Protein — Modified Albuminoids 67 



Some of the properties of this body have been noticed 

 in discussing the action of ferments. It has others 

 which it is well to mention. In the first place, casein 

 is not soluble in water. It is not in solution as it ex- 

 ists in milk, but is regarded as being in a swollen con- 

 dition. Again, it does not coagulate when milk is 

 boiled. While the skin which forms on the surface 

 of milk at a boiling temperature contains casein as 

 one component, the only genuine coagulation that oc- 

 curs is of the albumin present. Every housewife has 

 noticed that when vinegar is added to milk in a small 

 quantity the milk curdles. This is because the casein 

 is modified by a weakly acid medium. A generous 

 quantity of common salt, or of certain other salts, 

 would have a similar effect. 



The nuclein, which forms a part of casein, can be 

 split into an albumin and phosphoric acid, and is an 

 illustration of a class of compounds which are gen- 

 erally distributed in plant and animal tissue. The name 

 is suggestive of the fact that these bodies exist in the 

 nuclei of living cells, having an intimate relation to the 

 protoplasm. Nucleins are also found in milk and eggs, 

 and it appears quite possible that they take a pecu- 

 liarly important place in nutrition, especially with 

 young animals and milch cows. 



Another compound widely distributed in the animal 

 kingdom is mucin, a prominent constituent of the 

 slimy secretions of the mucous membranes that line 

 the passages of the animal body, such as the throat 

 and the intestines. This substance appears somewhat 

 anomalous in being a combination of a proteid and a 



