206 DIGESTION 



unknown substance secreted by the intestinal wall, the materials being 

 these amido-acids. This synthesis may occur as the amido-bodies are 

 absorbed through the protoplasm of the epithelium, or by other agency 

 elsewhere. The theories of proteolysis have already been given in 

 discussing the action of pepsin. 



The action of the diastase (amylopsin) is not unlike that of ptyalin 

 described above (page 188), but it is more vigorous, and it has the power 

 of dissolving the cellulose covering of starch grains, thus allowing it to 

 digest many uncooked vegetables and fruits more or less indigestible by 

 saliva in its usual environment. It is doubtful if the diastatic amylopsin 

 converts all the dextrin formed to maltose, this likely enough being in 

 part a product of the synthesizing action of an enzyme in the absorptive 

 mechanism of the gut-wall. 



The action of pancreatic juice on fats by means of its lipase (steapsin) 

 like some of its other functions, has probably a precedent in the stomach 

 of perhaps much smaller importance. This enzyme is a very unstable 

 one, and has not been isolated in even approximate purity. Loevenhart 

 has recently claimed to have found evidence that lipase exists in all the 

 tissues, especially in the liver, milk, blood, lymph, and intestinal juice, 

 and he supposes that the tissue-fats (stearin, olein, and palmitin) are 

 built up in the tissue-cells through its agency from free fatty acids cir- 

 culating in the blood and lymph as soaps. In general, it has been pre- 

 sumed that the action of lipase was the usual one of hydrolysis followed 

 immediately by cleavage into a glycerin and the fatty acid. Thus, in case 

 of the triglyceride palmitin the reaction would be 



C 3 H 5 (OC 15 H 31 CO) 3 + 3H 2 O = 3C 15 H 31 COOH + C 3 H 5 (OH) 3 



Palmitin. Water. Palmitic acid. Glycerin. 



the acid then uniting with potassium, sodium, or calcium either in the 

 gut-wall or in the tissues to produce the corresponding soap. This in a 

 nut-shell is the important process of saponification carried on by steapsin. 

 In addition, the lipase indirectly emulsifies fats, this being a mechanical 

 process (chemically performed by the fatty acids), while the other is a 

 distinctly chemical process. The exact relation of these two processes 

 has not even yet been well determined. Especially is it in doubt whether 

 all the fat destined for food is saponified or whether some of it is emulsi- 

 fied only and directly absorbed. For the most part emulsification is 

 probably due to the action of the fatty acids formed by saponification, so 

 that very likely both processes go on side by side in the duodenum. In 

 this process the bile, poured into the gut with the pancreatic juice, plays 

 an important part. It probably aids saponification by means of its 

 cholic acid. Furthermore, the action of lipase (steapsin) is at its best in 

 the presence of bile plus hydrochloric acid, according to Rachford, so 

 that the reagents on fat in the duodenum are very complex and the 

 reactions correspondingly complicated. The actions of the supposed 

 milk-curdling (rennin) and sugar-inverting enzymes (maltase, etc.) of 



