Chapter 11 



HORMONES 



The hormones have been defined by Houssay as "specific chemical 

 substances produced by an organ or tissue which, after being discharged 

 into the circulating fluids, may reach all parts of the organism and in 

 small amounts markedly influence the functions of other organs or 

 systems without themselves contributing important quantities of matter 

 or energy." Thus they resemble the vitamins very closely, differing only 

 by being produced in the body rather than having to be supplied ready- 

 made in the food. The hormones are produced by specialized organs 

 called the glands of internal secretion, or endocrine glands, such as the 

 pancreas, thyroid, ovaries, and others. Hormone manufacture and secre- 

 tion is the physiological function of these glands, and the effects which 

 follow their removal or alteration are merely the result of too little or 

 too much hormone production. Often it is possible to overcome the 

 effect of glandular lack by supplying the necessary hormone from an 

 outside source (for example, insulin). The various hormones were dis- 

 covered by showing that the effects of removing certain endocrine glands 

 could be counteracted with extracts of the same glands from other 

 individuals. 



Chemical types 



The known hormones of higher animals are sometimes grouped roughly 

 into three chemical types. Those of the pancreas and pituitary, plus a 

 few others, are proteins or -peptides. The sex hormones and adrenal cortex 

 hormones are steroids. The third group is made up of adrenalin and 

 thyroxine, which are classified together as phenolic compounds, although 

 they are otherwise quite dissimilar. Most of the plant and insect hor- 

 mones do not fit into any of these classes. Some of the former are con- 

 sidered briefly in Chap. 15. 



Control of hormone production 



The various glands of internal secretion and the hormones they produce 

 make up a closely interrelated system, which is delicately balanced and 

 responsive to many influences. The functioning of this system helps 



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