260 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



Fig. 15.4. Brook trout (Salveliniis fontinalis) with swollen gills and an external 

 goiter, a disease at one time prevalent among carnivorous fishes raised in hatcheries. 

 The disease was finally prevented by food and water containing iodine, largely 

 through the suggestions of Dr. David Marine (1910). This treatment has also 

 been applied with success to certain types of human goiter. (Courtesy, Marine 

 and Lenhart, J. Exp. Med. 12:311-335, 1910.) 



former broken into capillaries which surround the follicles that compose the 

 bulk of the gland (Fig. 15.5). The follicles are held together by loose con- 

 nective tissue. The wall of each one is formed by a single layer of epithelial 

 cells that produce the jellylike colloidal secretion. The activity of the thyroid 

 depends upon the diet, temperature, and conditions of special physiological 

 stress, and is primarily under the control of the pars anterior of the pituitary 

 gland. Sea food with its high content of iodine reduces thyroid activity, and 

 heavy meats, fats, and proteins increase it. It also responds to conditions of 

 the body such as activity of the reproductive organs and to climatic changes. 



The varying states of the thyroid, its diseases, and the results of experiments 

 all show its close relation to the general metabolism of the body. High activity 

 of the body, rapid oxidation, and quickened heartbeat all go with an over- 

 active thyroid. The secretion thyroxin (C15H11O4NI4) has been isolated and 

 synthesized. The adult human body contains a little less iodine than there 

 would be in ten drops of a medical solution of it. There is iodine in the skele- 

 ton, muscle, and liver, but the small thyroid gland itself contains about one- 

 fifth of the total iodine content of the body. 



Diseases of Deficient Thyroids. Too little thyroid secretion is due to injury 

 or underdevelopment of the gland, to some defect in pituitary control, to 

 accident or disease, and commonly to lack of iodine in the food. However, 

 iodine should not be taken without expert advice, nor should "iodized salt" 

 be put into general use. The latter has nearly disappeared from the markets. 



