BACTERIA OF MILK 



77 



represents 500,000 per cubic centimeter if the bacteria are 

 evenly distributed. As a matter of fact they are not uniformly 

 distributed, and it is necessary to count a number of fields for 

 accurate results. The total number of bacteria found in ten 

 fields, multiplied by 50,000, or the total number in 100 fields, 

 multiplied by 5000, gives the total number of bacteria per cubic 

 centimeter. 



The 



Exercise 112. The Examination of Milk for Body Cells 



The microscopical examination of milk sediment often reveals 

 the presence of leucocytes in greater or less numbers, 

 presence of these cells has 

 been regarded as important, 

 because it was assumed that 

 they showed the presence of 

 inflammation and pus forma- 

 tion in the cow's udder. Later 

 work, however, has cast con- 

 siderable doubt upon this as- 

 sumption. As a matter of fact 

 our methods do not enable us 

 to distinguish between leuco- 

 cytes and pus cells in the milk. 

 In some cities milk may not F i< ; .82. Field of the Thoma-Zeiss 

 be sold which contains more blood counter 



than 500,000 cells per cubic 

 centimeter. 



1. Fill 10-cc. centrifuge 



tubes with milk and heat to 70-75 C. for ten minutes. Shake 

 well to break up aggregates of fat globules. 



2. Centrifuge the tubes at high speed for ten minutes. Re- 

 move the upper layers of cream and milk with a pipette and 

 refill the tubes with distilled water. Centrifuge again for three 

 or four minutes. 



3. Draw off all except J- cc. of liquid in the point of the 

 centrifuge tube. Wipe out the upper part of the tube with a bit 

 of absorbent cotton fastened to a glass rod with a rubber band. 



The area of all the squares is 1 sq. mm. 

 Each of the 400 squares represents a vol- 

 ume of Woo cu. mm. 



