760 NITROGENOUS METABOLISM. [BOOK n. 



excreted by the kidneys, being brought to them in the blood, 

 they apparently, beyond the simple act of excretion, doing no more 

 than merely contributing to the stock of urea in so far as they 

 are masses of proteid material undergoing proteid metabolism as 

 part of their general life. What are the immediate antecedents 

 of urea we do not clearly know ; but it is probable that they 

 are not one but several and indeed possibly many. We have 

 reason to think that urea may be formed out of amides or 

 amido-acids, or out of ammonia itself by a synthetic process ; and 

 we have indications that this synthesis is effected in the liver 

 by the agency of the hepatic cells. But we do not know whether 

 this synthesis bears only on particular nitrogen-holding sub- 

 stances of food or of the body, or whether it comes into play in 

 the normal metabolism of proteid material. If the kreatin 

 which is so conspicuous a constituent of muscular and nervous 

 structures is a stage in the direct line to urea, then the synthesis 

 would affect only the sarcosin which the kreatin in becoming 

 urea sets free. But we have seen that it is by no means clear 

 that kreatin is such a stage. 



The evidence as far as it goes tends to shew that the meta- 

 bolism of proteid is very complex and varied, that a large number 

 of nitrogen-holding substances make a momentary appearance in 

 the body, taking origin at this or that step in the downward stairs 

 of katabolic metabolism and changing into something else at the 

 next step, and that the presence in various parts of the body and 

 even in the urine, in small quantities, of so many varied nitro- 

 genous crystalline substances, forming a large part of what are 

 known as extractives, has to do with this varied metabolism. 

 Possibly the transformations by which nitrogen thus passes 

 downwards take place to a certain extent in such organs as the 

 liver and the spleen which are remarkably rich in these extrac- 

 tives. But the whole story of proteid metabolism consists at 

 present mostly of guesses and of gaps. 



