MILK. 301 



the mammary glands have been little investigated, but among them 

 we find considerable amounts of xanthin bodies. 



As human milk and milk of animals are essentially of the same 

 constitution, it seems best to speak first of the one most thoroughly 

 investigated, namely, cow's milk, and then of the essential proper- 

 ties of the remaining important varieties of milk. 



Cow's Milk. 



Cow's milk forms, as all milks do, an emulsion which consists 

 of very finely-divided fat suspended in a solution consisting chiefly 

 of albuminous bodies, milk-sugar, and salts. Milk is non-trans- 

 parent, white, whitish yellow, or in thin layers somewhat bluish 

 white, of a faint, insipid odor and mild, faintly-sweetish taste. The 

 reaction is regularly amphoteric, sometimes with a stronger action 

 on the red and sometimes on the blue litmus-paper. The specific 

 gravity is 1.028 to 1.0345 at + 15 C. 



Milk gradually changes when exposed to the air, and its reaction 

 becomes more acid. This depends on a transformation of the 

 milk-sugar into lactic acid, which is produced partly by the pres- 

 ence of a special enzyme originating in the glands but not yet 

 positively detected, but which is chiefly produced by micro- 

 organisms. 



Entirely fresh, amphoteric milk does not coagulate on boiling, 

 but forms a skin consisting of coagulated casein and lime-salts, 

 which rapidly re-forms after being removed. Even after passing a 

 current of carbon dioxide through the fresh milk it does not coagu- 

 late on boiling. In proportion as the formation of lactic acid ad- 

 vances this behavior changes, and soon a stage is reached when the 

 milk, which has previously had carbon dioxide passed through it, 

 coagulates on boiling. At a second stage it coagulates alone on 

 heating ; then it coagulates by passing carbon dioxide alone with- 

 out boiling; and lastly, when the formation of lactic acid is suffi- 

 cient, it coagulates spontaneously at the ordinary temperature, 

 forming a solid mass. It may also happen, especially in the warmth, 

 that the casein-clot contracts and a yellowish or yellowish-green 

 acid liquid (acid whey) is separated. 



If the milk is sterilized by heating and contact with micro- 



