Integrative Systems 



221 



' 'JJ*A'i _ CONNECTI VE 

 VX//'T ISSUE 



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$% BLOOD VESSEL 



FOLLI CLE 



.CU BO I DAL 

 EPITHELIUM 



Fig. 188. A portion of a section of the thyroid gland, enlarged, showing the 

 secretory epithelium and the colloid-filled follicles. (Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



lies ventral to the trachea and just posterior to the larynx. It is usually 

 a single median body but in some cases, especially among amphibians 

 and mammals, it becomes more or less definitely bilobed. 



The thyroid is formed of numerous masses of glandular tissue, 

 separated from one another by connective tissue (Fig, 188). The glan- 

 dular tissue consists of rounded follicles, each enclosed by a single 

 layer of cuboidal epithelium. Loose connective tissue filled with blood- 

 vessels and lymphatics binds the follicles together. Each follicle is 

 filled with a jelly-like colloid material which has a strong affinity for 

 acid dyes such as eosin. 



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

 the thyroid gland is an iodine-containing compound, thyroxin, whose 

 empiric chemical formula is C15H11O4NI4. A substance with identical 

 properties has been made synthetically, and it is generally the synthetic 

 drug which is used in medical practice. 



One function of thyroxin is to control metabolism, especially that 

 of the carbohydrates. Also, it regulates growth before birth, through 

 infancy, and at puberty. Tadpoles fed with thyroid may become frogs 

 prematurely, but of abnormally small size. Excessive activity of the 

 human gland is a common malady, manifested by extreme nervous- 

 ness, rapid pulse, insomnia, and basal metabolism above normal, so 

 that the patient loses weight. As too much thyroxin speeds up the 



