APPENDIX. 171 



2. The Action of Saliva on Starch. Make a thin paste of good 

 arrowroot (which is almost pure starch) with boiling water. Let it 

 cool. 



a. Add two or three drops of this starch paste to half a test tube- 

 ful of cold water; next add three or four drops of solution of caustic 

 potash and two or three drops of dilute watery solution of blue vitriol 

 (cupric sulphate). Mix thoroughly and boil over a spirit lamp. No 

 orange-red precipitate will result. This shows that there is no grape 

 sugar in the starch paste. 



b. Rinse the mouth thoroughly and then collect a small quantity of 

 saliva in a test tube. Dilute with water. Add caustic potash and 

 cupric sulphate solutions as above; mix thoroughly and boil. The 

 mixture will become violet, but give no orange-red precipitate; there- 

 fore there is no grape sugar in saliva. 



c. Take now three drops of the starch paste and a teaspoonful of 

 saliva; mix with a half test tubeful of water. Place the mixture in 

 a moderately warm place for five minutes. Then add a few drops of 

 the caustic potash and cupric sulphate solutions; mix and boil. An 

 abundant orange or brick -red precipitate will be thrown down, prov- 

 ing the presence of grape sugar, which has been produced by the 

 action of the saliva on the starch. 



3. Gastric Digestion. a. Obtain a pig's stomach. Cut it open 

 and wash away its contents with a gentle stream of water. Then 

 dissect off the mucous membrane from its middle part, mince and 

 put aside for a couple of days in three or four ounces of glycerine. 

 The glycerine dissolves the pepsin. Then strain off the glycerine 

 through muslin. 



b. Get a butcher to " whip" some fresh drawn blood for you with 

 a bunch of wire or twigs. The blood fibrin will collect on these 

 (p. 181), and when thoroughly washed with water, forms a good proteid 

 for digestion experiments. One lot of it thus obtained and washed 

 may be put aside in 50 per cent, alcohol, and will provide material for 

 digestion experiments for years. 



c. Add a teaspoonful of muriatic acid to a pint of water. 



d. Dilute a teaspoonful of the pepsin solution a with two table- 

 spoonfuls of water. Fill a test tube with the mixture; add a few 

 shreds of washed fibrin, and set aside in a warm but not hot place for 

 twenty-four hours. No change will occur, showing that pepsin alone 

 will not dissolve proteid s. 



e. Put some shreds of fibrin in a test tube of the mixture c in a 

 warm place for twenty-four hours. The fibrin will swell up and 

 become translucent, but will not dissolve. This shows that dilute 

 acids will not in a short time dissolve proteids. 



