MAMMARY SECRETION. 617 



solution. This proves that the milk contains a ferment which is capable 

 of splitting up milk-sugar into lactic acid. Such a ferment is widely 

 distributed in the animal body. It may be often extracted from the 

 gastric mucous membrane, and is entirely distinct from pepsin or the 

 milk-curdling ferment, since it preserves its action even after the other 

 ferments are destroyed by dilute caustic soda. The ferment may be 

 obtained from milk after dialysis by precipitation with alcohol, and, 

 after drying, dissolving the precipitate in water. Such a precipitate will 

 coagulate milk and will cause the development of lactic acid in solu- 

 tions of milk-sugar. 



This ferment has a neutral reaction and is soluble in glycerin. It is 

 destroyed by boiling, and appears to regain its activity when treated 

 with oxygen. This would indicate that it is a ferment generator and 

 not a true ferment, and explains why boiled milk may be kept longer 

 than unboiled ; while the fact that even boiled milk will ultimately coag- 

 ulate is to be attributed to the subsequent development of the ferment 

 through the action of the air. 



Increase of temperature increases the activitj' of the ferment. Milk- 

 sugar is not directly fermentable; that is, it cannot be directty converted 

 into aleohol, though by the action of dilute sulphuric or hydrochloric 

 acids it may be partly transformed into a fermentable lactic acid. This 

 process is concerned in the manufacture of koumiss from mares' milk, 

 which contains a large amount of sugar. A similar fermented liquid 

 may be obtained from cows' milk through the action of the yeast-plant. 



4. Fat and Cream. — Fat is present in milk in the form of minute 

 globules, the average percentage being 3 to 3| per cent.,. although it 

 may vary from 2^ to 5^ per cent. 



The following fatty acids have been found in milk : Butyric, caproic, 

 caprylic, caprinic, myristic, palmitic, stearic, and oleic. The percentages 

 of palmitic, oleic, and stearic acids vary. So the melting point of butter 

 also varies, as does, also, its specific gravity. Normally, the melting point 

 varies from 32° to 37.5° C. 



Cows' butter contains about 68 per cent, palmitin, 30 per cent, 

 olein, and 2 per cent, other fats, the solid fats apparently being more 

 abundant in winter than in summer. The butter may be separated from 

 milk by mechanical agitation (churning), the enveloping layers of casein 

 being thus ruptured. It was formerly supposed that the fatty globule 

 of milk had a solid envelope, because the fat does not pass into solution 

 in ether unless caustic potash had been previously added. At most, 

 however, we may assume that the fat is surrounded by an envelope of 

 fluid casein, rendered more consistent here than elsewhere by the mo- 

 lecular attraction exerted by the fat-globules. Casein is not present in 

 the milk in the form of a true solution, but rather in a high degree of 



