506 VERTEBRATE LIFE AND ORGANIZATION 



be proteins, amino acids or steroids; thus, we cannot deRne a hormone 

 as a member of some particular chiss ol organic compound. All of the 

 hormones are required for normal body function and they must be 

 present in certain optimal amounts. Either a hyposecretion (deficiency) 

 or hypersecretion (excess) of any one may result in a characteristic 

 pathologic condition. 



Some practical knowledge of endocrinology, such as the results of 

 the castration of men and animals, has existed for several thousand 

 years. However, it was not until 1849 that Berthold, from clear-cut 

 experiments in which testes were transplanted from one bird to an- 

 other, postulated that these male sex glands secrete some blood-borne 

 substance which is essential for the differentiation of the male sec- 

 ondary sex characters. In 1855 the British physician, Thomas Addison, 

 describetl the signs and symptoms of the human disease which now 

 bears his name, "and realized that this was associated with the deteri- 

 oration of the cortex of the adrenal. The first attempt at endocrine 

 therapy was made in 1889, when the French physiologist, Brown- 

 Sequard, injected himself with testicular extracts and claimed that they 

 had a rejuvenating effect. Epinephrine was the first hormone to be 

 isolated and chemically identified (1902). Many of our theoretical con- 

 cepts regarding endocrines stem from the classic work of Starling and 

 of Bayliss with secretin during the first two decades of this century. 



The basic problem of just how a hormone may act upon a tissue to 

 regulate its activities remains to be solved. It would appear that hor- 

 mones are not essential for the survival of individual cells, for many 

 kinds of cells can be grown in tissue culture indefinitely without added 

 hormones. It has been postulated that hormones produce their effects 

 by directly stimulating or inhibiting one or more of the intracellular 

 enzyme systems, or by modifying in some way the permeability of the 

 cell membrane so that substances can enter more readily to be me- 

 tabolized. The tissues in various parts of the body differ greatly in their 

 sensitivity to particular hormones, but the explanation for this phe- 

 nomenon is lacking. It is not clear at present whether a hormone is 

 used up in the process of regulating metabolism in a target cell. Hor- 

 mones are gradually inactivated and eliminated from the blood stream, 

 and hence must be continually replaced by the appropriate endocrine 

 gland. Both the synthesis and the inactivation and degradation of hor- 

 mone molecules are enzymatic processes. 



248. Methods of Investigating Endocrines 



The complete understanding of the role of an endocrine gland 

 requires information about (1) the number and kinds of hormones it 

 secretes, (2) what chemical and physical properties each of these hor- 

 mones has, (3) where and how they are made within the endocrine 

 organ, (4) what factors control their production, (5) what stimulates 

 their secretion by the gland, (6) how they are transported to the target 

 organ, (7) how they act to alter the metabolism of the target organ, (8) 

 how they are broken down and eliminated from the body, (9) how they 



