CHAPTER 22 



j - Endocrine Mechanisms 



rTI 



I HE PRODUCTION and dispersal within the organism of chemical sub- 

 I stances which subserve definite integrating and coordinating roles, 



.JL — and thereby supplement the activity of the nervous elements, are 

 characteristic of all living things. Such substances may be referred to de- 

 scriptively as chemical coordinators, in the broadest sense, every substance 

 which enters the body fluids from the external environment or from the con- 

 stituent cells of a higher organism and thus contributes to the normal com- 

 position of the internal medium is a chemical coordinator. Oo and CO2, for 

 example, certainly operate importantly in the coordination of organismic ac- 

 tivities. Other coordinatory substances, such as the D vitamins, may enter 

 the body or may under certain circumstances be svnthesized within certain 

 cells of the organism and thence be liberated into the blood. Some chemical 

 substances, e.g., secretin, appear to be more restricted in their region of 

 origin within the body and adaptively participate in a specialized activity 

 within the organism, namely, stimulation of the liberation of pancreatic 

 juice in response to the presence of food in the duodenum. Finally, many 

 groups of higher organisms have differentiated specialized glandular cells, 

 tissues, or organs which elaborate coordinatory substances for the organism 

 as a whole. This latter development seems to have paralleled the general 

 specialization and restriction within the organism of numerous other organs 

 and organ systems; these developments could come about as soon as an 

 effecti\'e mechanism of internal transport was provided in the form of a cir- 

 culatory system. It is therefore not surprising to find most described instances 

 of the existence of specialized endocrine organs among the annelids, mol- 

 luscs, and arthropods,^^- ^'' "•^' •''*• '•'''• "''* in addition to the vertebrates. 



The terms hormones and endocrines are applied to special chemical co- 

 ordinators which are produced at some more or less restricted region or regions 

 within the organism and which possess specific phvsiological action at that 

 or other regions within the body. Thev are usuallv, but by no means al- 

 ways, produced in well defined glandular organs. No two endrocrinologists 

 may agree on how broadly one should interpret such a definition of a hor- 

 mone. Obviously, in view of the complete intergradation of all types of 

 chemical coordinators with one another it is impossible to make a sharp 

 natural division between hormones, on the one hand, and other chemical co- 

 ordinators, on the other. Ascorbic acid may fulfill the definition of a hormone 

 for the rat, for example, and yet fulfill the definition of a vitamin for man. 



Our knowledge of the modes of action of chemical coordinators is still far 

 too scanty to permit an attempt to make any distinction in terms of how the 

 substance exerts its influence, e.g., whether it actively participates in chem- 



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