The Endocrine Glands - 403 



below the larynx (Fig. 22-3). The entire thy- normal. Conversely, an excess of thyroxin 

 roid weighs only about an ounce; and the elevates the metabolic rate considerably 



above normal levels. 



Many of the severe symptoms that ac- 

 company thyroid derangement probably rep- 



gland is purely endocrine in function. 

 -LARYNX 



ALVEOLI 

 CONTAINING COLLOID 



TRACHEA 



GROSS 

 VIEW 



Fig. 22-3. Thyroid gland: structure and position. 



MICROSCOPIC 

 VIEW 



The thyroid arises in all vertebrate em- 

 bryos as an outgrowth from the floor of the 

 pharynx. All vertebrates also possess para- 

 thyroid glands (p. 405), which arise from the 

 wall of the pharynx. In many species the 

 parathyroids lie some distance from the thy- 

 roid, but in man the parathyroids are four 

 small bean-shaped bodies, embedded in the 

 thyroid tissue. 



The importance of the thyroid was sus- 

 pected even in ancient times; but in 1885 

 replacement experiments gave definite proof 

 of a thyroid hormone; and in 1916 thyroxin 

 (C 15 H u 4 NI 4 ) was isolated for the first time. 

 Thyroxin successfully counteracts the effects 

 of thyroidectomy, although the hormone ap- 

 pears to be carried in the blood stream as a 

 thyroxin-protein compound. :i 



The primary effect of the thyroid hormone 

 is upon the rate of metabolism in the body 

 tissues. With inadequate thyroxin, the body 

 consumes less oxygen and produces less meta- 

 bolic wastes; and in extreme cases the basal 

 metabolism falls to less than 30 percent of 



3 Recently a series of compounds, closely related 

 to thyroxin, have been isolated from thyroid tissues; 

 among these, tri-iodothyronine (T 3 ) is about five 

 times more active than thyroxin. 



resent secondary effects, resulting from the 

 drastic metabolic changes in the various or- 

 gans. Also the net results are different, de- 

 pending upon whether the thyroid deficiency 

 (or excess) is experienced before or after the 

 animal reaches maturity. 



Hypothyroidism in young animals retards 

 development very profoundly (Fig. 22-4). In 





Fig. 22-4. Thyroidectomized cretin lamb about 14 

 months old and a normal sheep of the same age. 

 The thyroids had been removed from the cretin about 

 12 months previously. (After Sutherland Simpson. From 

 The Living Body, by Best and Taylor. Holt, Rinehart 

 and Winston, Inc.) 



tadpoles, for example, metamorphosis into 

 adult frogs does not occur if the thyroid is 

 destroyed at an early stage. This situation 

 has a parallel in the axolotl, a kind of sala- 

 mander which inhabits the highlands of 

 Mexico. This species possesses a hereditary 

 delect of the thyroid and does not develop 

 beyond the gill-bearing "tadpole stage." 

 However, if thyroxin is administered, the 

 axolotl completes development like other 

 salamanders. Soon it loses its gills and be- 

 comes a true land animal — a sort of "new 

 species," produced artificially (Fig. 22-5). 

 Overdosage experiments also give striking 



