382 MUCIN. [BOOK n. 



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 (01 p.c.) solutions of 

 potassium hydrate, more slowly in solutions of alkaline salts. In 

 order to filter a mucin solution, 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 a 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 diges- 

 tion, 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. 



198. 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 

 organisms which are always present in the mouth may produce 

 much more profound 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 understand this change, the reader 



