DIGESTION 389 



the blood-supply, although various circumstances suggest the 

 probability of a more direct action. All the digestive juices show 

 a certain adaptation to the nature of the food, although it has not been 

 demonstrated that this is due to a specific sensibility of the mucous 

 membranes for each kind of food-stuff. The action of one juice 

 on the secretion of another is also of great significance. Thus, 

 the water of the saliva directly excites a flow of gastric juice when 

 it reaches the stomach ; the acid of the gastric juice excites a flow 

 of pancreatic juice when it reaches the duodenum ; and the pan- 

 creatic juice excites the intestinal mucous membrane to the pro- 

 duction of enterokinase, the most characteristic constituent of the 

 succus entericus. In all the glands the blood-flow is increased 

 during activity ; in some (salivary glands] this is known to be 

 caused through vaso-motor nerves. In the salivary glands electro- 

 motive changes accompany the active state, and more heat is pro- 

 duced. Both in the salivary glands and the pancreas it has been 

 shown that much more carbon dioxide is given off, and much more 

 oxygen used up, during secretion than during rest. In the other 

 glands we may assume that the same occurs. This is one proof 

 that work is done in the separation or manufacture of the con- 

 stituents of the various secretions. 



IV. Digestion as a Whole. 



Having discussed in detail the separate action of the digestive 

 secretions, it is now time to consider the act of digestion as a 

 whole, the various stages in which are co-ordinated for a common 

 end. The solid food is more or less broken up in the mouth 

 and mixed with the saliva, which its presence causes to be 

 secreted in considerable quantity. Liquids and small solid 

 morsels are shot down the open gullet without contraction 

 of the constrictors of the pharynx, and reach the lower portion 

 of the oesophagus in a comparatively short time ( T V second) ; 

 while a good-sized bolus is grasped by the constrictors, then by 

 the oesophageal walls, and passed along by a more deliberate 

 peristaltic contraction. 



Chemical digestion in man begins already in the mouth, a 

 part of the starch being there converted into dextrins and sugar 

 (maltose), as has been shown by examining a mass of food con- 

 taining starch just as it is ready for swallowing (p. 424). This 

 process is no doubt continued during the passage of the food 

 along the cesophagus. 



The first morsels of a meal which reach the stomach find it 

 free from gastric juice, or nearly so. They are alkaline from the 

 admixture of saliva ; and the juice which is now beginning 

 to be secreted, in response to the psychical excitement, and 



