DIGESTION. 285 



it is mixed; while,, with these are mingled gastric fluid, and fluid that has 

 been swallowed, together with such portions of the food as are not digest- 

 ible, and will be finally expelled as part of the fseces. 



On the entrance of the chyme into the duodenum, it is subjected to 

 the influence of the bile and pancreatic juice, which are then poured out, 

 and also to that of the succus entericus. All these secretions have a more 

 or less alkaline reaction, and by their admixture with the gastric chyme 

 its acidity becomes less and less until at length, at about the middle of 

 the small intestine, the reaction becomes alkaline and continues so as far 

 as the ileo-csecal valve. 



The special digestive functions of the small intestine may be taken in 

 the following order: 



(1.) One important duty of the small intestine is the alteration of the 

 fat in such a manner as to make it fit for absorption; and there is no 

 doubt that this change is chiefly effected in the upper part of the small 

 intestine. What is the exact share of the process, however, allotted re- 

 spectively to the bile, to the pancreatic secretion, and to the intestinal 

 juice, is still uncertain, probably the pancreatic juice is the most impor- 

 tant. The fat is changed in two ways. (a). To a slight extent it is 

 chemically decomposed by the alkaline secretions with which it is mingled, 

 and a soap is the result, (b). It is emulsionized, i.e., its particles are 

 minutely subdivided and diffused, so that the mixture assumes the condi- 

 tion of a milky fluid, or emulsion. As will be seen in the next Chapter, 

 most of the fat is absorbed by the lacteals o2 the intestine, but a small 

 part, which is saponified, is also absorbed by the blood-vessels. 



(2.) The albuminous substances which have been partly dissolved in 

 the stomach, and have not been absorbed, are subjected to the action of 

 the pancreatic and intestinal secretions. The pepsin is rendered inert 

 by being precipitated together with the gastric peptones and parapeptones, 

 as soon as the chyme meets with bile. By these means the pancreatic fer- 

 ment trypsin is enabled to proceed with the further conversion of the 

 parapeptones into peptones, and of part of the peptones (hemipeptone, 

 Ktilme) into leucin and tyrosin. Albuminous substances, which are 

 chemically altered in the process of digestion (peptones), and gelatinous 

 matters similarly changed, are absorbed by both the blood-vessels and 

 lymphatics of the intestinal mucous membrane. Albuminous matters, 

 in a state of solution, which have not undergone the peptonic change, are 

 probably, from the difficulty with which they diffuse, absorbed, if at all, 

 almost solely by the lymphatics. 



(3.) The starchy, or amyloid portions of the food, the conversion of 

 which into dextrin and sugar was more or less interrupted during its stay 

 in the stomach, is now acted on briskly by the pancreatic juice and the 

 succus entericus; and the sugar, as it is formed, is dissolved in the intes- 

 tinal fluids, and is absorbed chiefly by the blood-vessels. 



