86 THE CHEMICAL COMPOSITION OF THE BODY 



the calcium compound of soluble casein. Caseinogen may be prepared 

 by adding dilute hydrochloric acid to milk until the mixture is distinctly 

 acid, when a flocculent precipitate of caseinogen will be thrown down and 

 may be separated by nitration. The fat which is carried down with this 

 precipitate may be removed by washing with alcohol and then with ether. 



Caseinogen may also be prepared by adding to milk an excess of crys- 

 tallized magnesium sulphate or sodium chloride, either of which salts causes 

 it to separate out. Caseinogen gives the biuret ' and Millon's reactions. 

 It is soluble in distilled water, dilute or strong alkalies, and sulphuric acid, 

 but insoluble in sodium chloride and 0.2 per cent of hydrochloric acid. 



Vitellin. Vitellin is prepared from yolk of egg by washing with 

 ether until all the yellow matter has been removed. The residue is then 

 dissolved in 10 per cent saline solution, filtered, and poured into a large 

 quantity of distilled water. The precipitate which falls is impure vitellin. 

 It gives the same tests as myosin, but is not precipitated on saturation with 

 sodium chloride; it coagulates at about 75 C. 



Albuminoids. The albuminoids belong to the simple tissues of 

 the body which are derived from the epiblast and are characterized by a 

 lack of any degree of activity, either physiological or chemical. They are 

 nitrogenous bodies derived from proteid matter in the cells, and give crys- 

 talline amido-acids and nitrogenous bases on decomposition, but differ from 

 true proteids in not having their nitrogen in a form fit for the physiological 

 needs of the body. In other words, they are not true nitrogen-supplying 

 foods, though gelatin has a certain indirect value as it protects the body 

 proteids from work in many ways. The albuminoids are soluble in dilute 

 acids or alkalies; they may be distinguished from albumin or globulin by 

 being insoluble in water or salt solution respectively. Typical albuminoids 

 are gelatin, elastin, chondrin, keratin, etc. 



Gelatin. Gelatin is contained in the form of collagen, its anhy- 

 dride, in bone, ossein, teeth, fibrous connective tissues, tendons, ligaments, 

 etc. It may be obtained by prolonged action of boiling water or of dilute 

 acetic acid. 



The percentage composition is O 25.24 per cent, H 6.56 per cent, N 

 17.81 per cent, C 50 per cent, SO 25 per cent. It contains more nitrogen 

 and less carbon and sulphur than proteids. It is amorphous, and trans- 

 parent when dried. It does not dialyze; it is insoluble in cold water, but 

 swells up to about six times its volume; it dissolves readily on the addition 

 of very dilute acids or alkalies. It is soluble in hot water, and forms a jelly 

 on cooling, even when only i per cent of gelatin is present. It is also soluble 

 in hot salt solution. Prolonged boiling in dilute acids or in water destroys 

 the power of forming a jelly on cooling. On decomposition it gives 2 per 

 cent of leucin and 2.6 per cent of argenin, but no tyrosin, and a large amount 

 of glycocoll (amido-acetic acid or glycin), a crystalline substance. 



