DIGESTION AND ABSORPTION IN THE INTESTINES. 787 



Secretin is not an enzyme, since its activity is not destroyed by boil- 

 ing nor by the action of alcohol. The experimental evidence at 

 present favors the view that the normal sequence of events is as 

 follows: The acid of the gastric juice upon reaching the duodenum 

 produces secretin; this in turn is absorbed Ijy the blood, carried to 

 the pancreas, and stimulates this organ to activity. The pan- 

 creatic secretion furnishes, therefore, a second example of the group 

 of substances designated by Starling as hormones (p. 772) . Accord- 

 ing to the evidence at present in our possession we must believe 

 that the pancreatic secretion, like the gastric secretion, consists 

 of two parts: 1, A nervous secretion caused by the secretory 

 fibers in the vagus and splanchnic; 2, a chemical secretion due to 

 the action of the secretin. These two secretions are said to 

 present quite different characters.* The former is thick, opales- 

 cent, rich in ferments and proteins, but poor in alkalies. . The 

 trypsin contained in it may be secreted in active form, and the 

 secretion is suspended by the action of atropin. Administration 

 of pilocarpin, on the contrary, excites this secretion. The chemical 

 secretion, on the contrary, is thin and watery, contains relatively 

 little ferment or proteins, and is rich in alkali. The trpysin in 

 it is secreted in inactive form (see next paragraph), and the 

 secretion is not affected by the administration of atropin. The 

 normal relation of these two forms of secretion in an ordinary 

 meal is not so apparent as in the case of the gastric secretion. 

 The existence of a nervous secretion is somewhat uncertain. 

 We know nothing as to the nature or place of action of the afferent 

 stimuli responsible for it, but assuming its existence as a part of 

 the normal machinery we may infer that the nervous reflex is 

 preparatory and provides for a flow of pancreatic juice in the 

 earlier stages of intestinal digestion, while the action of the se- 

 cretin serves to maintain this flow until the stomach is completely 

 emptied. 



Activation of the Trypsin — Enterokinase. — It was discovered 

 in Pawlow's laboratory (Chepowalnikow) that the pancreatic juice 

 obtained from a fistula may have little or no digestive action on 

 proteins, but if brought into contact with the duodenal membrane 

 or an extract of this membrane it shows at once powerful pro- 

 teolytic properties. This discovery has been 3onfirmed repeatedly. 

 Evidently the proteolytic enzyme of the juice is secreted in a 

 zymogen or pro-enzyme form (trypsinogen), which is activated or 

 converted to trypsin by something contained in the mucous mem- 



* Sawitsch, "Zentralblatt f. d. ges. Physiol, u. Pathol, d, Stoffwechsels," 

 10, 1, 1909. 



