120 HANDBOOK OP PHYSIOLOGY. 



character. The other, and larger part, is known as soluble casein and 

 does not solidify in the absence of calcium salts ; as these are always 

 present in milk, it there unites with them and forms insoluble calcium 

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

 compound of soluble casein. Caseinogen may be prepared by adding di- 

 lute hydrochloric acid to milk until the mixture is distinctly acid ; a floe- 

 culent precipitate of caseinogen will be thrown down and may be sepa- 

 rated by filtration ; 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 salt 

 causes it to separate out. 



Caseinogen gives the Biuret and Millon's reactions showing the pres- 

 ence of proteid substances, much the same tests as alkali-albumin. It is 

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

 but insoluble in sodium chloride and .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 dis- 

 solved 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 011 saturation 

 with sodium chloride ; it coagulates between 70 and 83 C. 



Albumenoids or Proteoids. The albumenoids belong to the sim- 

 ple tissues of the body which are derived from the epiblast and are char- 

 acterized by a lack of any degree of activity, either physiological or 

 chemical. They are proteid derivatives, nitrogenous bodies derived from 

 proteid matter in the cells, and give crystalline amido-acids and nitrogen- 

 ous 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 foods, though gelatin has a certain indirect 

 food value as it protects the body proteids from work in many ways, at 

 times. The albumenoids are soluble in dilute acids or alkalies ; they may 

 be distinguished from albumin or globulin by being insoluble in water or 

 salt solution respectively. 



Gelatin. Gelatin is contained in the form of collagen, its anhydride, 

 in bone (ossrin), teeth, fibrous connective tissues, tendons, ligaments, etc. 

 It may be obtained by prolonged action of boiling water in a Papin's di- 

 gester or of dilute acetic acid at a low temperature (15 C.). 



Properties. The percentage composition is 0, 25.24 per cent, H, 6.56 

 percent, 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 amor- 

 phous, and transparent when dried. It does not dialyse ; it is insoluble 



