456 



COMPARATIVE ANATOMY 



for acid dyes such as eosin. The colloid is evidently a secretion of the 

 cuboidal epithelium and is generally assumed to be a store of the hormone. 



Physiology. 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. 



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

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

 Many commercial flesh-reducing preparations, therefore, contain thyrox- 

 ine, often in dangerous quantity. It regulates also growth before birth, 



iLOOD VESSEL 



Fig. 379. — A portion of a section of the thyroid gland enlarged, showing the secretory- 

 epithelium and the colloid-filled follicles. 



through infancy, and at puberty, so that, in no small measure, 

 we are what we are in 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 basic metabolism above 

 normal, so that the patient loses weight. In man, exophthalmic goitre 

 is one of the common manifestations. 



As too much thyroxine speeds up the life processes, too little slows them 

 down. Basal metabolism is low, so that the tendency is to put on fat. 

 In the young, both growth and development are retarded; and if the 

 deficiency is very great, a child, unless given thyroid artificially, may 

 become an idiotic dwarf not unlike a cretin. 



Iodine is needed for the production of thyroxine, and in regions where 

 iodine is lacking in the food, as in the central states of America and in the 

 Alps, the glands may attempt to make up the deficiency by enlargement 

 and increased secretion. This results in the more usual form of goiter. 

 The remedy is to add potassium iodide to the diet. 



Development. In man, as in other Vertebrates, the thyroid gland 

 arises as a median ventral outgrowth from the floor of the pharynx at the 



