Secretines 125 



would provoke secretion of pancreatic juice. This had been put down to a 

 reflex effect through nerves and nerve-centres, but the experiments of Bayliss 

 and Starling made it clear that the effect is due to the formation and absorption 

 into the blood of an organic chemical agent which is dialysable and not des- 

 troyed by boiling, and is presumably of a fairly simple chemical constitution. 



Hustin finds that perfusion of pancreas with Locke's solution + secretine 

 is without effect, but if blood + secretine is used an abundant secretion, 

 containing all the ferments, is poured out by the duct. 



That there is some mutual interaction between pancreas and duodenum 

 would appear from the observations of Evans, who found that pro-secretine 

 disappears from the duodenum after complete extirpation of pancreas, but 

 not if enough pancreas is left to prevent the occurrence of glycosuria. 



Duodenal extracts do not act only on the pancreas ; the}' also increase 

 the flow of bile and of succus entericus, but to a less extent. 



Cow has made the interesting observation — already referred to in con- 

 nexion with the pituitary body — that intravenous injection of duodenal ex- 

 tract produces a discharge of pituitary secretion into the cerebro-spinal fluid. 



In 1906 Edkins announced that a substance of similar nature to secretine, 

 but acting upon the glands of the stomach, can be extracted from the 

 pyloric mucous membrane by boiling water or dilute hydrochloric acid, or 

 by solutions of dextrin, maltose, or albumose. According to Edkins extracts 

 of the mucous membrane of the fundus do not yield a similar substance. 

 The hormone in question is termed gastric secretine or gastrine. It exists 

 in the mucous membrane in the form of a precursor (pro-gastrine) which is 

 activated by the above reagents. These observations have recently been 

 confirmed by experiments in Pawlow's laboratory. 



Atropine, which, in small doses, operates through the nerve endings of secret- 

 ing organs, does not inhibit the action of these secretines. Their activity is 

 therefore probably manifested directly upon the cells of the organ they stimulate 

 and not through the nerve terminations. 



The Internal Secretion of the Pancreas. The Islets 

 of Langerhans 



The pancreas contains, besides its alveoli and the ducts which conduct 

 their secretion into the duodenum, a peculiar epithelial tissue occurring 

 in most animals in the form of small isolated masses interspersed through- 

 out the gland, and known from their discoverer (1869) as the islets of 

 Langerhans (fig. 88). Although quite distinct in appearance and in the 

 characters of their cells from the epithelium of the alveoli, the study of 

 their development shows that they originally grew out from the budding 

 ducts, and that their cells have therefore an origin in common with those 

 of the alveoli. But they have no open communication with the ducts or 

 alveoli. The number of islets in the pancreas is very variable, and this 

 variability has led to inferences being drawn regarding their appearance 



