210 



GENERAL ZOOLOGY 



below the larynx (Fig. 132). Histologically the thyroid gland is 

 composed of a large number of follicles of a more or less spherical 

 shape and filled with a viscous secretion known as colloid, in 

 which the thyroid hormone is dissolved (Fig. 133). The thyroid 

 gland has a very rich supply of blood vessels. 



In the frog, removal of the thyroid glands of the tadpole pre- 

 vents metamorphosis. In mammals, removal of the thyroid 



of the young results in a lowered 

 rate of metabolism and a re- 

 tarded physical, mental, and 

 sexual development. When 



Fig. 132. — Diagrammatic view of Fig. 133. — Section of human thy- 



human thyroid gland in position against roid gland. B. blood vessels; f, folli- 

 anterior end of trachea. cles filled with colloid. (After Stohr.) 



thyroidectomy is practiced on adult mammals, the main effect 

 is lowered metabolic rate, manifested by a lowered heat produc- 

 tion. Impaired functions and retarded growth can be restored 

 to normal by implanting or feeding thyroid tissue. 



The active principle of the internal secretion of the thyroid 

 gland is thyroxine, whose formula is C15H11O4NI4. This com- 

 pound has been synthesized and the synthetic substance has the 

 same physiological action as natural thyroxine. Thyroxine fed 

 to tadpoles results in a rapid loss of weight and the completion of 

 metamorphosis in from four to five days. Thyroxine may be 

 used effectively as a substitute for thyroid gland in cases of 

 thyroid deficiency, natural or experimental. 



