442 A MANUAL OF PHYSIOLOGY 



(5) In acute yellow atrophy, and in extensive fatty de- 

 generation of the liver, urea may almost disappear from the 

 urine, and leucin and tyrosin may appear in it along with a 

 much larger amount of ammonia than normal. The leucin 

 and tyrosin are formed in the intestine, and, passing 

 unchanged through the degenerated liver, are excreted by 

 the kidney. 



If it be granted, as in the face of the evidence it must, that the 

 liver plays an important part in the formation of urea, we have still 

 to ask what the materials are upon which it works, and in what 

 organs they are formed before being brought to the liver. To the 

 latter question it may be at once replied that proteid metabolism, 

 although its final stages may be .worked out in the hepatic cells, 

 must go on in all the organized elements of every tissue. The living 

 substance everywhere contains proteid ; proteid is everywhere and 

 at all times breaking down. In the muscles especially nitrogenous 

 substances on the road to urea must be constantly produced. Can 

 we lay our finger on any such intermediate substances ? Can we 

 with certainty state that any of the separate links of the chain of 

 proteid metabolism, except the first and the last, have actually been 

 discovered, identified, and labelled ? The answer is that a whole 

 series of bodies containing nitrogen, simpler than proteids and with 

 a greater proportion of oxygen, more complex and less oxidized than 

 urea, has been found in muscle and other tissues ; but we cannot 

 say definitely that any or all of them, although they are undoubtedly 

 stages in the downward course of worn-out proteids, have arisen the 

 one from the other, or must necessarily pass into the form of urea 

 before being finally excreted. 



Such substances are : 



Guanin, C 5 H 5 N 5 O ... ... ... In the pancreas, liver, blood, 



and muscles. 



Sarkin, or hypoxanthin, C 5 H 4 N 4 O ... ( In spleen, liver, muscles, 



thymus, bone-marrow, and 



Xanthin, C 5 H 4 N 4 O 2 ... 



Uric acid, C 5 H 4 N 4 O 3 



urine. 



In spleen, liver, muscles, 

 brain, pancreas, thymus, 

 and in the urine. 



In liver, spleen, lungs, pan- 



creas, brain, and in urine. 

 Kreatin, C 4 H 9 N 3 O 2 ... ... ... In muscles, blood, brain. 



The increase in the proportion of oxygen from guanin to uric acid 

 is very striking, and particularly the regular series formed by hypo- 

 xanthin, xanthin and uric acid. But while there is good evidence 

 that the first three bodies in the list can be converted into uric acid, 

 there is no reason to believe that they are stages on the way to urea. 

 Kreatin exists in the body in greater amount than any of these, 



