A HOSPITAL 



METABOLISM, NUTRITION AND DIETETICS 499 



Living and Dead Proteins. Carried to the tissues, the decom- 

 position 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 necessary 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 transforma- 

 tion 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 

 living protein molecule in the whirl of flying atoms which we call 

 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 a molecule 

 of a particular amino-acid, or perhaps a polypeptid group inter- 

 mediate in complexity between the simple amino-acid and the pro- 

 tein, coming within the grasp of the molecular forces 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 sperma- 

 tozoon 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 the case of the more highly developed 

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

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

 it may be ill-fed, it may be starved ; but its essential peculiarities 

 remain as long as it continues to live. Its organization dominates 

 its nutrition and function. 



The speculation of Pfluger, that the nitrogen of living protein 

 exists in the form of cyanogen radicals, whilst in dead protein it is in 

 the form of amides, and that the cause of the characteristic instability 

 of the living substance its prodigious power of dissociation and 

 reconstruction is the great intramolecular movement of the atoms 

 of the cyanogen radicals, is interesting and ingenious, but it remains, 

 and is likely to remain, a speculation. And the same is true of the 

 suggestion of Loew and Bokorny, that the endowments of living 

 protoplasm depend on the presence of the unstable aldehyde group 

 H-C=O. Nor do the known differences of chemical composition 

 in dead organs give any insight into the peculiarities of organization 

 and function which mark off one living tissue from another. In 

 any case, the living protein molecule, whatever function it may 

 have been fulfilling in the organized elements of the body, has 

 certainly a much greater tendency to fall to pieces than the dead 

 protein molecule. And it falls to pieces in a fairly definite way, 



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