METABOLISM OF PROTEINS 



567 



know is that now and again a protein molecule or an aggregate of such 

 molecules incorporated in the colloid mass which constitutes the proto- 

 plasm of a muscle-fibre, or a gland-cell, or a nerve-cell, must fall to 

 pieces. Now and again a molecule of protein, hitherto dead (or perhaps, 

 to speak moce correctly, hitherto not a constituent of living protoplasm, 

 since protoplasm is certainly more than protein), or a molecule of a 

 particular amino-acid, or perhaps a polype plide group intermediate in 

 complexity between a.mino-acid and protein, coming within the grasp 

 of the molecular forces or chemical affinities of the living substance, is 

 caught up by it, takes on its peculiaynotions, acquires its special powers, 

 and is, as we phrase it, made alive. [Each cell has the power of selecting 

 and, if necessary, further decomposing or further synthesizing the 

 protein materials offered to it; so that a particle of serum-albumin or a 

 mixture of amino-acids may chance to take its place in a liver-cell and 

 help to form bile, while an exactly similar particle or mixture may 

 furnish constituents to an endothelial scale of a capillary and assist in 

 forming lymph, or to a muscular fibre of the heart and help to drive on 

 the blood, or to a spermatozooji and aid in transferring the peculiarities 

 of the father to the offspring/ And just as a tomb and a lighthouse, a 

 palace and a church, may be, and have been, built with the same kind 

 of material, or even in succession with the very same stones, so every 

 organ builds up its own characteristic structure from the common 

 quarry of the blood. 



It is not any difference in the kind of protein offered them which 

 determines the difference in structure and action between one organ 

 and another. In this quarry alongside of the plasma proteins the tissue 

 cells find what is probably more important for their individual nutrition, 

 the building-stones of the shattered food-protein molecules. They are 

 only under exceptional circumstances confronted with intact molecules 

 of food -proteins. ' The body cells do not know what the kind of food 

 was ' (Abderhalden). In the case of the more highly developed tissues, 

 at least, no mere change of food will radically alter the structure of the 

 cells, nor even, as we have seen, the composition of the tissue proteins. 

 A cell may be fed with different kinds of food, it may be overfed, it may 



