THE THYROID 193 



either side of the voice box or larynx, and usually connected by a 

 narrow strip of tissue. The entire structure weighs between one 

 and two ounces. The substance secreted by the thyroid is called 

 thyroxin and it contains about 65 per cent of iodine. It passes 

 directly into the blood since there is no duct to convey it from the 

 gland. Thyroxin influences the rate of oxidation in the body. 

 The method of testing this rate is by determining the person's basal 

 metabolism. Basal metabolism means the heat or energy expended 

 by the body when there is almost complete absence of absorption 

 from the digestive tract, and almost complete muscular and mental 

 repose. A normal person who has fasted for fifteen hours preced- 

 ing the test would use a definite amount of oxygen, depending on 

 his weight, height, and age. This amount has been determined, 

 and is a measure of normal basal metabolism. Doctors can tell 

 whether or not a thyroid gland in any person is overactive by 

 performing this basal metabolism test. A large amount of thyroid 

 secretion increases the metabolism in the body, which is noted by 

 an increase in the amount of oxygen used, as compared with the 

 amount used by a normal person. A decrease in thyroid secre- 

 tion results in a lower rate of basal metabolism; that is, less 

 oxygen is used than that usually required by a normal person. 



The location of the ductless glands in the rat. 



Thyroids have been removed from animals, and harmful effects 

 were observed. Other thyroids were grafted in another section 



