118 HANDBOOK OF PHYSIOLOGY. 



upon other proteids ; the temperature necessary to produce coagulation 

 varying in the manner previously indicated. They may also be produced 

 by the prolonged action of alcohol upon proteids ; the process is one of 

 dehydration. They are soluble in strong acids or alkalies; slightly so in 

 dilute; are soluble in digestive fluids (gastric and pancreatic). Are in- 

 soluble in water or saline solutions (except fibrin) . 



Fibrin. Fibrin is formed by the action of fibrin ferment on fibrinogen 

 and can be obtained as a soft, white, fibrous, and very elastic substance 

 by whipping blood with a bundle of twigs and washing the adhering mass 

 in a stream of water until all the blood-coloring matter is removed. It 

 is soluble to a certain extent in strong saline solutions. 



Compound Proteids. The compound proteids are compounds of a 

 simple proteid with some other molecule. According to their chemical 

 composition and characteristics they are divided into several classes, viz. : 



Chromo- proteids. A combination of a proteid substance with some 

 form of pigment. For example, haemoglobin is a combination of a globu- 

 lin with haematin, an iron-containing radicle. 



Gluco-proteids. A combination of a proteid substance with a carbo- 

 hydrate radicle. Examples are mucin, which is found in mucous secre- 

 tions ; and mucoids, which are found in certain tissues, cartilages, etc. 



Nmleo-proteids. A combination of a proteid substance with a nucleic 

 acid : they are divided into two groups according to the character of the 

 acid. The true nucleo-proteids contain true nucleic acid; the para-nucleo- 

 proteids, or pseudo-nucleo-proteids contain para-nucleic acid. Both acids, 

 and therefore both groups, contain phosphorus ; but the true nuclo-proteids 

 yield nuclein (xanthin) bases while the para-nucleo-proteids do not. They 

 are found in the nucleus and protoplasm of every cell, and also in milk, 

 as caseinogen, and in the yolk of egg, as vitellin. 



Gluco-nudeo-proteids. A combination of a nucleo-proteid with a car- 

 bohydrate radicle. 



Mucin. Mucin is a compound of a globulin with a carbohydrate 

 radicle, and is the characteristic component of mucus ; it is contained also 

 in foetal connective tissue, in tendons, and salivary glands. It can be 

 obtained from mucus by diluting it with water, filtering, treating the 

 insoluble portion with weak caustic alkali, and reprecipitating with acetic 

 acid. The mucins derived from different sources probably have different 

 compositions. 



Properties. Mucin has a ropy consistency. It can be coagulated; is 

 insoluble in water, salt solution, and very dilute muriatic acid ; is soluble in 

 alkalies and concentrated sulphuric acid, It gives the proteid reaction 

 with Millon's reagent and with nitric acid. Neither mercuric chloride 

 nor tannic acid gives a precipitate with it (?). It does not dialyse. 



