THE CONCEPT of chemical messengers or "hormones" was first recognized with the pioneering studies of Bayliss and Starling1 in 1902 when they showed that perfusion of the intestine with acid resulted in pancreatic secretion, and that injections of crude extracts of duodenal mucosa produced a similar stimulation of the pancreas. Classically, a hormone has been defined as "any substance normally produced in the cells of some part of the body and carried by the blood to distant parts, which it affects for the good of the organism as a whole."1 With the increasing recognition that many hormones act locally as well as at distant sites, hormones have been operationally defined as "chemical messengers which are dissolved in extracellular fluid and carry information between cells."2 Recent studies have suggested that we may have to modify further our concept of what a hormone is and how it reaches its
Morley JE. Food Peptides: A New Class of Hormones? JAMA. 1982;247(17):2379–2380. doi:10.1001/jama.1982.03320420029026
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