DIGESTION IN THE SMALL INTESTINE 399 



It seems probable the other two are also secreted as zymogens. The 

 potency of freshly secreted juice or of a glycerine extract of fresh 

 pancreas is feeble. Treatment with dilute acetic acid greatly increases 

 it. A glycerine extract increases in potency with keeping, and its 

 power is accelerated by dilution with a weak solution of salts, and 

 especially by the addition of another potent extract. The transforma- 

 tion of the zymogens of steapsin and amylopsin (if transformation there 

 be) must be rapid, that of trypsinogen is slow. The activation of 

 trypsinogen does not normally take place until the small intestine is 

 reached. If trypsin were active in the juice, it would destroy steapsin 

 and amylopsin. In the intestine, these enzymes are protected by the 

 presence of the bile and the proteins of the chyme. How the activa- 

 tion of trypsinogen is brought about by enterokinase, calcium salts, 

 and probably bacteria, has been already mentioned (p. 370). 



The Digestive Action of the Pancreatic Juice Trypsin. Most 

 proteins are readily hydrolyzed by this enzyme acting in an alkaline 

 medium. Trypsin, unlike pepsin, does not attack collagen and 

 gelatin. Like pepsin, it digests elastin but little, and keratin not at 

 all. Nucleo-protein is split into nucleic acid and a protein. In the 

 digestion of other proteins, little alkali metaprotein is formed, and 

 digestion proceeds so quickly that primary proteose is not as a rule 

 found; the peptone stage is quickly reached. The peptones are hydro- 

 lyzed to polypeptides, and these in turn to amino-acids, ammonia, 

 pyrimidin, and pyrollidin bases. The following table shows this: 



PROTEIN >- ALKALI METAPROTEIN 



PROTEOSES (mainly secondary) 



i 



PEPTONES 



t 

 POLYPEPTIDES 



AMINO- ACIDS (see p. 43) 



This digestion takes place in stages. Some amino-acids, such as 

 tyrosin, are split off earlier than others. 



Starch is digested by amylopsin, passing to maltose through 

 the same stages as in the case of digestion by ptyalin. The pan- 

 creatic diastase, however, exerts a considerable action upon unboiled 

 starch. This is not the case with ptyalin (see p. 71). 



