434 THE HUMAN BODY. 



it must be carried off somehow. The urine, however, con- 

 tains very little kreatin, or its immediate derivative, krea- 

 tinin, and what it does contain depends mainly on the 

 feeding, since it varies with the diet and vanishes during 

 starvation; so it is probable that this substance is con- 

 verted into urea and excreted in that form. This conver- 

 sion must occur elsewhere than' in the muscles, which con- 

 tain no urea; also, very little, if any, exists in the brain. 



Where the kreatin is finally changed into urea is doubt- 

 ful. It may be in the kidneys by the renal epithelium, 

 or it may be elsewhere, and the urea produced be merely 

 picked up from the blood and passed out by the kidney- 

 cells; or both may occur; histologically the distinctly 

 secretory epitheliums of the convoluted parts of the tubules 

 and of Henle's loops, differ so much as to suggest an en- 

 tirely different function for them. 



On the whole, the evidence seems to show that urea i& 

 merely separated and not produced in the kidneys; a priori 

 this is more probable, since in the degradation of kreatin to 

 yield urea energy is liberated and this might very well be 

 utilized in some organ; while if the process took place in 

 the kidney tubules the force set free would be wasted. 

 The blood always contains urea, and renal-artery blood 

 apparently more than renal-vein blood, which shows that, 

 urea is removed from the blood in the kidneys. Moreover, 

 if a mammal's kidneys be extirpated urea accumulates in 

 its blood, which could not be the case if urea were nor- 

 mally produced only in the kidneys; and if urea be injected 

 into a vein it is rapidly picked up and carried off in the 

 urine, showing that the kidney-cells have a selective power 

 with respect to it. 



While the urea resulting from, further changes in the 

 kreatin formed in the tissues is a measure of the wear 

 and tear of their protoplasm, part of the urea excreted 

 has probably a different source; being due to the oxidation 

 of proteids, as energy liberators or respiratory foods, before 

 they have ever formed a tissue. When plenty of proteid 

 food is taken the urea excretion is largely increased and 

 that very rapidly, within a couple of hours for example,. 



