The fundamental conception involved in the theory of hormone activity in the organism has proved to be popular and fruitful in its applications. Instead of ascribing all sorts of physiologic performance to the effect of stimulation through nervous channels, the possibility that chemical substances poured into the blood stream may travel through the body in the circulation and, reaching specific organs and tissues, may act directly as stimuli to receptive structures, has become recognized. That hormones may thus act as excitants of the secretory glands discharging into the alimentary tract has been demonstrated beyond question; and it has become customary to speak of secretins or secretagogues which promote flow from such glands. The latter may be influenced in their physiologic activities, therefore, by both nervous and chemical stimuli.
One of the early demonstrations of such possibilities was furnished by the English physiologist Edkins,5 who discovered that acid extracts of