THE DIGESTIVE SYSTEM. 2QI 



THE STAGES OF DIGESTION. 



The activities of the digestive tract are two in number. First, there 

 is a motor mechanism by means of which the contents of the tract are 

 moved progressively from mouth to oesophagus, stomach, small intestine, 

 and large intestine, and are finally expelled. Secondly, there is a series 

 of secretory glands, which produce the digestive juices met with in the 

 different regions of the tract. These juices have no effect on water and 

 inorganic salts, but their enzymes bring about hydrolytic changes in 

 the protein, carbohydrate, and fatty constituents of the various food- 

 stuffs. Food is mixed with saliva in the mouth and is then quickly 

 passed on to the stomach, in which it remains for some time. During 

 the early part of its stay in the stomach, the starch of the food under- 

 goes the preliminary stages of digestion through the agency of the 

 saliva. Gastric juice is also secreted, and originates digestive changes 

 in the proteins and, to a certain extent, in the fats, while it gradually 

 destroys the salivary enzyme. 



The next stage consists in the forwarding of the stomach contents 

 into the small intestine, where the pancreatic juice and bile carry the 

 digestive changes in the carbohydrates and proteins a stage further, and 

 complete the digestion of the fats. 



Finally, the intestinal juice effects the concluding stages of 

 carbohydrate and protein digestion, and the hydrolytic products of 

 carbohydrates, proteins, and fats, together with inorganic salts and water, 

 are absorbed through the wall of the small intestine. Undigested 

 substances and certain waste matters reach the large intestine, where 

 much of the remaining water is absorbed, the residue constituting the 

 faeces. Under normal conditions, such of the constituents of a meal as 

 are not absorbed begin to reach the large intestine four to five hours after 

 the ingestion of the meal, and the residues are finally expelled from 

 twelve to twenty hours later, so that all the processes described above 

 are going on simultaneously, there being as a rule an interval of not 

 more than four or five hours between meals during the day, 



SECTION II. 

 CHANGES IN THE FOOD IN THE MOUTH. 



The changes which take place in the food in the mouth are chiefly 

 mechanical, its stay in this region being so brief that the saliva has not 

 time to produce an appreciable chemical effect. Salivary digestion 

 takes place mainly in the stomach in the interval before the enzyme is 

 destroyed by the gastric juice. In the mouth the food is masticated, 

 that is, it is broken up and formed into a pulpy mass by the vertical, 



