78 A MANUAL OF BACTERIOLOGY 



Mix thoroughly the remaining sediment and liquid with the 

 rounded end of the glass rod. 



4. Obtain a clean Thoma-Zeiss blood corpuscle counting cell. 

 The cell represents a volume of 0.1 cu. mm., and each square 

 represents --$$-$ cu. mm. With a rod transfer a drop of thor- 

 oughly mixed milk and sediment to the counting cell and cover 

 with a clean cover glass. Place the cell under a J objective of 

 good working distance. 



If the number of leucocytes is low, the entire area of the cell 

 may be counted, using a mechanical stage to move the slide. If 

 their number is large, five or six small squares may be counted 

 and averaged. The average number per small square, multiplied 

 by 200,000, gives the number of leucocytes per cubic centimeter 

 in the original milk. 



Exercise 113. A Direct Microscopical Method for Determining the Number 

 of Body Cells in Milk 



Recent work lias cast some doubt upon the accuracy of the 

 method using the sediment from centrifuge tubes (see Prescott 

 and Breed. Jour. Infect. Dis. 7 : 632. IdlO). 



The direct method is the same as that employed for counting 

 of bacteria described in Exercise 111, except that the film should 

 be somewhat overstained with methylene blue and decolorized 

 with alcohol. 



Exercise 114. The Germicidal Action of Fresh Milk 



The freshly drawn milk of some, if not of all, cows shows a 

 distinct decrease in the total germ content during the first few 

 hours. Part of the decrease in numbers may be due to the fact 

 that many bacteria fall into milk which are not able to live there, 

 and so perish and pass out of sight. Well-controlled inocula- 

 tion experiments, however, have shown that milk may have dis- 

 tinct germicidal power for seven to eighteen hours after it is 

 drawn (see Lohnis, Handbuch d. landw. Bakteriologie, p. 144. 

 Berlin, 1911), but this is not always easy to demonstrate. 



1. Procure in sterile flasks samples of freshly drawn milk 

 from several cows. Keep the milk samples at 20 C. 



