THE DIGESTIVE SYSTEM. 323 



the intestine, 60 per cent, of the fat of a meal passes into the faeces, 

 as compared with about 5 per cent, under normal conditions. (4) Bile 

 is said to stimulate the peristaltic movements of the intestine, and (5), 

 as already pointed out, the reabsorbed bile salts stimulate the liver to 

 further secretion. 



THE SECRETION OF BILE. 



The secretion of bile is a continuous process, and in periods when 

 digestion is not taking place, bile accumulates in the gall-bladder. 

 About the third hour after a meal is taken the gall-bladder is 

 emptied into the lumen of the duodenum, but the mechanism by 

 which the contents are expelled has not yet been ascertained. Bile 

 continues to flow into the intestine during the digestive process, and, 

 later, again accumulates in the gall-bladder. The rate of production of 

 bile has been studied in animals with experimental fistulse, and also in 

 man when fistulae have formed in the course of disease. In such cases, 

 however, the normal stimulus due to the reabsorption of bile salts is 

 wanting. It is found in man that something less than a litre of bile 

 can be collected from a fistula in twenty-four hours, an amount equal to 

 that of the juice secreted by the pancreas in the same time. 



So far as is known, the secretion of bile is independent of nervous 

 action, and is excited (1) by the reabsorbed bile salts, and (2) by 

 secretin. The flow from a fistula is fairly continuous, though it varies 

 in rate with the period of the day and with other conditions. The rate 

 of flow in such a case is, of course, unaffected by reabsorption of bile 

 salts. It is doubled by the introduction of dilute hydrochloric acid into 

 the duodenum, or by the injection of secretin into the blood stream. 

 The rate of flow, like that of the pancreatic juice, varies with the nature 

 of the food, and for the same reason (p. 318), being greatest when a 

 meat diet is taken, and least when the food consists mainly of carbo- 

 hydrates. Fatty food, which inhibits the secretion of gastric juice, 

 excites the secretion of the pancreas and liver, though not to the same 

 extent as does meat. The influence of fat in this direction is due to 

 the formation of soap in the duodenum, the latter substance acting as 

 a stimulus to the production of secretin. 



THE FINAL STAGES OF THE DIGESTIVE PROCESS. 



Saliva, gastric and pancreatic juice in turn carry the digestion of 

 the food-stuffs up to a certain point. The pancreatic juice completes 

 the digestion of the fats ; it converts proteins into peptones and ammo- 

 acids ; it is capable of converting starch completely into dextrose, but it 

 has no action upon cane sugar and lactose. The completion of the 

 digestive process is the function of the intestinal juice. s 



