COW'S MILK. 611 



also contain a proteolytic enzyme which, according to HiLDEBRANDT, 1 

 occurs to a much greater extent in the active gland as compared with 

 the inactive one. 



As human milk and the 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 properties 

 of the remaining important kinds of milk. 2 



Cow's Milk. 



Cow's milk, like every other kind, forms an emulsion which consists 

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

 protein bodies, milk-sugar, and salts. Milk is non-transparent, white, 

 whitish yellow, or in thin layers somewhat bluish white, of a faint, insipid 

 odor and mild, faintly sweetish taste. The specific gravity is 1.028 to 

 1.0345 at 15 C. The freezing-point is -0.54-0.59 C., average -0.563- 

 C., and the molecular concentration 0.298. 



The reaction of perfectly fresh milk is generally amphoteric toward 

 litmus. The extent of the acid and alkaline part of this amphoteric 

 re-action has been determined by different investigators, especially 

 THORNER, SEBELIEN, and COURANT. S The results differ with the indi- 

 cators used, and moreover the milk from different animals, as well as 

 that from the same animal at different times during the lactation period, 

 varies slightly. COURANT determined the alkaline part by N/10 

 sulphuric acid, using blue lacmoid as indicator, and the acid part by N/10 

 caustic soda, using phenolphthalein as indicator. He found, as an average 

 for the first and last portions of the milking of twenty cows, that 100 

 cc. milk had the same alkaline reaction toward blue lacmoid as 41 cc. 

 N/10 caustic soda, and the same acid reation toward phenolphthalein 

 as 19.5 cc. N/10 sulphuric acid. The actual reaction of cow's milk, 

 which follows from the electrometric estimation, is, on the contrary, 

 FoA 4 claims, nearly neutral, like the reaction of animal fluids and 

 tissues in general. 



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

 becomes more and more acid. This depends on a gradual transformation 

 of the milk-sugar into lactic acid, caused by micro-organisms. 



1 Bert, Compt. rend., 98; Thierfelder, Pfluger's Arch., 34, and Maly's Jahresber., 

 13; Hildebrandt, Hofmeister's Beitrage, 5. 



2 A very complete reference to the literature on milk may be found in Raudnitz's 

 "Die Bestandteile der Milch," in Ergebnisse der Physiol., 2, Abt. 1. The literature, 

 of the last few years may be found in the references by Raudnitz, Monatsschrift f., 

 Kinderheilkunde . 



3 Thorner, Maly's Jahresber., 22; Sebelien, ibid.', Courant, Pfluger's Arch., 50. 



4 Compt. rend. soc. biolog. (58), 59, 51. 



