Internal Secretions and Internally Secreting Organs 3 



little by the organism into simple oxidisable substances, but is passed out 

 at once into the blood and extracted from the blood by the kidneys, pro- 

 ducing glycosuria. It may be added that our present knowledge of the 

 causation of diabetes is based mainly on these observations. 



Now it is known that the pancreas possesses, besides the secreting 

 alveoli which form the enlarged and blind terminations of its ducts, a 

 special kind of secreting cells which are massed together into islets of 

 irregular shape and variable number and size, having a special kind of 

 blood supply. These islets, which were first described as a distinct element 

 of pancreatic tissue by Langerhans, have been originally developed from 

 and may retain a connexion with the ducts of the organ, in this respect 

 resembling the ordinary alveoli. But in the course of growth they have 

 lost all open connexion with the ducts ; their cells have acquired specific 

 properties ; and their function is without doubt different from the ordinary 

 cells of the gland. With some show of reason the special internal secreting 

 function above mentioned has been ascribed to them, and in support of this 

 it may be stated that in many if not most cases of diabetes these cells are 

 found to have undergone degeneration. They, in fact, form an organ within 

 an organ, and collectively may be regarded as belonging to the group of 

 internally secreting or endocrine glands. 



An example of a tissue devoted to the formation of both an external 

 and an internal secretion is found in the epithelium which lines the 

 duodenum. The functions of this epithelium which have been longest 

 known are those of promoting the absorption of digested food materials 

 and of helping to furnish the secretion known as the intestinal juice. But 

 in 1902 it was discovered by Bayliss and Starling that if an extract of the 

 duodenal epithelium is boiled with dilute hydrochloric acid and, after 

 neutralisation, is injected into the blood stream of an animal, a rapid flow 

 of pancreatic juice is determined. It had already been known that the 

 gush of acid gastric juice through the pylorus, or the painting of the mucous 

 membrane of the duodenum with dilute acid, would determine a flow of 

 pancreatic juice, but this flow had been supposed to be brought about as 

 a reflex act by excitation of a local nervous mechanism by the acid. The 

 observations of Bayliss and Starling rendered it clear, however, that this 

 is not the correct explanation of the phenomenon ; but that the flow must 

 rather be regarded as due to the absorption of some internal secretion into 

 the blood : the material of this internal secretion being produced in an in- 

 active form by the epithelium cells, and becoming so altered by the dilute 

 acid as to be converted into an agent which, after absorption into the blood 

 stream, excites the secreting cells of the pancreas to activity. 



To the active substance which is yielded by the epithelium cells of the 

 duodenal mucosa the name secretine was given by Bayliss and Starling, 

 whilst they termed pro-secretine the inactive material contained within 

 the cells before the action of dilute acid upon them. The active material 

 is obviously of the nature of an internal secretion ; it appears, however, 



