THE ENDOCRINE SYSTEM 627 



is not indispensable for ACTH release; the latter can occur normally 

 after removal of the adrenal medulla. 



The hypothalamus provides a third, and very important, control of 

 pituitary fvmction. It is cvirrently believed that axons from certain 

 centers in the hypothalamus end in the median eminence (Fig. 30.7). 

 The tips of these axons secrete some neurohumor which is carried by 

 the portal veins to the hypophysis, where it stimulates the release of 

 ACTH. Evidence of ACTH secretion is obtained when the median 

 eminence is stimulated electrically, but not when the stimulus is ap- 

 plied to the supraoptic nuclei whose axons pass to the posterior lobe 

 of the pituitary. The electrical stimulus is ineffective if the blood ves- 

 sels between the hypothalamus and pituitary are cut. If the nerve 

 fibers to the median eminence are destroyed, ACTH is no longer re- 

 leased in response to stresses. Some investigators believe that the re- 

 lease of other pituitary hormones— growth hormone, thyrotropin and 

 the gonadotropins— is also under hypothalamic control. 



All the living vertebrates have pituitaries which are basically 

 similar, and they all appear to secrete the same battery of hormones. 

 The intermediate lobes of birds and mammals secrete intermedin, al- 

 though these forms have no chromatophores; birds secrete luteinizing 

 hormone but have no corpora lutea; and all vertebrates secrete prolactin, 

 but only mannnals have its target organ, the mammary glands. 



254. The Testis 



In between the seminiferous tubules of the testes are hormone- 

 secreting cells, the interstitial cells of Leydig. Although Berthold con- 

 cluded in 1849 that the testis produces a blood-borne substance needed 

 for the development of male sex characters, no effective testicular ex- 

 tract was prepared until 1927. Extracts of human urine with androgenic 

 activity were made in 1929, and by 1934 two hormones, androsterone 

 and dehydroepiandrosterone, had been isolated from urine and identi- 

 fied. A new androgen, testosterone, six times more potent than andros- 

 terone, was extracted from testicular tissue in 1935. All of these 

 androgens are steroids. It has recently been demonstrated that the testis 

 will synthesize carbon^* labeled testosterone if provided with C^^ labeled 

 acetate. The testis also produces estrogenic substances. 



Testosterone has a general effect on metabolism, inducing growth 

 by stimulating the formation of cell proteins. The administration of 

 androgens leads to an increase in body weight due to the synthesis of 

 protein in muscle and to a lesser extent in liver and kidney. 



Testosterone and other androgens stimulate the development and 

 maintenance of the secondary male characters: the enlargement of the 

 external genitals, the growth of the accessory glands such as the prostate 

 and seminal vesicles, the growth of the beard and of body hair, and the 

 deepening of the voice. The secondary sex characters of other animals, 

 the antlers of deer and the combs, wattles and plumage of birds, are 

 controlled by androgens. Male sex hormones are responsible in part for 

 the development of mating behavior. 



