374 GROWTH 



per day. A child of that age has an intake on the average of 

 about 500 cals. per day. That is, the energy used for growth 

 amounts to about 7 per cent, of the total energy intake. 



It has been calculated that each gram of infant's body sub- 

 stance has a value of 1-87 cals. Thus, if the child adds 20 grams 

 a day, it " fixes " 20 x 1-87 =37-4 calories per day, a result closely 

 approximating to that just given. 



Rubner has formulated the following law regarding the energy 

 of growth. 



Law of constant energy consumption. " The number of 

 calories required to double the weight of a new born animal of all 

 species (except man) is practically the same in spite of the 

 enormous differences in time taken in attaining the double weight." 

 Analysis has shown that each kilogram of body substance contains 

 about 113 grams of protein and 120 grams of fat having an energy 

 value of 1726 calories. Experiment has shown that in building 

 this up the animal uses about 4800 calories. Man is an ex- 

 ception and requires six times the amount. 



The growth quotient is the percentage of the energy intake 



which is " fixed " in the animal tissues. It is about 36 ( - - ) for 



\ 48 / 



all animals except man, who is able to " fix " only 6 per cent, of 

 his energy intake. 



The question now arises as to why growth stops. If it is an 

 autocatalytic reaction, growth should stop when the process is 

 balanced, i.e. when anabolism and catabolism mathematically 

 cancel one another. According to Loeb, this happens when the 

 organism has reached a certain size, or a certain number of cells 

 have been formed. Rubner's second law, that of length of life, 

 is suggestive rather of the cessation of positive growth after the 

 cell had expended a certain amount of energy. It is certain that 

 if growth is inhibited during a certain period, i.e. if energy which 

 would normally have been expended on building up tissue is not 

 used for this purpose, the whole " growing time " may be pro- 

 longed. 



The healing of a wound, the regeneration of tissues and the 

 growth of tumours, etc., bear a close resemblance to animal 

 growth as a whole. They may be modified by the same factors, 

 and investigation of the various processes involved has shed 

 considerable light on the mechanics of growth. 



Leo Loeb and his co-workers have made an extensive study of 

 the healing of a skin wound. They found that if the skin is 



