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Cow's milk is more opaque than woman’s milk, although woman’s 
milk may contain a greater percentage of fat. This is due to the 
greater content of calcium salts in cow’s milk by reason of its greater 
proportion of casein. 
In reaction cow’s milk, though slightly alkaline or amphoteric 
when freshly drawn, soon becomes somewhat acid, while woman’s 
milk is amphoteric or alkaline. 
As there is very little difference in total solids between the two, 
their specific gravity is about the same. 
The sugar of cow’s milk and woman’s milk is lactose in complete 
solution. They differ, however, in quantity, as woman’s milk con- 
tains 6 to 7 per cent, while cow’s milk has usually 4.5 per cent. The 
greater part of the fat in cow’s milk is neutral fat, as in woman’s 
milk; but cow’s milk contains far greater quantities of the volatile 
fatty acids, of which there are but traces in woman’s milk. It is 
in the proteids that the chief difference between cow’s milk and 
woman’s milk is manifest. Cow’s milk has on the average 3.50 per 
cent of proteid to 1.50 per cent in woman’s milk. The reason for this 
difference is obvious. As all growth is dependent upon proteid ma- 
terial, and animals are unable to build up proteids within themselves 
from the nonnitrogenous portions of their diet, they are dependent 
for their supply upon the ingestion of proteid in their food. As the 
ratio of growth of the calf compared to the infant is about as 2 is to 1, 
it follows that the calf requires relatively twice as much proteid as 
does the infant. Moreover, owing to differences in their respective 
digestive tracts, the proteid in cow’s milk is of a different composition 
from that of woman’s milk. In the human being the stomach forms 
20 per cent of the intestinal tube and digestion is chiefly intestinal. 
In the cow the’ stomach forms 70 per cent of the digestive tract and 
digestion is chiefly gastric. Under the action of the active rennet 
ferment present in the stomach of the calf, cow’s milk forms a large 
curd which remains in the calf’s stomach until digestion is complete. 
In the infant the soft, flocculent curd of mother’s milk is adapted 
to easy transit from the stomach to the duodenum, and it is alto- 
gether likely that a portion of the milk ingested does so pass out 
before the nursing is finished. 
In view of the foregoing, as casein is the curding proteid in milks, 
we should expect to find, as is actually the case, that the proteid of 
cow’s milk is richer in casein than the proteid of human milk. Koenig 
gives the following, composition of the proteids in cow’s milk and 
human milk : 
