PHYSIOLOGY OF MILK SECRETION. 



201 



According to Koenig, the limits of variation as collected from about 

 800 analyses taken from all parts of the world are as follows: 



Maximum. Minimum. 



Water .' 90 . 69 80 .32 



Fat 6.47 1.67 



Casein 4.23 1.79 



Albumen 1 . 44 .25 



. Sugar 6.03 2.11 



Ash 1.21 .35 



While the range of variation seems considerable, some of the constitu- 

 ents, notably that of fat, may show even greater variation, in milk secreted 

 by normal, healthy cows. Babcock found the milk of a single cow giving 

 a small quantity, contained as high as 10 per cent, of fat. He further 

 states that no analysis showing more than 9 per cent, of fat is recorded 

 from a cow giving more than 15 pounds of milk per day. 



When milk is examined under the microscope, it appears as a clear 

 liquid in which are suspended an immense number of spherical bodies 

 that are of a light yellowish appearance and are highly refractive. These 

 constitute the fatty part of the milk and are known as milk globules. Their 

 small size and the viscous nature of the milk serum, tend to prevent the 

 coalescence of the globules. The size of these globules varies in all milks 

 varying from 2 mmm. up to 30 mmm.'s in diameter (1 mmm. equals 1 

 25,000 of an inch). The size also varies with the milk drawn, whether 

 it be first, middle or last drawn, as may be seen by the following records 

 given in table 3. TABLE 3. 



NUMBER OF GLOBULES OF EACH SIZE PER THOUSAND GLOBULES. 



As the period of lactation advances the globules increase in number 

 and the average size diminishes, so that after several months the total 



