476 GROWTH 



Of their chemistry a Httle is known^ — of their physics or of the 

 mechanism of their action nothing positive can be said (Chap. 

 XIIT.). 



(i) Social position. The children of the well-to-do are generally 

 markedly heavier and slightly taller than those of the working 

 classes. Quite apart from any possible underfeeding of the 

 latter, one must take into account the more or less sheltered life 

 of the former and their freedom from those influences which tend 

 to put the burdens and responsibilities of the adult on the child 

 of the lower classes at a comparatively early age. 



(j) Endocrinetes. Considering the factors which influence the 

 rate of growth, and keeping well in mind the imthinkable com- 

 plexity of the polyphase solution of colloids and crystalloids 

 composing the animal body, one can hardly be surprised that so 

 little is known of the mechanism of growth. In some way, the 

 various alterations in size and shape are interrelated and regulated 

 through the blood and through the nervous system by various 

 secretions from endocrine organs. Growth in length is associated 

 with the secretion of the pituitary gland. Any alteration in this 

 gland causes alteration in the performance of other endocrine 

 organs, e.g. the gonads and thyreoid. The growth of cartilage and 

 bone are profoundly modified by alterations in the output of the 

 thyreoid gland, while the gonads, and in early life, the thymus, 

 control both growth and development, again by processes of which 

 the mechanism, from a physico-chemical standpoint, is quite 

 unknown. 



rv. The Energy of Growth. 



It is generally believed that young animals require much food 

 because they are growing. That this is not quite correct has been 

 shown in Chap. XXXII., where we saw that the young animal, 

 because of its large surface compared to its mass, lost heat most 

 rapidly. To prove this we need only examine the metabolic 

 balance sheet of the child. Camerer gives the following com- 

 position of a new-born child (in grams) : 



Total weight. Protein. Fat. 



2,820 320 348 



Now, as we have seen, in 180 days the child doubles its weight. 

 It does not, however, do so by adding equal quantities of the 

 material already present. In a child 180 days old, weighing 

 5,600 grams, there are 790 grams of protein and 733 grams of fat. 

 Thus 470 grams of protein and 385 grams of fat are added. In 

 Calories that gives 470 X 5-3 = 2,491 Calories + 385 X 9-3 = 

 3,580-5 Calories = a total of 6,071-5 Calories for the period, or about 



