2 THE PHYSIOLOGY OF PROTEIN METABOLISM 



the various steps, however, of the intermediate metabolism which lead 

 to the formation and the excretion of the different products we know 

 but little. 



Faced with the question as to the nature of the processes which 

 lead to the building up of the material commonly known as " tissue 

 protoplasm " we are at the very outset hampered and confined in our 

 quest for accurate information by the imperfect knowledge which exists 

 as to the very nature of the material formed. Can we with right 

 assume that such a substance actually exists as a constant chemical 

 entity, a substance immutable in form but variable in quantity ? Does 

 it wax and wane as does a crowd, the units constituting the whole 

 inconstant in number but identical in nature, or is it a material unstable 

 alike in form and amount? As Sir Michael Foster wrote in 1885 : 

 " He (the biologist) may speak of protoplasm as a complex substance 

 but he must strive to realize that what he means by that is a complex 

 whirl, an intricate dance, of which what he calls chemical composition, 

 histological structure, and gross configuration are, so to speak, the 

 figures ; to him the renewal of protoplasm is but the continuance of the 

 dance, its functions and actions the transferences of figures." Strive as 

 we may, our insight into this intricate problem of the nature of living 

 matter is but faulty. We see through a glass darkly. 



Despite this scanty knowledge questions of such primary import- 

 ance arise that their solution must be attempted. Such a question is 

 that of the real demand for protein by the body. Does the organism 

 require a large intake of protein or can it subsist on a relatively small 

 one? We know by direct observation that under normal conditions 

 the amount of nitrogen excreted is directly dependent on the amount 

 of protein taken in the food, that is, if a definite amount of nitrogen in 

 the form of protein be ingested a similar amount of nitrogen, in the 

 form of waste material, from which practically all the energy has been 

 extracted, will appear in the urine. What has happened to the nitro- 

 genous material which left the lumen of the intestine in the form of 

 "digest" products? Has it all been utilized for the necessary repair 

 of tissue waste or has part of the material absorbed been stored and a 

 corresponding amount of material present in the protoplasmic complex 

 been excreted ? Must this material taken up by the blood from the 

 intestine be first converted into " living protoplasm " before it can be 

 available for use in the tissues, or can disruption of the absorbed mole- 

 cule occur without this synthesis? It is now more or less generally 

 accepted, irrespective of the mode of such decomposition, that soon 

 after absorption the protein products are split into a nitrogen-containing 



