40 UNIVERSITY OF WISCONSIN STUDIES 
the district nurse system of instructing mothers in uniform 
and regular feeding of babies suggests that babies respond 
similarly to like physiological stimuli. In the feeding of calves 
it is observed that calves like the human infant thrive when 
fed uniformly upon sweet milk or when fed uniformly upon 
sour milk but that diarrhea appears promptly when a system 
of feeding sweet milk is varied by an occasional feeding of 
sour milk. While frankly admitting the danger of reasoning 
by analogy, the similarity of the factors observed and of the 
results obtained in the feeding of calves and of babies strongly 
suggests that in both cases diarrheal conditions result from 
sudden changes in the acid reaction of the diet. If this rea- 
soning is correct, a satisfactory supply of baby milk is char- 
acterized among other things by a constant acid reaction. 
Under ordinary conditions the reaction which will be most 
satisfactory is approximate neutrality. 
If the above explanation of the evil results attending the 
use of occasional samples of partially sour milk is accepted, it © 
offers an excellent reason for insisting that the public milk 
supply shall have such a keeping quality as to maintain an 
essentially unchanged reaction during the time which would 
ordinarily elapse before its consumption. 
Since the increase in acid reaction of milk is due to the 
growth of bacteria, the problems of keeping quality are prob- 
lems of limiting the entrance and activity of germ life and 
of measuring the activity of the germs which enter. 
The work of Dr. Russell, above referred to,2° is one of the 
earliest inquiries as to the source of the bacteria which enter 
the milk and this type of inquiry has been continued by a 
number of his students. In connection with this series of 
studies it has developed that the barn and barn conditions 
exert surprisingly little influence upon the germ content of 
milk.*° While in a few instances the udder flora heavily 
See footnote 25. 
HH. A. Harding, G. L. Ruehle, J. K. Wilson and G. A. Smith, The Effect 
of Certain Dairy Operations upon the Germ Content of the Milk, Bulletin 
365, N. Y. Asr. Exp. Sta., 1913. , 
M. J. Prucha and H. M. Weeter, Germ Content of Milk as Influenced 
by the Factors at the Barn, Bulletin 199, Tl, Agr. Exp. Sta t9i7. 
i 
