576 



METABOLISM, NUTRITION AND DIETETICS 



Living and Dead Proteins. Carried to the tissues, the decomposition 

 products of the food -proteins, or the regenerated proteins of the plasma, 

 which in ordinary language are still to be regarded as dead material, 

 are built up into the living protoplasm, at any rate to the extent neces- 

 sary to make good its waste. In this form they sojourn for a time 

 within the cells, and then they become dead material again. The 

 nature of this tremendous transformation has, of course, been the 

 subject of speculation, but the truth is that we do not understand 

 wherein the difference between a living and a dead cell, between a living 

 and a dead particle in one and the same cell, really consists. All we 

 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 moBe 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 polypeptide group intermediate in 

 complexity between amino-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 peculiar motions, 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 spermatozoon 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. 



