THE MILK. 431 



stood for some time, however, and lactic acid fermentation has begun, 

 a tendency to coagulation soon becomes manifest, and at different stages 

 this may then be effected by boiling after saturation with carbon 

 dioxide, then by boiling alone, subsequently on treating with carbon 

 dioxide without boiling ; and finally, as I have stated, it occurs 

 spontaneously. Sterilization of the milk, with the subsequent exclu- 

 sion of micro-organisms, as also the addition of preservatives, such 

 as boric acid, salicylic acid, thymol, etc., will prevent lactic acid fer- 

 mentation, and consequently also coagulation referable to this source. 



On exposure to the air, milk is said to absorb its own volume of 

 oxygen within three days. 



Amount. The amount of milk furnished in the twenty-four hours 

 is, of course, different in different animals. It is largely dependent 

 upon the development of the glands, and accordingly is most abun- 

 dant in those animals in which by artificial selection a marked hyper- 

 trophy of the organs has been produced. Some cows may thus yield 

 24 liters of milk in the twenty-four hours. The amount is further 

 influenced by the age, as also by the character of the diet, the amount 

 of liquid ingested, etc. Especially important is the character of the 

 diet, and notably the amount of albuminous food that is ingested. 

 Where this is deficient the amount of milk is diminished, while, 

 caeteris paribus, larger amounts are furnished if an abundance of 

 albumins is ingested. 



Women furnish from 900 to 1000 grammes on an average during 

 the height of lactation ; 1500 grammes probably represent the maxi- 

 mum output. The amount seems to depend somewhat upon the 

 demand. It is thus not rare in institutions where one woman nurses 

 several babies that 4 liters of milk are secreted in the twenty- 

 four hours. Good cows commonly yield from 6 to 10 liters, goats 

 and sheep about 1 liter, in the twenty-four hours. With the gradual 

 cessation of lactation and the coincident atrophy of the mammary 

 glands the amount decreases, until finally the secretion is arrested 

 entirely. In women and cows the period of lactation usually lasts 

 about ten months. 



Specific Gravity. The specific gravity of the milk is largely de- 

 pendent upon the amount of fat present, and is much the same 

 in different animals. Its normal variations are seen in the accom- 

 panying table : 



Skimmed milk is, of course, specifically heavier than full milk, 

 and a higher specific gravity is accordingly also noted in milk which 

 is poor in fat than in rich milk. 



Reaction. Woman's milk and that of most herbivorous animals 

 owing to the presence of diacid and monacid phosphates in association 



