150 GROWTH 



physico-chemical basis. (7) Every organ or special tissue, by 

 virtue of its physiological activity, produces chemical by-prod- 

 ucts which, when distributed throughout the body of an animal 

 or plant, exert a definite influence on the rate of growth, de- 

 velopment, and activity of all other parts. (8) These inter- 

 organ reactions are infinite in variety, but are broadly of two 

 classes, namely, those which stimulate or augment reactions and 

 those that inhibit reactions. Not only the growth in mass but 

 the growth and development of activity in the parts of the ani- 

 mal are conditioned by the physiological factors' expressed in 

 this interaction of the parts of an animal or plants. (9) Ex- 

 treme over- or underproduction of special substances elaborated 

 by the organs of the body itself may stimulate or retard growth 

 to such an extent as to produce extreme deviation from the aver- 

 age normal, i.e., abnormal body form, undersized pigmies, and 

 overgrowth or giants. 



/. The Hormones 



In mammals and the higher vertebrates the chief chemicals 

 that control the growth processes of rate, volume, and the 

 growth time cycle to so great a degree have developed into 

 special substances produced by special organs within the differ- 

 entiating body itself. These substances as a class received the 

 name of "hormones" by Starling 4 in 19 12. They are present 

 only in minute amounts and exert influence only after distri- 

 bution through the circulatory system to all growing structures. 

 It is this class of substance such as that produced by the pan- 

 creas in the illustration already used, that we wish especially to 

 emphasize. 



In discussing the influence of hormones one must recognize 

 that the usual studies of growth and the growth curves assume 

 average standard conditions. In other words, the human growth 

 curve shown by Professor Brody is a curve of the standard aver- 

 age growth of the human as a type. All the factors that I am 

 emphasizing are theoretically acting normally, and to that ex- 



