CHEMICAL BASIS OF THE ANIMAL BODY. 1207 



well-characterized classes of substances, to one of which the 

 name of nucleo-albumin is given. Whereas nuclein occurs 

 chiefly in the nuclei, nucleo-albumin is present chiefly in the 

 cell-protoplasm but is also found in large quantity in certain 

 secretions, such as milk and bile. 



The more characteristic reactions of the nucleo-albumins may 

 be stated as follows. Soluble in very dilute alkalis they are 

 readily reprecipitated by acetic acid and the constancy in prop- 

 erties of the product obtained by repeated solution and precipi- 

 tation shews that they are not mere mixtures of nuclein and 

 proteid. Their behaviour towards alkalis and acetic acid is 

 such as to lead to an easy confusion with the mucins, from 

 which however they differ by being highly phosphorized and not 

 yielding a carbohydrate as a product of hydrolytic decomposition. 

 When digested with pepsin they yield peptones and albumoses 

 and a phosphorized residue which is in most respects identical 

 with nuclein, but since it does not yield products of the xanthine 

 series when decomposed by .acids, is known as pseudo- or para- 

 nuclein. They are, like the globulins, precipitated from solu- 

 tion by neutral salts, the precipitate becoming swollen and 

 slimy when the precipitant is sodium chloride or magnesium 

 sulphate, but not so when sodium sulphate is employed. 



Casein. 



This is the well-known proteid existing characteristically 

 in milk and in no other fluid or secretion of the body. 



It may be readily obtained by diluting milk with four vol- 

 umes of water and adding acetic acid to faint acidity. The 

 casein is thus precipitated and after several washings by decan- 

 tation is freed from fats by extraction with alcohol and ether. 



Pure casein is a fine, snow-white powder. It is practically 

 insoluble in water, but is soluble in alkalis, carbonates and 

 phosphates of the alkalis, lime- and baryta-water. From these 

 solutions it may be precipitated by excess of neutral salts such 

 as sodium chloride, and by dilute acids, in which it is again 

 soluble if any excess of acid is present. Its reactions thus 

 correspond closely to those of acid- and alkali-albumin, but it 

 is a perfectly distinct substance. Solutions of pure casein are 

 not coagulated by boiling, but if heated to 130 — 150° in 

 sealed tubes a coagulation is obtained. 



When casein is digested with pepsin in presence of hydro- 

 chloric acid it leaves a phosphorized residue of nuclein, which 

 since it cannot be made to yield any members of the xanthine 

 series by boiling with mineral acids is characterized by the 

 name of paranuclein or pseudonuclein. Casein is therefore 

 now regarded as a compound of a true proteid with this nuclein 

 and is the typical and best-known instance of a nucleo-albumin. 



