34 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



0.54 to 0.57 C. (31.02 to 30.9 F.) . It varies with 

 the amount of the dissolved substances contained in the 

 milk, especially the salts. When water is added to milk 

 the freezing point rises, while in disease it is sometimes 

 lowered and sometimes raised. The determination of 

 the freezing point has as yet proven of no practical value 

 in routine milk examination. Surface tension and elec- 

 trical conductivity are likewise of no practical impor- 

 tance. 



MICROSCOPICAL APPEARANCE OF MILK AND MILK SEDIMENT 



When examined under the microscope, milk is found 

 to contain numerous fat globules with a few cells, cell 

 fragments, and free nuclei scattered among them. If 

 a small quantity of milk is placed in a sediment tube and 

 centrifugalized, only a part of the cells and cell remnants 

 are thrown down to the bottom of the tube with the 

 heavier constituents of the milk; many adhere to the 

 fat globules and are carried to the top, while the others 

 remain in the intermediate fluid. According to Prescott 

 and Breed, only about one-fourth is contained in the 

 sediment, one-half being in the cream and the remainder 

 in the milk. Heating the milk to 60 C. (140 F.) or 

 above before centrifugalizing will increase the cellular 

 content of the sediment. If some of the sediment is 

 spread out in a thin layer on a glass slide, dried in the 

 air, fixed by heating, and stained, the cellular bodies can 

 be more readily studied. 



Cellular Content. It will then be observed that the 

 cells are of two principal kinds: leucocytes and epithe- 

 lial cells. The leucocytes are of the polymorphonuclear 

 and lymphocyte varieties, while the epithelial cells are 

 of the pavement, cuboidal and cylindrical types. Fre- 



