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TEXTBOOK OF ZOOLOGY 



of the trachea just below the larynx, or voice box, the two lobes 

 being connected by a saddle-shaped isthmus. The thyroid is well 

 supplied with blood, receiving in proportion to weight, three and 

 one-half times as much blood as the brain; this permits an easy 

 access of the hormone to all parts of the body. The thyroid is 

 normally not visible externally, but the pathogenic condition of 

 the organ caused by overgrowth, and known as goiter, is familiar 

 to everyone. 



A. B. C. 



Fig. 362. — Illustrating- cretinism. A, a cretin, 23 months old and B, the same 

 child after having received thyroid treatment for eleven months. C, an untreated 

 cretin 15 years old. (From Zoethout, Textbook of Physiology, published by The 

 C. V. Mosby Company, after Osier.) 



The functions of the thyroid gland, as recognized today, are two : 

 that of control, in conjunction with other endocrine organs, of the 

 growth and development of the body; and, second, a most important 

 role in the regulation of metabolism. Any upset of the normal 

 functioning of the gland or the removal of the gland results in 

 serious physical and mental disorders, if not in death itself. The 

 hormone produced by this organ, designated thyroxine, has been 

 prepared from the fresh thyroid glands removed from various ani- 

 mals and has also been manufactured synthetically. Chemical diag- 



