MUCIN 85 



contain phosphorus; but the true nucleoproteids yield nuclein (xanthin) 

 bases while the para-nucleoproteids do not. The nucleoproteids are found 

 in the nucleus and protoplasm of every cell. The para-nucleoproteids are 

 found in milk, as caseinogen, and in the yolk of egg, as vitellin. Glyco- 

 proieids. Glycoproteid is 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. 



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

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

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

 from mucus by diluting 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. 



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. It does not dialyze. When treated with sulphuric acid 

 and then neutralized with solid potassium hydrate, it will give both the biuret 

 test, denoting the presence of proteid matter, and also Fehling's test, show- 

 ing the presence of a sugar. 



Nucleins. The substance known as nuclein and found in all cells 

 is really a compound proteid and consists of a series of bodies made up of pro- 

 teid and nucleic.acid in varying proportions; there is almost no limit to the 

 possible variations. At one end of the series is nucleic acid (C 30 H 52 N 9 P 3 O 17 , 

 according to Kossel), a body containing the maximum (9 to n per cent) 

 of phosphorus but without any proteid, and found as such only in sper- 

 matozoa; in the middle are the nucleins proper; and at the other end 

 are the nucleoproteids, containing the minimum of phosphorus. As phos- 

 phorus is the characteristic component of nucleic acid, its amount will meas- 

 ure the amount of the acid present in any molecule. 



The chemical differences in the action of cytoplasm and karyoplasm 

 toward solvents are due also to the proportion of nucleic acid and proteid 

 which they contain. These differences are qualitative and not quantitative. 

 All of the nucleoproteids in the cell body are true ones in that they yield 

 nuclein bases. 



Caseinogen. Caseinogen, the chief proteid of milk, yields para- 

 nuclein on digestion. It bears the same relation to casein that fibrinogen 

 does to fibrin. When acted on by rennin it splits into two parts of which 

 one, the smaller, is peptone-like in character. The other, and larger part, 

 is known as soluble casein and does not solidify in the absence of calcium 

 salts. As calcium is always present in milk, it there unites with it and forms 

 insoluble calcium casein; strictly speaking, therefore, the curd of milk is 



