CH. xxxix.] ROLE OF AMINO-ACIDS 59? 



economy, compares it to the part played by -the salts of the food. 

 Neither salts nor extractives simply pass into the urine without ful- 

 filling a useful purpose on their way ; but the exact and specific use 

 of each, whether on the synthetic or analytic side of metabolic pheno- 

 mena, must be the subject of renewed research. (Eead again in this 

 connection the last paragraph on p. 520, which describes certain 

 recent researches that shew that leucine and similar simple 

 substances may be actually synthesised into protoplasm. If this 

 is ultimately found to be correct, the opinions expressed in the 

 first part of the present paragraph will need modification.) 



