330 



CHORDATE ANATOMY 



The most important, if not the exclusive, endocrinal secretion of the 

 thyroid gland is an iodine compound, thyroxine, the empirical chemical 

 formula for which is C15H11O4NI4. A substance with identical properties 

 has been made synthetically, and it is generally the synthetic drug which 

 is used in medical practice. 





Fig. 293. 



. CONNECTIVE 



VjO .''''TISSUE 



COLLOID 



'•Jj BLOOD VESSEL 



FOLLI CLE 



.CUBOIDAL 

 EPITHELIUM 



-A portion of a section of the thyroid gland, enlarged, showing the secretory 

 epithelium and the colloid-filled follicles. 



Fig. 294. — Longitudinal section of head of 19-day Petromyzon embryo showing the 

 relation of the thyroid anlage to the floor of the pharynx, ch, optic chiasma; ep, 

 epiphysial outgrowth; h, hypophysial ingrowth; mcs, mesenteron; n, nasal epithelium; 

 nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord; th, thyroid. 

 (From Kingsley's "Comparative Anatomy of Vertebrates.") 



One function of thyroxine is to control metabolism, especially of the 

 carbohydrates, a thirtieth of a grain increasing oxidation by one percent. 

 It regulates also growth before birth, through infancy, and at puberty, so 

 that, in no small measure, we are what we are by virtue of our thyroid 

 glands. Tadpoles fed with thyroid may become frogs no larger than flies. 

 Excessive activity of the gland is a common malady, manifested by 

 extreme nervousness, rapid pulse, insomnia, and basal metabolism above 



