186 COMPOSITION OF THE DIGESTIVE JUICES. [cH. VIII. 



destroyed by boiling. Also the lipase of the pancreas 

 requires the presence of some soluble, heat-stable sub- 

 stance to allow it to act. Bile salts have this property, as 

 has been seen in a previous chapter. The action of the 

 enzymes can be retarded by certain substances. These 

 are of two classes : paralysers and anti-enzymes. The 

 paralysers are generally salts of the heavy metals, which 

 probably alter the physical state of the colloidal enzymes. 

 The anti-enzymes are of an organic nature. They probably 

 combine with the enzyme and thus prevent it from acting 

 on the substrate. Examples are seen in the case of the 

 anti-trypsin of normal serum, of the intestinal mucous 

 membrane and of the tissues of intestinal parasitic worms. 



A. Saliva. 



Saliva is of value as a lubricant in the act of degluti- 

 tion, and in some animals this is its sole function. 



232, Collect about 5 cc. of your own saliva in a small beaker. 

 Test the reaction with neutral litmus paper : it is alkaline. 



NOTE. The first portion of saliva collected is very apt to be neutral, 

 or even slightly acid, probably owing to bacterial decomposition in the mouth. 

 But if the secretion is free, that collected later is invariably alkaline. 



233. Transfer the saliva to a test-tube and add strong acetic 

 acid. A stringy precipitate of mucin is formed, insoluble in excess 

 of acid. Stir the mixture vigorously with a glass rod : the mucin 

 forms a clump which can be removed by the rod. To the clear 

 fluid remaining add some Millon's reagent and heat cautiously. 

 Only a slight red precipitate is formed, showing that the proteins 

 of saliva consist almost entirely of mucin. 



B. Ptyalin. 



Ptyalin, or salivary amylase, is an enzyme that acts 

 on boiled starch and certain other polysaccharides, the 

 chief end product being the disaccharide maltose. It is 

 possible that small amounts of glucose are also formed. It 

 is claimed by certain workers that for the complete hydroly- 

 sis of starch three ferments are necessary, viz., amylase, 



