244 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the relative amounts depending upon the completeness of digestion. As has 

 previously been said, there are indications that under the favorable conditions 

 of natural digestion all the starch may be changed to maltose, but possibly 

 it is not necessary that the action should be so complete in order that the 

 carbohydrate may be absorbed into the blood, as will be shown when we come 

 to speak of the further action of the intestinal secretion upon maltose and the 

 dextrins. The amylolytic action of the pancreatic juice is extremely import- 

 ant. The starches constitute a large part of our ordinary diet. The action of 

 the saliva upon them is probably, for reasons already given, of subordinate 

 importance. Their digestion takes place, therefore, entirely or almost entirely 

 in the small intestine, and mainly by virtue of the action of the amylopsin 

 contained in the pancreatic secretion. The action of the amylopsin is supple- 

 mented to some extent, apparently, by a similar enzyme formed in small 

 quantities in the intestinal wall itself, the nature of which will be described 

 presently in connection with intestinal secretion. 



Steapsin. Steapsin is the name given to a fat-splitting enzyme occurring 

 in the pancreatic juice. It is of the greatest importance in the digestion and 

 absorption of fats. The peculiar power of the pancreatic juice to split neutral 

 fats with the liberation of free fatty acid was first described by Bernard. His 

 discovery has since been corroborated for different animals, including man, 

 by the use of normal pancreatic juice obtained from a fistula, or by the aid of 

 the tissue of the fresh gland, or, finally, by means of extracts of the gland. 

 When neutral fats (see Chemical section for the composition of fats) are 

 treated with an extract containing steapsin, they take up water and then 

 undergo cleavage (hydrolysis), with the production of glycerin and the free 

 fatty acid found in the particular fat used. This reaction is explained by the 

 following equation, in which a general formula for fats is used : 



C 3 H 5 (C n H 2n+1 COO) 3 + 3H 2 = C 3 H 5 (OH) 3 + 3(C n H 2n+1 COOH). 



Fat. Glycerin. Free fatty acid. 



The reaction in the case of palmitin would be 



C 3 H 5 (C 15 H 31 COO) 3 + 3H 2 = C 3 H 5 (OH) 3 + 3(C 15 H 31 COOH). 



Palmitin. Glycerin. Palmitic acid. 



While this action is undoubtedly caused by an enzyme, it has not been possible 

 to isolate the so-called " steapsin " in a condition of even approximate purity. 

 As a matter of fact also, ordinary extracts of pancreas, such as the laboratory 

 extracts in glycerin, do not usually show the presence of this enzyme unless 

 special precautions are taken in their preparation. It would seem that steapsin 

 is easily destroyed. With fresh normal juice or with pieces of fresh pancreas 

 the fat-splitting effect can be demonstrated easily. One striking method of 

 making the demonstration is to use butter as the fat to be decomposed. If 

 butter is mixed with normal pancreatic juice or with pieces of fresh pancreas, 

 and the mixture is kept at the body-temperature, the several fats contained in 

 butter will be decomposed and the corresponding fatty acids will be liberated, 



