372 PRACTICAL PHYSIOLOGY 



filtrate add a few drops of Millon's reagent and boil. The result shows 

 the presence of protein. 



EXPERIMENT IV. Add to some saliva in a test tube a drop of a 

 weak solution of ferric chloride (Liq. Ferri. Perchlor. B.P.) and a drop 

 of hydrochloric acid. A red colour is sometimes produced. This is 

 due to the production of ferric sulphocyanide by interaction between 

 the ferric chloride and a sulphocyanide which is contained in saliva. 

 The red colour is discharged by adding a few drops of a solution of 

 mercuric chloride (1-1000). A more sensitive way of performing this 

 test is to place a drop of saliva at one end of a piece of filter paper, 

 and then to allow a drop of ferric chloride solution (acidified with HC1), 

 spread to the edge of the saliva drop; a deep red stain will result 

 where the two moistened areas meet. 



EXPERIMENT V. If some saliva be allowed to stand for an hour or 

 so, it becomes milky or a thin surface film forms on it. This is due to 

 the precipitation of calcium carbonate, which exists in fresh saliva in a 

 soluble state as calcium bicarbonate. On standing exposed to the air, 

 however, carbonic acid gas is given off, in consequence of which the 

 bicarbonate changes into carbonate, which is insoluble. A similar pre- 

 cipitation of calcium carbonate, carrying with it a certain amount of 

 calcium phosphate, sometimes occurs in the ducts of the glands and 

 leads to the formation of calculi, or it may form on the teeth, where it 

 leads to the formation of tartar. 



II. To Study the Action of the Ferment Ptyalin. 



EXPERIMENT VI. Place a few cubic centimetres of a 0*5 per cent, solu- 

 tion of starch in two test tubes, a and b. To b add about an equal amount 

 of saliva, and place both a and b in the water-bath heated to body 

 temperature. Note that in a very few minutes the solution in b loses 

 its opalescence and becomes clear. By means of a glass rod transfer 

 drops from each solution, about once a minute, to a white slab or dry 

 evaporating dish, and add to each drop a little iodine solution. In the 

 drops from the test tube b the blue colour becomes at first purplish and 

 then reddish brown, and ultimately disappears. When this stage has 

 been reached, apply Trommer's or Fehling's test to the contents of the 

 test tube, and note that reduction occurs. In the case of a the blue 

 colour persists throughout and reduction of cupric salts does not occur. 



What has occurred in b is that the ptyalin has hydrolysed the poly- 

 saccharide starch (blue with iodine and no reducing power), first into a 

 simpler polysaccharide dextrin (red with iodine, no reducing power), 

 and then into the disaccharide maltose (no colour with iodine, reduces 

 cupric salts). If left in contact with the maltose for some time the 

 saliva can invert this, yielding dextrose. This indicates the presence 



