CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 599 



though it deserves to be borne in mind, is too obscure to be 

 dwelt on here. 



391. We may now briefly sum up the varied discussions 

 which have occupied us in the present section. 



Urea is the main end-product of proteid metabolism. Un- 

 like hippuric acid and some other constituents of urine, urea is 

 simply 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 pro- 

 teid 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 possi- 

 bly 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 substances of food or of the body, or whether 

 it comes into play in the normal metabolism of proteid mate- 

 rial. 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 ap- 

 pearance 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 vari- 

 ous parts of the body and even in the urine, in small quantities, 

 of so many varied nitrogenous 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 remarka- 

 bly rich in these extractives. 



