344 A MANUAL OF PHYSIOLOGY 



requiring a relatively high temperature to inactivate it. The 

 smallest trace of enterokinase will convert a large quantity of 

 trypsinogen into trypsin if time be given. At the same time, 

 although to a much smaller extent, the fat-splitting and starch- 

 digesting activity of the pancreatic juice is increased. The 

 secretion of the duodenum causes a greater increase in the proteo- 

 lytic power than that of the other portions of the small intestine, 

 while no such difference has been made out in the case of the 

 amylolytic and lipolytic functions. It is probable that the 

 enterokinase, which is secreted mainly in the upper part of the 

 small intestine, and solely by the intestinal epithelium, acts only 

 on the trypsinogen, and that the amylopsin and steapsin are 

 aided in some other way. Enterokinase is only found in the 

 intestinal juice when pancreatic juice is present in the gut. It 

 is therefore secreted in response to the presence of trypsinogen 

 or of some other constituent of the pancreatic juice. 



Delezenne has attempted to explain the interaction of entero- 

 kinase and trypsinogen as an adaptive phenomenon of the same 

 kind as the formation of antitoxins and haemolysins (p. 27). Ac- 

 cording to him, enterokinase acts like a complement in haemolysis, 

 while trypsinogen plays the part of an intermediary body or ambo- 

 ceptor which enables the enterokinase to attack the protein mole- 

 cule. He asserts that enterokinase, or a substance which produces 

 a similar effect on trypsinogen, is contained not only in the mucous 

 membrane of the intestine, but also in leucocytes, in fibrin (one of 

 whose properties it is to pick out ferments from liquids containing 

 them), in lymph-glands, in snake venom, and even in certain anae- 

 robic bacteria. On this view trypsin would not be a definite sub- 

 stance produced by the interaction of enterokinase and trypsinogen, 

 but only an expression for these two bodies acting together. Strong 

 evidence against this view, and in favour of the independent existence 

 of trypsin, has been brought forward by Bayliss and Starling, and 

 it does not seem to merit further consideration. 



According to Pawlow, the reason why the trypsin is not secreted 

 in the active form is that active trypsin readily destroys the 

 amylolytic and lipolytic ferments. In the intestine, where trypsin 

 is rendered active by enterokinase, these ferments are protected 

 from its attack by the proteins of the food and by the bile. 



Having now finished our review of the chemistry of the diges- 

 tive juices, our next task is to describe what is known as to their 

 secretion the nature of the cells by which it is effected and their 

 histological appearance in activity and repose, and the manner in 

 which it is called forth and controlled. 



III. The Secretion of the Digestive Juices. 



The digestive glands are formed originally from involutions of the 

 mucous membrane of the alimentary canal, the salivary glands 

 from the ectoderm, the others from the endoderm (Chap. XIV.). 

 Some are simple unbranched tubss, in which there is either no dis- 



