PRINCIPLES AND PRACTICE OF MILK HYGIENE 



which are peculiar to milk. They also take from th 

 blood, water and other substances which are commo 

 to milk and blood. In histological sections, the milk fa 

 may be observed within the actively secreting cells i 

 the form of small fat droplets ( Fig. 1 ) . The other cor 

 stituents of milk, being without form, cannot be seer 

 but their presence in the cells is indicated by a granula 

 or turbid condition of the protoplasm. When the cell 

 of an alveolus become filled with secretion, the cell-prc 

 toplasm contracts and the contents is discharged into th 

 lumen of the alveolus, after which secretion again begin 

 The different stages of cell activity do not occur in all c 

 the alveoli at the same time; different alveoli in the sam 

 lobule may show various stages. Secretion continue 

 until the cells are exhausted, and it then ceases unt 

 the cells can recuperate. 



In the course of time, after repeated periods of actii 

 ity, exhaustion and rest, the epithelial cells of the alveo 

 are worn out and secretion stops in one alveolus aft* 

 another. This change is called involution. In youn 

 cows the worn-out cells are regenerated. The alveo 

 then present the same appearance microscopically i 

 is observed at the time of parturition (see Fig. 2) . Tl 

 interalveolar connective tissue is increased in volume an 

 contains many leucocytes. Leucocytes are also numei 

 ous in the alveoli, together with exfoliated epithelii 

 cells, free nuclei, parts of disintegrated cells, fat globule 

 and coagulated casein. Fat droplets may be observe 

 within some of the leucocytes. By their amoeboid movi 

 ment the leucocytes migrate from the interalveolar coi 

 nective tissue into the interior of the alveoli, passin 

 between the epithelial cells; and the same movemei 

 enables them to take up fat droplets and carry the] 



