DIGESTION IN THE MOUTH 149 



and by reason of the mucin present, serves as a lubricating agent 

 to aid in the manipulation and swallowing of the food. It also 

 cleans and preserves the teeth, both by washing away particles 

 of food which otherwise might decay, producing acids which 

 would attack the teeth, and also by neutralizing acids introduced 

 into the mouth. The saliva also performs an important digestive 

 function by reason of the enzyme ptyalin which it contains. 

 Ptyalin is an enzyme which acts on starches. Little is known of 

 its chemical nature, but evidence seems to indicate that it is 

 made up of a substance resembling a protein combined with a 

 carbohydrate gum. Perhaps ptyalin is not a single enzyme, but 

 a mixture of two or more enzymes, each of which is responsible 

 for one of the steps in the digestion of starch. Ptyalin is a 

 member of the group of amylases, or starch splitting enzymes. 

 These enzymes are widespread in nature, and are found in 

 vegetable cells and also in various animal secretions and tissues. 

 They convert starch or starch-like carbohydrates into interme- 

 diate decomposition products and finally into maltose. The 

 different amylases are not identical, however, as they are de- 

 stroyed at different temperatures, and show other variations 

 in their properties. The composition of the amylases is of 

 course unknown, but they have been much studied. They may 

 be extracted from tissues by water and precipitated with mag- 

 nesium sulphate, the method of Lintner. Ptyalin itself is very 

 difficult to isolate and purify. Practically all amyla.se prepara- 

 tions contain phosphate. It is likely that' the phosphate is 

 essential to the action of the enzyme. As stated above, ptyalin 

 appears to be combined with a carbohydrate gum, which may 

 act as a carrier. Ptyalin is secreted by the parotid glands. It 

 acts best at a temperature of 40 -45 C. and is destroyed by 

 heating rapidly to 75 C. Ptyalin acts best in a very faintly 

 acid solution, the optimum acidity being 10' 5 - 3 X normal. If 

 the acidity is sufficient to turn congo red to violet (N X lO^ 4 ) 

 the action of ptyalin is inhibited. It also will act in a weakly 

 alkaline solution such as the saliva. The presence of some salt 

 favors its action. If the salt is removed by dialysis, the ptyalin 



