Composition of Milk. 33 



for the doubling of the weight of the young and the percentage of proteids in the milk; 

 between the proportion of certain salts and the ash constituent, and the rapid growth of 

 the young ; between the growth of the brain and the supply of proteids and lecithin. 



Milk consists of dissolved constituents, and this solution con- 

 tains substances in suspension ; in the entire mixture there are also 

 undissolved substances in emulsion. 



The dissolved and suspended substances are designated as 

 milk plasma, which after coagulation separates in milk serum and 

 coagulum. The fat is present in an emulsion; there are in addi- 

 tion to this several salts, coagulums, cells, etc., undissolved or in 

 a precipitated condition. In coagulation the casein which at first is 

 in suspension, thickens, and carries down the undissolved sub- 

 stances, separating more or less from the milk serum in which the 

 soluble salts, milk sugar, certain proteids, ferments, coloring mat- 

 ter, etc., remain. 



The principal constituents of the milk, which constitute as well 

 the principal properties of the glandular secretions, are the parts 

 which have received the most thorough study. 



The proteids. Casein, milk albumen, and milk globulin (traces 

 of lactomucins, and possibly traces of other proteid substances, 

 which remain after acid precipitation and boiling, being known 

 collectively as lactoproteins) are the protein constituents of milk. 



The fat; the milk sugar. The milk further contains lecithin, 

 sarcin, kreatinin, nuclein, urea and sulphocyanic acid. 



Nothing is known at the present time of some of these constit- 

 uents, whether they occur originally in the milk, or whether they 

 are only split products, which result during the final production of 

 the various principal constituents, or through bacterial action in 

 the milk ; of such substances may be mentioned peptone, ammonia, 

 leucin, etc. 



Of non-nitrogenous substances milk also contains citric acid, 

 cholesterin and under certain conditions free lactic acid, alcohol 

 and acetic acid. 



Gases which occur free in milk are oxygen, nitrogen, and oc- 

 casionally carbonic acid ; the salts are combinations of the bases of 

 sodium, potassium, magnesium, calcium, and iron, with hydro- 

 chloric acid, sulphuric acid, phosphoric acid, carbonic acid, and 

 citric acid. 



Principal Constituents. 



Casein is a proteid especially characteristic of milk, occur- 

 ring almost exclusively in the milk gland secretion of mammalia, in 

 quantities of from 2 to 4 per cent. 



(It is supposed to occur also in the secretion of the sebaceous 

 glands of mammalia and in the coccygeal gland of birds.) 



The origin of casein is unknown. It was formerly supposed that it originated from 



an enzymic change of serum albumen produced by the action of enzyme-like bodies 



upon the albumen. However, since it has been found that the assertion of Kemmerich, 



relative to the increase of the casein at the expense of the lactalbumen, after the di- 



3 



