HORMONES 173 



ticed ; but sometimes it becomes enlarged — a condition 

 known as goitre — in which case the change in size of the 

 neck is readily perceived. The thyroid gland manufac- 

 tures a hormone which has been identified chemically 

 and has been given the name of thyroxin. The rate of 

 the basic metabohsm of the body cells is determined by 

 the amount of this substance in the blood stream. When 

 the thyroid departs from its normal production of 

 thyroxin, so that the blood contains too little or too much 

 of the hormone, the metabolism is affected one way or 

 the other and the individual in whom this happens is no 

 longer normal. A description of the bodily changes thus 

 caused is given in the next chapter, where the effects of 

 disturbances in correlation are taken up. 



Hormones Affecting Growth Metabolism. — Re- 

 ference was made at the beginning of the chapter to the 

 fact that growth is a kind of process which lends itself 

 particularly to chemical regulation. Our knowledge of 

 the various hormones which control growth is very in- 

 complete, but two illustrations will be cited to show 

 something of the method of action of the growth- 

 regulating agencies. 



The Hormones of the Pituitary Body. — In the 

 space between the roof of the mouth and the base of the 

 brain and behind the nasal passages is a hormone-manu- 

 facturing gland known as the pituitary body. This is a 

 complex structure and probably secretes several hor- 

 mones. We are interested here only in the one which is 

 known to influence growth. The manner in which this 

 particular influence is exerted is through an effect upon 

 the size of the skeleton. It is a fact that on the whole 

 the size of an animal is determined by the size of his 

 skeleton, the soft parts adapting themselves of necessity 

 to the growth of the bones. Giants and dwarfs have 

 always been more or less common, but the cause of over- 

 or under-growth has only recently come to light. It is 

 now believed that a considerable percentage of the giants 



