304 THE EXAMINATION OF MILK 



Wash, dry and mount in Canada Balsam. 



(a) Examine with a J in. objective and No. 4 eye piece and 

 count the number of leucocytes in ten separate fields (Fig. 49) ; 

 calculate the average. 



(b) Note the number and forms of bacteria present. Are 

 chains of streptococci abundant ? 



(c) Prepare another film and after treatment with ether and 

 alcohol, examine for presence of acid-fast organisms (see p. 325). 

 Stain for several minutes with hot carbol-fuchsin, wash and de- 

 colourize with 25 per cent, sulphuric acid and counterstain for 

 ten seconds with methylene blue. 



Wash, dry, and mount in Canada Balsam, and examine with 

 T a 2 in. objective. 



3. Bacteria : number and kind. In addition to 

 blood, pus-cells and inert substances, the milk contains 

 various kinds of living bacteria, some of them derived 

 from the interior of the udder, others being introduced in 

 the particles of dung, fodder and other materials already 

 enumerated. 



The number of bacteria present at the time of delivery 

 to the consumer is partly controlled by the amount of 

 the initial contamination but is also dependent upon the 

 temperature at which the milk has been kept, and the 

 time which has elapsed since it was first drawn from 

 the cow. 



The fixation of a standard based upon the number 

 of organisms is therefore difficult, and is of less moment 

 than the examination of milk in regard to the kinds 

 which it contains. Some of them render milk acid very 

 rapidly, or otherwise alter it so that it soon becomes 

 unfit for domestic use : others may set up decompositions 

 which result in the formation of poisonous compounds 

 or cause disease, An abundance of Streptococcus lacticus 



