288 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the saliva, which we have already found to exist in the saliva and the 

 aqueous extract of the salivary glands of most groups of animals. 



Experiments as to the diastatic action of tlie saliva may, therefore, he made 

 either with fresh, filtered saliva, with an aqueous or glycerin infusion of the sali- 

 vary glands, or with an aqueous solution of pure ptyalin. A mucilage for testing 

 the diastatic action of saliva may be made by mixing one grain of powdered starch 

 into a thin paste with a few drops of cold water, and then adding the paste to 100 

 cubic centimeters of boiling water and allowing it to boil for ten minutes. Then, 

 after standing until the sediment has settled, the clear supernatant fluid is filtered 

 off and is ready for use. Equal quantities of cold starch-mucilage are poured 

 into three test-tubes, which are numbered one, two, and three ; tube No. 1 con- 

 tains starch-mucilage alone ; to tube No. 3 a few drops of filtered saliva are added; 

 an equal quantity of saliva is boiled thoroughly for a few minutes and added .to 

 No. 3 ; in tube No. 4 is poured a small quantity of saliva alone. The four test- 

 tubes are placed in the hot-water bath or an oven at a temperature of about 38° 

 or 39° C. After a few moments the tvibes may be removed for testing. If to 

 tube No. 1, which contained starch-mucilage alone, a few drops of dilute iodine 

 solution are added, a characteristic blue color is developed, showing the presence 

 of starch, while Fehling's solution will demonstrate the absence of sugar. If to 

 tube No. 2, which contains starch-mucilage and saliva, a few drops of the same 

 solution of iodine are added, no blue color will be developed, showing the absence 

 of starch, and the fluid will either remain colorless or may take on a more or less 

 marked reddish tint from the presence of dextrin, showing that the starch has 

 disappeared. If to another portion of the same fluid contained in tube No. 2 a 

 few drops of Fehling's solution are added and the fluid boiled, a copious yel- 

 lowish-red precipitate, due to the reduction of cupric to cuprous oxide, will be 

 formed, showing the presence of a considerable quantity of sugar. Sugar has, 

 therefore, in this test-tube replaced the starch. If a few drops of iodine are added 

 to the fluid of test-tube No. 3, which contained starch-solution and boiled saliva, 

 the reaction of starch will still be developed, and Fehling's fluid will show the 

 absence of sugar. Boiling, therefore, has prevented the conversion of the starch 

 by the saliva into sugar. The fluid of test-tube No. 4, which consists of saliva 

 alone, will give no reaction with iodine, while no sugar will be found with 

 Fehling's test, though the blue color may be turned to a violet from the presence 

 of proteids. 



Starch-mucilage, when subjected to the action of saliva at a temper- 

 ature about that of the blood for a few moments, is converted into sugar. 

 This conversion is not instantaneous, although it was taught by Bidder 

 and Schmidt that momentary contact with saliva and starch was all that 

 was necessary to turn starch into sugar. An experiment which has been 

 long used to substantiate this view, and which appears at first to dem- 

 onstrate its truth, is really by no means conclusive. The experiment is 

 as follows : — 



If into a beaker which contains a little saliva warmed up to 40° C. 

 is added, drop by drop, a solution of starch which has been colored blue 

 by iodine, as each drop falls it is decolorized. The view, however, that 

 the loss of color is clue to the conversion of the starch into sugar is 

 erroneous, as was pointed out by Schiff. He showed that the decolori- 

 zation was due to the conversion by the saliva of the iodine into 

 hydriodic acid, and that many other organic fluids which would not con- 

 vert starch into sugar would decolorize the iodide of starch; thus, the 

 addition of morphine solution or of dog's urine to the iodide of starch 



