THE ENDOCRINE SYSTEM 507 



may be produced synthetically and (10) what use they may have in 

 the treatment of disease. The assembling of all of this information 

 requires the efforts of anatomists, histologists, physiologists, biochem- 

 ists, pharmacologists and clinicians. 



The fact that a certain gland has endocrine function is frequently 

 first learned as a result of its accidental or deliberate removal. The 

 deprivation of the organism of its normal source of the hormone 

 usually results in readily observable abnormalities. As we shall see, the 

 normal functioning of any given organ is usually the result of the ef- 

 fects of a number of different hormones, some of which work together 

 (act synergistically) while others oppose the action of the first (act 

 antagonistically). It may be incorrect to attribute the effects of the 

 surgical removal of one gland to the simple lack of its hormone; they 

 may result from the unopposed action of hormones secreted by other 

 glands. It may require a complex experimental design, including the 

 removal of several endocrine glands and the replacement of their secre- 

 tions by injected pure hormones, to elucidate the role of each. 



Further information about endocrine function is obtained by re- 

 placing the surgically extirpated gland by transplanting a gland from 

 another animal, by feeding dried glands, or by injecting an extract or 

 a purified compound obtained from the gland. The administration 

 of one hormone frequently suppresses or stimulates the secretion of 

 hormones by other glands. By proper experimental design, one can 

 distinguish between the primary effect of the injected hormone and 

 its possible secondary effects via the stimulation or inhibition of other 

 endocrines. 



Another experimental approach to the endocrine problem is the 

 extraction and purification of the hormone by chemical and physical 

 procedures from the gland itself or from the blood or urine of the 

 organism. Only an extremely small amount of hormone is required to 

 produce its normal effects, and the amount present in the endocrine 

 gland, or in the blood and urine, is usually quite small. The isolation 

 of a pure hormone is a difficult procedure; more than two tons of pig 

 ovaries had to be extracted to yield a few milligrams of estradiol, the 

 female sex hormone, and to get 15 mg. of androsterone, a male sex 

 hormone, it was necessary to extract over 5000 gallons of urine! 



Much has been learned about endocrine function by careful ob- 

 servation of the symptoms of human diseases resulting from the hypo- 

 or hypersecretion of hormones. Further information has been derived 

 from the careful study of strains of rats, mice and other animals with 

 particular endocrine abnormalities-dwarf mice, obese mice, diabetic 



mice, and so on. 



The location of the human endocrine glands is shown in Figure 

 30.1. Their relative position in the body is much the same in all the 

 vertebrates. The source and physiologic effects of the principal hor- 

 mones are listed in Table 7. It must be kept in mind that hormones 

 are not found solely in vertebrates, but occur as well in such inverte- 

 brates as insects, crustaceans, annelids and molluscs. 



