SECRETION AND ACTIVATION OF DIGESTIVE FLUIDS 360 



provision of this juice in the duodenum. It is suggested that, for 

 the other parts of the small intestine, the absorption of the products 

 of the digestion in the parts above evokes a messenger which, absorbed 

 into the blood, calls forth a flow of appropriate juice in the regions 

 lower down the tract. 



The bile takes an important share in the preparation of the food 

 for intestinal digestion for example, in the emulsification of fats 

 and the precipitation of protein from acid solution. A quick flow 

 of bile is therefore required. In animals where a gall-bladder exists, 

 the first flow of bile is probably provided by the contraction of the 

 gall-bladder, which is excited by a nervous reflex. The reflex arises 

 from the stimulus of food passing the pylorus. A further supply of 

 bile is provided from the liver by the action of " secretin." This 

 insures its presence in the intestine in amounts adequate to the food 

 Avhich is arriving there to be digested. Possibly, too, the products of 

 digestion reaching the liver cause a further flow of bile. The reab- 

 sorption of bile salts from the intestine stimulates the liver to secretion, 

 but this is usually after the period of active digestion, and the bile 

 secreted by this mechanism is generally stored in the gall-bladder, 

 there being a correspondingly active secretion to replenish the depleted 

 store. The following chart shows how different juices are provided: 



Juice. Mechanism. 



Saliva . . . . . . Xervous reflex. 



Gastric juice .. .. (1) Nervous reflex. 



(2) Liberation of gastrin (chemical reflex). 

 Bile .. .. .. (1) Probably nervous reflex contraction of gall-bladder. 



(2) By secretin (chemical reflex). 



(3) By products of digestion reaching liver. 



(4) By absorption of bile salts. 

 Pancreatic juice .. .. (1) By secretin. 



(2) Probably also by nervous reflex. 

 Succus entericus . . . . (1) By acid chyme. 



(2) By absorption of products of digestion. 



We have, therefore, to bear these mechanisms in mind when thinking 

 of the digestive disorders which may possibly arise. It may well be 

 that in some conditions the flow of one or other of these juices is not 

 evoked adequately, owing to a failure of the proper stimulus for its 

 secretion. 



The Activation of the Juices. In most of the juices the digesting 

 agent, or enzyme, is in the form of a precursor, or zymogen. This 

 zymogen must be converted into the enzyme, or " activated," as it is 

 termed, before it becomes potent. The enzyme of the saliva 

 " ptyalin " is probably activated by the bacteria of the mouth, or 

 some other body present in the mouth, since in the horse it has been 

 found that if the saliva be collected aseptically it manifests no digestive 

 action. It is only when bacteria arc allowed to enter the saliva that 

 the enzyme attains its digestive power. Bacteria probably play a 

 similar and important part in other parts of the alimentary tract, 

 even when specific activators of the zymogens are secreted there. 



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