116 EXPERIMENTAL DAIRY BACTERIOLOGY 



after twenty-four hours' incubation for chains of cocci, by 

 preparing stained preparations. 



The student should become familiar with the morphologi- 

 cal, cultural, and physiological characteristics of Streptococcus 

 pyogencs and Staplilococcus pyogenes aureus, the common yel- 

 low pus organism. 



Microscopical examination of milk. In the milk of all 

 cows is to be found a greater or less number of cellular ele- 

 ments, such as colostrum cells, epithelial cells, and white 

 blood corpuscles. Whenever there is any inflammation in 

 the udder, whether the cause is the presence of pyogenic bac- 

 teria or a physiological disturbance such as the influence of 

 cold, bruising, etc., the number of leucocytes is greatly in- 

 creased. Considerable significance is attached to the pres- 

 ence of large numbers of these cells. The milk drawn from 

 perfectly healthy animals varies so widely in content of 

 leucocytes that it is difficult to establish a standard and to 

 determine what significance they possess. The milk of nor- 

 mal, healthy cows may contain from a few hundred leucocytes 

 to several million per cubic centimeter. In many cities a limit 

 of 500,000 leucocytes per cubic centimeter has been adopted. 

 Milk containing a greater number than this is looked upon as 

 containing pus. While the milk of individual animals in a 

 perfect state of health may frequently exceed this number, 

 it is certain that the mixed milk from a herd of considerable 

 size (twelve animals or more) will not show a higher number 

 unless one or more animals have udder trouble. Standards 

 of this character should, however, be regarded as suggestive 

 rather than as final. Where negative findings are determined 

 they may be taken as indicating a normal, healthy condition ; 

 where results show an excess of leucocytes, it should lead to 

 a careful physical examination of the herd. 



