APPENDIX 489 



color indicating the presence of starch. Pour a second portion 

 into another tube, add sodium hydrate and copper sulphate, 

 and boil. If the yellow color appears it indicates that some 

 of the starch has already been digested by the saliva, i.e., has 

 been changed to grape sugar, which remains dissolved in the 

 fluid in the test tube. If the yellow color does not appear on 

 the first trial, make another after an interval of a few minutes. 



(4) To show that digested starch is capable of absorption, 

 while undigested starch is not 



Prepare two dialyzers. The parchment, or parchment paper, 

 which in each dialyzer separates the contents of the inner from 

 the contents of the outer jar, may be considered to represent 

 roughly the membrane lining the alimentary canal, through 

 which membrane substances are absorbed into the system. 

 Into the inner jar of one dialyzer put a solution of grape sugar ; 

 into the inner jar of the other put some thin starch paste. 

 After an hour or two test the water in the outer jar of the first 

 dialyzer for the presence of grape sugar : that in the outer jar 

 of the other dialyzer for starch. It will be found that grape 

 sugar i.e., digested starch dialyzes, while undigested starch 

 does not. In other words, undigested starch cannot be ab- 

 sorbed. The experiment may be varied by putting both grape 

 sugar and starch paste into the same dialyzer. Or, a mixture 

 of starch paste and saliva may be put into the one, while starch 

 paste alone is put into the other dialyzer. 



GASTRIC DIGESTION 



(i) Some of the chemical reactions of undigested albuminous 

 substances (proteids) 



Into a bowl or beaker break the white of an egg, cut it to 

 pieces with a pair of scissors, add fifteen or twenty times its 

 bulk of water, mix thoroughly by stirring, but do not beat it, 

 then strain through muslin to remove the fine flakes of coagu- 

 lated matter. 



