FOOD AND DIGESTION". 399 



melted, but not dissolved at all; starch very slowly in process of con- 

 version into sugar, and as it becomes sugar, also dissolving in the fluids 

 with which 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 digestible, and will be finally expelled as part of the faeces. 



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 contin- 

 ues so as far as the ileo-caecal 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, al- 

 lotted respectively to the bile and to the pancreatic secretion, is still un- 

 certain. 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 par- 

 ticles are minutely subdivided and diffused, so that the mixture assumes 

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

 Chapter, most of the fat is absorbed by the lacteals of 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 chiefly to the 

 action of the pancreatic juice. The pepsin is rendered inert by being 

 precipitated together with the gastric peptones and proteoses, as soon as 

 the chyme meets with bile. By these means the pancreatic ferment 

 trypsin is enabled to proceed with the further conversion of the proteo- 

 ses into peptones, and part of the peptones (hemipeptone) into leucin 

 and tyrosin. Albuminous substances, which are chemically altered in 

 the process of digestion (peptones) and gelatinous matters similarly 

 changed, are absorbed by the blood-vessels and lymphatics of the intes- 

 tinal mucous membrane. Albuminous matters, in 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 maltose was more or less interrupted during their stay in the 

 stomach, are now acted on briskly by the pancreatic juice and the succus 

 entericus; and the sugar in the form of maltose is dissolved in the intes- 



