DIGESTION OF FOOD: THE INTESTINES 103 



contrast to the acid secretions of the gastric glands, the 

 pancreatic secretion is strongly alkaline. 



Food as it leaves the stomach is by no means completely 

 digested. The starch is only partially changed to sugar; much 

 of the proteid passes through the stomach without change, 

 and the fats, though melted and emulsified in part, have been 

 only partially digested, if at all. 



The fluid derived from the pancreas has the power to dis- 

 solve and change any kind of food, this being accomplished for 

 the most part by three different ferments (enzymes), as follows: 



1. Trypsin, which starts the digestion of proteids by 

 changing many of them to peptones, thus supplementing the 

 work of the gastric juice. 



2. Amyolopsin, which converts starches into sugars, thus 

 completing the action of the saliva. 



3. Lipase (steapsin), under the action of which fats are 

 split up and made absorbable, i. e. digested. 



The influence of trypsin is similar to that of pepsin although 

 more complete; it also acts upon the proteids which have 

 been partly changed into peptones, and breaks them up still 

 further chemically; this action continues till all proteid mat- 

 erial has been reduced to very simple substances called ammo- 

 acids. These differ from proteids in that they are composed 

 of much smaller molecules, are soluble in water, do not 

 coagulate with heat, and easily pass through membranes. 

 Probably most, if not all, the peptones are changed into amino- 

 acids before absorption, this final step being brought about 

 by a ferment, erepsin, secreted by the intestinal wall glands. 



About eighteen different amino-acids are now recognized as 

 resulting from the complete digestion of different sorts of pro- 

 teid foods. Some of these, e. g. arginine and glutaminic acid, 

 are abundant, i. e. derived from many proteid sources; others, 

 e.g. cystine, are seldom formed ;tryotophane (derived from milk 

 and wheat) and lysine (from milk and eggs) are very essential 

 to growth, while glutaminic acid (from milk, eggs, wheat, peas. 



