52 THE PROTEINS. 



can be considered, then, as unions of an albuminous sub- 

 stance with some other substance. Most of them are coagu- 

 lated by boiling. 



THE MUCINS. 



Mucins are found in some of the secretions of the body, 

 especially in those of the mucous membrane and saliva, 

 and also as a constituent of the tendons and umbilical cord. 

 In their composition they resemble the albuminous sub- 

 stances, but contain less nitrogen. Their characteristic 

 property is that when boiled with a dilute mineral acid they 

 .are decomposed, giving two substances: an albuminous com- 

 pound and a compound containing little or no nitrogen and 

 having the power of reduction, as is shown by its changing 

 cupric hydrate to cuprous oxid in an alkaline solution. 

 By this they can be distinguished from all similar albu- 

 minous compounds. There are several kinds of mucin, 

 although as yet they are not well differentiated from one 

 another. 



The mucins are colloid substances, insoluble in pure 

 water, but soluble in small amounts of dilute alkalies, such 

 as calcium hydrate. They are mucilaginous and can be 

 drawn out into threads. They are precipitated by the 

 addition of acetic acid, if neutral salts are absent. They 

 are not coagulated by boiling, but give many of the reac- 

 tions of the albumins. Like the nucleoalbumins, they are 

 acid in reaction. 



There have been sometimes included with the mucins a class 

 of substances which resemble them in being decomposed by acids 

 into albuminous substances and substances with the power of 

 reduction, but which are not precipitated by acetic acid. They 

 are more properly called mucoids. Such are the pseudomucin, 

 found in ovarial liquids; chondromucoid of cartilage, and a few 

 others. 



