4 io DIGESTION 



although it has not been demonstrated that this is due to a specific sensi- 

 bility 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 pancreatic 

 juice excites the intestinal mucous membrane to the production 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 produced. 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. 



SECTION VI. SURVEY OF 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 consider- 

 able 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 (y 1 ^ 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. ^xf-^XA^.^vt^eXi^^o 



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 containing starch 

 just as it is ready for swallowing (p. 450). This process is no doubt 

 continued during the passage of the food along the oesophagus. 



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

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

 mixture of saliva ; and the juice which is now beginning to be secreted, 

 in response to the psychical excitement, and reflexly through the 

 presence of the food and the water of the saliva in the stomach, is for 

 a time neutralized, and amylolytic digestion still permitted to go on. 

 For 20 to 40 minutes after digestion has begun there is no free 

 hydrochloric acid in the stomach, although some is combined with 

 proteins, and during this period the ptyalin of the swallowed saliva 

 will be able to act even better than in the mouth, being favoured by 

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