502 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



taiK'ously. The quantity of available substrates or Nutrient -level of 

 the tissue-fluids is, however, not unlimited but adjusted, as we have 

 seen, by a dynamic equilibrium, to the average needs of the body as a 

 whole. In the competition for these materials, therefore, the most 

 specifically rapid syntheses will have a decided advantage over the 

 specifically slower syntheses, and when the nutrient-level sinks below 

 the normal, as in starvation, the more rapidly metabolizing tissues 

 will maintain their integrity for relatively prolonged periods at the 

 expense of the more slowly metabolizing tissues. 



If we now turn to the question of the origin of the varying metabolic 

 rate of different tissues, we can only infer that the rapidly metabolizing 

 tissues produce Endogenous Catalyzers of growth which are either more 

 efficient accelerators than those which are produced by other tissues 

 or else are produced in greater amount. We may thus clearly look to 

 the nervous system and the tissues of the heart as the origin of very 

 powerful or abundant catalyzers of growth. Since the majority of 

 catalyzers, and probably the growth-catalyzers also, 1 accelerate both 

 the forward and the backward reaction, both the anabolism and the 

 catabolism of such tissues are exceptionally rapid. 



Since the effect of starvation is to favor the rapidly metabolizing 

 tissues at the expense of those of slower metabolic rate the result must 

 be to increase the proportion of rapidly metabolizing tissues in an 

 animal and the production of growth-catalyzers per kilo of body-weight. 

 Corresponding with this fact Osborne and Mendel found that a period 

 of starvation greatly improves the subsequent utilization of foodstuffs, 

 so that in a growing rat the total growth attained in a period of starva- 

 tion followed by a period of feeding may exceed that attained by normal 

 animals in a like period of time. A second period of starvation even 

 enhances this effect. The same effect may often be noted in infants 

 as a result of a period of subnutrition or of a lowered nutritional level 

 due to the enhanced exogenous metabolism in fevers. 



From quite another avenue of experimental investigation the conclu- 

 sion may also be drawn that a period of starvation increases the pro- 

 portion of vigorously metabolizing tissues in the body. Embryonic 

 Tissues and rapidly growing tissues generally have been shown by many 

 observers, and particularly by Cramer, to contain a high proportion of 

 Water, while those which metabolize most slowly and suffer most in any 

 severe competition for nutrients contain a relatively low proportion 

 of water. The nervous system, for example, contains an exceptionally 

 high percentage of water. Now Aron has shown that a period of starva- 

 tion or subnutrition leads both in children and in animals to a greater 

 loss of nitrogen and calories than would normally be equivalent to the 

 loss of body-weight; in other words the tissues are becoming progres- 

 sively more dilute and of less calorific value. 



We are led again in this connection to recall the important observa- 



1 We may infer this from the symmetry of the curve of growth. 



