488 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



the above data afford a very remarkable demonstration of the correct- 

 ness of the view that growth is determined by an underlying auto- 

 catalyzed chemical process. It is furthermore clear that the form of 

 the curve of growth in normal infants is determined by two separate 

 groups of factors. The one, analogous to the absolute mass of the 

 reacting substances in a chemical reaction, being dependent upon the 

 environment and probably largely influenced by the abundance or 

 deficiency of the habitual dietary; while the other, analogous to the 

 specific velocity of a chemical reaction, is relatively, if not absolutely, 

 independent of environmental or nutritional conditions, and, being 

 expressive of the nature of the growth-process itself as distinguished 

 from the availability of the materials for growth, is distinctively modi- 

 fied by race and sex. 



THE SUBSTRATES OF GROWTH. 



The substrates of growth, i. e., the material out of which living tissues 

 are synthesized, are the Foodstuffs, namely oxygen, water, inorganic 

 salts, carbohydrates, fats and proteins. In the period of biochemical 

 research which immediately followed the fundamental discoveries of 

 Liebig and Voit, the application of the laws of the conservation of 

 matter and energy to the phenomena of growth and metabolism 

 appeared to supply all of the necessary clues for the interpretation of 

 the relationship of the foodstuffs to the maintenance of life. But with 

 the increasing refinement of our knowledge of the intimate chemical 

 structure of the foodstuffs themselves it has become increasingly appar- 

 ent to us during the recent decades that it is not sufficient merely to 

 supply an animal or a human being with a sufficiency of nitrogen, 

 carbon and calories to replace his daily waste in order to maintain the 

 equilibrium between waste and repair in his tissues, nor is it even 

 sufficient to supply these desiderata in digestible and assimilable form; 

 it is furthermore necessary to supply irreducible minima of specified 

 atomic groupings or complexes of nitrogen, carbon, hydrogen and so 

 forth which, it appears, are essential constituents of living matter, 

 and yet are not synthesizable by animal tissues. Thus the Pyrrole 

 grouping, for example (see Chapter XV), which is an essential building- 

 stone of Hemoglobin, would appear to be as much an elementary 

 requirement of animals as nitrogen or carbon itself, inasmuch as, 

 according to Abderhalden, they are unable to synthesize it from other 

 carbon or nitrogen complexes in the diet and, lacking it, are just as 

 assuredly suffering starvation as if they were lacking one of the more 

 elementary desiderata. 



The variety of these essential constituents of the diet with which we 

 are acquainted is already very great and is unquestionably destined to 

 grow with increasing scope and refinement of investigation. It is 

 highly probable that many of the raw materials from which the various 



