PHYSIOLOGY 



has been found that this thyroid gland manufac- 

 tures a complex material which it pours out into 

 the blood in very, very small quantities, but which 

 is absolutely indispensable not only to health but 

 to life itself. It is customary to call these chemical 

 messengers that minister to distant parts of the 

 body ' hormones ' (stimulators). Some of them 

 do not stimulate, some of them do the reverse, but 

 this particular hormone that the thyroid gland 

 makes is a stimulator, and is necessary for healthy 

 growth. It is known now that, by feeding them 

 upon thyroid, animals not accustomed to that diet 

 I mean creatures like tadpoles may be made to 

 grow at a greatly increased rate. The particular 

 material that is formed in the thyroid gland, which 

 has received the name of ' thyroxin,' contains a 

 small amount of an element which was supposed 

 in former days to be exclusively confined to the 

 sea-weeds, or at any rate to the plant world I 

 mean the element Iodine. This iodine-complex 

 is necessary to maintain health. If too much of it 

 is formed there is a certain kind of disease ; if too 

 little is formed there is the opposite disease, in 

 which the symptoms are the converse. 



To give you some example of how little there 

 is of this material, I may allude to some work which 

 has been recently carried out in America, and 

 which has resulted in the preparation of thyroxin 

 in a pure form and in sufficient quantities to enable 



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