METABOLISM 13 



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ing to a new pattern. A more recent view L 

 simpler. This is to the effect that it is the L 

 acids rather than the proteins of the plasma on \ 

 various tissues depend for their renewal. We ieft 

 somewhat in doubt as to the real value of the blood 

 proteins. 



During starvation it is known that some organs are 

 sustained while others are suffered to waste. Thus the 

 heart and the brain are preserved almost intact while the 

 spleen and the liver lose largely in weight. At such 

 times we may picture the proteins of the less essential 

 organs becoming resolved into amino-acids which can be 

 incorporated into those which must be protected. A 

 striking case is that of the female salmon when the time 

 for spawning is approaching. The muscles steadily 

 atrophy and there is no doubt that their substance is 

 made to contribute to the growing mass of roe. 



It used to be held that one protein must be equivalent 

 to another. The impression was a natural one so long as 

 attention was directed only to the percentage composition 

 of proteins from different sources. Their content of 

 oxygen, nitrogen, carbon, etc., varies but little. When 

 the significance of the " building stones" came to be 

 appreciated it became clear that certain proteins may 

 be much more serviceable than others for the general 

 work of nutrition. If they do not furnish all the con- 

 structive units called for they may be hopelessly inade- 

 quate. One example of an inadequate or defective 

 protein has long been known. This is gelatin from the 

 connective tissue of meat and from bone. 



Gelatin was found long ago to analyze like protein in 

 general. It seemed to carry the standard quantity of 

 nitrogen and the other elements. Yet it never could 

 replace other proteins in the diet of animals or men 

 without initiating a decline of weight and condition that 

 would continue to the end of the trial. We have learned 

 now that the difficulty with gelatin is the absence of 

 one or two amino-acids from the list of its cleavage- 



