THE MILK. 409 



lactic acid from lactose in consequence of the activity of certain 

 micro-organisms. 



Perfectly fresh milk does not coagulate on boiling, but it will be 

 noted that a skin forms on the surface of the milk, which is rapidly 

 reformed when removed. This consists of coagulated casein in com- 

 bination with mineral salts, and especially phosphates of calcium. 

 Actual coagulation does not occur, even if a current of carbon dioxide 

 has previously been passed through the liquid. If the milk has 

 stood for sometime, 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, 

 cceterls paribuz, 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. 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 : 



Woman 

 Cow . 

 Goat 

 Sheep 



Mure 

 Bitch 



.028-1.034 



.029-1.034 

 .030-1.034 

 .037-1.040 

 .029-1.035 

 .028-1.034 

 .034-1.040 



