SALIVA AND GASTRIC JUICE. 247 



mass, which is mucin, if stirred carefully with a glass rod, shrinks, becoming 

 opaque, clings to the glass rod, and may be thus removed from the fluid. If 

 the quantity of mucin be small and the saliva be violently shaken or stirred 

 while the acid is being added, the mucin is apt to be precipitated in flakes, 

 and may then be separated by filtration. It may be added that the precipi- 

 tation of mucin by acid is greatly influenced by the presence of sodium 

 chloride and other salts ; thus, after the addition of sodium chloride, acetic 

 acid, even in considerable excess, will not cause a precipitate of mucin. 



Mucin, thus prepared and purified by washing with acetic acid, swells 

 out in water without actually dissolving ; it will, however, dissolve into a 

 viscid fluid readily in dilute (0.1 per cent.) solutions of potassium hydrate, 

 more slowly in solutions of alkaline salts. In order to filter a mucin solu- 

 tion, great dilution with water is necessary. 



Mucin is precipitated by strong alcohol and by various metallic salts ; it 

 may also be precipitated by dilute mineral acids, but the precipitate is then 

 soluble in excess of the acid. 



Mucin gives the three proteid reactions mentioned in 15, but it is a 

 very complex body, more complex even than proteids, for by treatment with 

 dilute mineral acids and in other ways, it may be converted into some form 

 of proteid (acid-albumin when dilute mineral acid is used), while at the 

 same time there is formed a body which appears to be carbohydrate and 

 resembles a sugar in having the power of reducing cupric sulphate solutions. 

 Solutions of mucin, moreover, on mere keeping are apt to lose their viscidity 

 and to become converted into a proteid not unlike the body peptone, which, 

 as we shall see, is the result of gastric digestion, and into a reducing body. 

 Several kinds of mucin appear to exist in various animal bodies, but they 

 seem all to agree in the character that they can by appropriate treatment 

 be split up into a proteid of some kind and into a carbohydrate or allied 

 body. 



184. The chief purpose served by the saliva in digestion is to moisten 

 and soften the food and to assist in mastication and deglutition. In some 

 animals this is its only function. In other animals and in man it has a 

 specific solvent action on some of the food-stuffs. Such minerals as are 

 soluble in slightly alkaline fluids are dissolved by it. On fats it has no effect 

 save that of producing a very feeble emulsion. On proteids it has also no 

 specific action, though pieces of meat, cooked or uncooked, appear greatly 

 altered after they have been masticated for some time ; the chief alteration, 

 however, which thus takes place is a change in the haemoglobin and a 

 general softening of the muscular fibres by aid of the alkalinity of the saliva. 

 Of course, when particles of food are retained for a long time in the mouth, 

 as in the interstices or in cavities of the teeth, the bacteria or other organ- 

 isms which are always present in the mouth may produce much more pro- 

 found changes, but these are not the legitimate products of the action of 

 saliva. The characteristic property of saliva is that of converting starch 

 into some form of sugar. 



Action of saliva on starch. If to a quantity of boiled starch, which is 

 always more or less viscid and somewhat opaque or turbid, a small quantity 

 of saliva be added, it will be found after a short time that an important 

 change has taken place, inasmuch as the mixture has lost its previous 

 viscidity and become thinner and more transparent. In order to under- 

 stand this change the reader must bear in mind the existence of the follow- 

 ing bodies, all belonging to the class of carbohydrates : 



1. Starch, which forms with water not a true solution but a more or less 

 viscid mixture, and gives a characteristic blue color with iodine. The for- 

 mula is C 6 H 10 O 5 , or more correctly (C 6 H 10 O 5 ) n since the molecule of starch 



