314 MUCIN. [BOOK n. 



Mucin, thus prepared and purified by washing with ac3t:c 

 acid, swells out in water, without actually dissolving; it will 

 however dissolve into a viscid fluid readily in dilute (O'l p.c.) 

 solutions of potassium hydrate, more slowly in solutions of alka- 

 line 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. 

 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. 



176. 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 inter- 

 stices, 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 tur- 

 bid, 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 must bear in mind the existence of the following 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 colour 

 with iodine. The formula is C 6 H 10 5 or more correctly (C 6 H J0 5 ) n 



