NORMAL AND PATHOLOGICAL GROWTH 151 



tent are taken into consideration in the establishment of average 

 growths. 



It is only by the extraordinary action of the hormones in in- 

 dividual cases in which variations from the normal type occur 

 that we secure the knowledge from which we are able to deduce 

 the function of each individual hormone. It is easy to under- 

 stand that the study of abnormalities of growth is largely the 

 study of individuals of extreme variation from type. 



In mammals and the higher vertebrates the hormonal mecha- 

 nisms for the control of growth processes have evolved much 

 further than the secondary and casual impress of the chemical 

 waste products of the processes in other organs or parts. Hor- 

 mones may, therefore, be defined as particular chemical sub- 

 stances produced in certain tissues of man and animals which 

 have become differentiated in such a way as to exert a degree of 

 chemical control of other organs through an adaptation of the 

 one to the other. Hormones occur in minute amounts only. The 

 origin and significance of the more important hormones will be 

 briefly discussed. 



//. The Hormones of the Thyroid 



Everyone is familiar with the gland of the neck known as 

 the thyroid, and its excessive overgrowth called goiter. Certain 

 regions of the United States are known as goiter territory be- 

 cause of the high prevalence of disturbed development from 

 abnormal functioning of this gland. 



There are two types of thyroid disturbances which produce 

 a profound influence on growth. These are known physiologi- 

 cally as hyper- and hypothyroidism and follow in train with 

 corresponding changes in function of the thyroid gland. The 

 hyperthyroid individual is characterized by spareness of body, 

 underweight, by excessive muscular energy, by aggressiveness, 

 by nervousness, and by extreme irritability. If we physiologi- 

 cally analyze further we find a high heart rate, excessive blood 

 pressure, overactive motility of the alimentary canal, and, most 



