202 THE HUMAN BODY 



thought, when first discovered, to be the hormone of the gland, 

 but fuller study showed that iodothyrin as such is not the hormone 

 although it probably has to do in some way with it. Although 

 the chemistry of the hormone is not perfectly known its physiology 

 can be studied indirectly by observing the effect of changes in the 

 amount present in the Body. These changes may be brought 

 about experimentally or may occur as the result of disease. 



Studies thus made show that the hormone of the thyroid gland 

 has a great deal to do with the proper carrying on of those chemical 

 activities of living cells which constitute their " vital" processes and 

 which are grouped together under the term metabolism. The 

 nervous system is peculiarly dependent upon this hormone for its 

 proper development and for the proper carrying on of its metabolic 

 activities. This fact appears strikingly in cases in which the 

 hormone is deficient in amount. In adults a condition known as 

 myxedema is the result of such deficiency; its chief manifestation is 

 distressing mental deterioration. Sometimes children are born in 

 whom the thyroid gland fails to develop properly; they grow into 

 dwarfish, misshapen idiots. To such a condition the name cretinism 

 is applied. The sufferers are called cretins. Thanks to the dis- 

 covery that by simple feeding of thyroid material the hormone can 

 be supplied in ample quantity, sufferers from myxedema and 

 cretinism are now restored to perfectly normal condition; although 

 it is said that for the treatment to be wholly successful for cretins 

 it must be begun quite early in life. 



There is a disease known as exophthalmic goiter (Grave's dis- 

 ease), named from the protrusion of the eyes which is a prominent 

 symptom. This disease is due to an increase in the amount of the 

 thyroid hormone. The effects on the Body are just the opposite of 

 those seen in myxedema. There is heightened nervous activity, 

 often proceeding so far beyond the normal as to constitute mental 

 instability. One of the triumphs of modern surgery is the establish- 

 ment of a method whereby enough of the thyroid can be removed 

 to reduce the hormone to normal amount, and so cure the com- 

 plaint. In this connection only the effects of the hormone on the 

 nervous system are discussed. In a later chapter (p. 513) its in- 

 fluence on general metabolism is considered. 



Emergency Action of the Thyroid. An interesting fact of re- 

 cent discovery is that during the outpouring of autonomic in- 



