36 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
milk of normal cows are likely to balance one another 
when the milk of several cows is mixed together; con- 
sequently different samples of market milk show less 
variation in the number of leucocytes than individual 
milk. On the other hand, when the milk from one cow 
affected with mastitis is mixed with the milk of other 
cows in the herd which are in normal condition, the cell 
content of the mixed milk is not likely to be very much 
increased unless the herd is a very small one or the milk 
from the diseased cow contains an enormous number of 
cells. 
Several methods have been devised for detecting an 
excessive number of cells in milk (see pages 281 to 284). 
When used to examine the milk of individual cows, 
these methods are of great assistance in discovering cases 
of mastitis before clinical symptoms or visible milk 
changes appear, but when applied to samples of mixed 
market milk they cannot be depended upon entirely for 
the purpose of detecting mastitis in the herds supplying 
the milk. 
BIOLOGICAL PROPERTIES OF MILK 
Ferments or Enzymes.—Milk contains a number of 
ferments or enzymes. Some of them resemble the di- 
gestive ferments in their action. This class includes a 
proteolytic ferment called galactase, and diastase, an 
amylolytic ferment. These ferments are believed to 
assist in the digestion of milk. A tripsin-like ferment 
and fat-splitting ferments or lipases have been reported, 
but their existence is questioned. There are also oxidiz- 
ing ferments: the oxydases and peroxydase, and reduc- 
ing ferments: catalase and reductase. The diastase, 
