2 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
which are peculiar to milk. They also take from the 
blood, water and other substances which are common 
to milk and blood. In histological sections, the milk fat 
may be observed within the actively secreting cells in 
the form of small fat droplets (Fig. 1). The other con- 
stituents of milk, being without form, cannot be seen, 
but their presence in the cells is indicated by a granular 
or turbid condition of the protoplasm. When the cells 
of an alveolus become filled with secretion, the cell-pro- 
toplasm contracts and the contents is discharged into the 
lumen of the alveolus, after which secretion again begins. 
The different stages of cell activity do not occur in all of 
the alveoli at the same time; different alveoli in the same 
lobule may show various stages. Secretion continues 
until the cells are exhausted, and it then ceases until 
the cells can recuperate. 
In the course of time, after repeated periods of activ- 
ity, exhaustion and rest, the epithelial cells of the alveoli 
are worn out and secretion stops in one alveolus after 
another. This change is called involution. In young 
cows the worn-out cells are regenerated. The alveoli 
then present the same appearance microscopically as 
is observed at the time of parturition (see Fig. 2). The 
interalveolar connective tissue is increased in volume and 
contains many leucocytes. Leucocytes are also numer- 
ous in the alveoli, together with exfoliated epithelial 
cells, free nuclei, parts of disintegrated cells, fat globules, 
and coagulated casein. Fat droplets may be observed 
within some of the leucocytes. By their amceboid move- 
ment the leucocytes migrate from the interalveolar con- 
nective tissue into the interior of the alveoli, passing 
between the epithelial cells; and the same movement 
enables them to take up fat droplets and carry them 
