of creatinin excreted is roughly proportional to the bulk of the 

 muscle tissues may be taken to indicate that all muscle-cells live 

 at about the same rate, the temperamental differences noted in 

 different individuals not involving differences in the metabolic 

 activities of their muscle tissues. 



The Purin Bodies, of which uric acid is the best known, are 

 other endogenous excreta found in urine. They show chemical 

 characteristics which indicate that they represent probably the 

 end products of the metabolism of cell nuclei. Caffein, the active 

 principle of coffee and tea, and theobromin, the active principle 

 of cocoa, are very closely related chemically to the purin bodies 

 excreted from the kidney. 



Since all the endogenous excreta are produced in the living 

 tissues they occur in the flesh of animals eaten for food. In fact 

 the flavor of meat is largely the result of their presence. When 

 eaten with meat they are, of course, absorbed into the blood from 

 the intestine and become part of the exogenous excreta. For 

 this reason it is often necessary, when studying metabolism ex- 

 perimentally, to exclude meat from the diet, so that the endoge- 

 nous excreta may be obtained pure. 



The Urinary Salts are chiefly sodium chlorid, and the sulphates 

 and acid phosphates of sodium, potassium, calcium, and magne- 

 sium. Whatever salt is taken with the food, unless stored perma- 

 nently in the Body, as in bone formation, finally is excreted by the 

 kidneys. The acid phosphates of sodium and potassium are in 

 part responsible for the acid reaction of urine. 



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 a Uriniferous Tubule. 

 The miscroscopic 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 



