272 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



to name only a few, is exerted by the growth and adrcnocorticotrophic 

 hormones. 



The hormones of the endocrine glands (gonads, thyroid, and adrenal cor- 

 tices), whose production is largely controlled by hormones from the pars 

 anterior of the pituitary, also exercise some control on the production of 

 these controlling hormones by the pars anterior. For example, the thyrotrophic 

 hormone (TSH) of the pars anterior stimulates the thyroid to produce 

 thyroxin and release it into the blood stream; in turn a high level of thyroxin 

 hormone (TH) in the blood causes the pars anterior to reduce its production 

 of thyrotrophin (TSH), while a low level causes it to increase its production. 

 This see-saw relation also occurs between the gonadotrophic hormones se- 

 creted by the pars anterior (FSH and LH or ICSH) and the sex hormones 

 secreted by the gonads (estrogen and progesterone of ovaries and testosterone 

 of testes) and between the adrcnocorticotrophic hormone (ACTH) of the 

 pars anterior and the cortical hormones of the adrenal cortices. 



After removal of the pituitary (hypophysectomy) of young animals, the 

 skeleton stops growing, the sex organs do not develop, and the thyroid and 

 cortices of the adrenal glands gradually shrink. However, if a fragment of 

 pars anterior is then transplanted daily into these animals, they will resume 

 growing, the sex organs will develop, and the thyroid and adrenal cortices 

 become normal. Extracts of pars anterior have been prepared which will 

 correct one or another defect caused by hypophysectomy; some of these 

 extracts also cause specific effects upon normal animals. Young rats and 

 puppies thus treated will grow to almost double the size of others in the same 

 litter (Fig. 15.15). Even immature mice or rats implanted with fresh pars 

 anterior or injected with gonad-stimulating hormones at weaning time will 

 become sexually mature in three to five days. 



Giants, Acromegalics, and Dwarfs. Giants and acromegalics have over- 

 active pituitaries (pars anterior) (Fig. 15.16), Giantism begins in very early 

 childhood, acromegaly in adult life. In acromegaly the nose and lower jaw 

 become abnormally prominent and the forehead and the skin thickened. Indi- 

 viduals dwarfed by underactivity of the pituitary are of two types; one kind 

 has a body like a normal child's (Fig. 15.17), the type of dwarf usually seen 

 on the stage and in circuses; the other has a short, heavy body overlaid with 

 fat. 



Function of the Pars Intermedia. In man the function of the pars inter- 

 media is not known. In frogs, toads, lizards, and some fishes it produces a 

 hormone, intermedin, which disperses the pigment in melanophores, the 

 ameba-shaped cells which contain black pigment (Fig. 15.18). Tadpoles from 

 which the pituitary has been removed are very pale but regularly darken 

 when the pars intermedia of normal tadpoles is implanted into them. By an 

 operation on embryos of the small spring-peepers {Hyla crucijer) the pars 



