VARIATIONS IN COW'S MILK 27 



in different degrees, the fat content varying the most, 

 the albumin, lactose and ash ingredients less. In 

 ' ' rich ' ' milk the fat content often considerably exceeds 

 the average, while the proportions of protein and lac- 

 tose are never much higher than the averages given. 

 On the other hand, with thin, poor milk the fat per cent, 

 may fall far below the average, while the proportions of 

 protein and lactose depart but slightly from the average. 

 These variations are not due merely to the elimination 

 of water from the milk, that is, to a simple concentration, 

 but rather to a qualitative variation in the activities of 

 the gland cells. 



Just as is the case with other glands, the milk 

 glands may be stimulated to greater activity, so that the 

 quantity of secretion increases, but its chemical compo- 

 sition is not disturbed, or only slightly, or temporarily, 

 until the gland tissue has accustomed itself to the 

 new condition. Drinking a large quantity of water 

 causes no increase in milk secretion; the gland cells do 

 not secrete more water than the quantity required to 

 maintain the proper proportion to the milk solids. Food 

 containing much water has no permanent effect upon 

 the composition and the quantity of the milk. The 

 influence of such materials as pilocarpine and atropine, 

 which increase or reduce other secretions in a great 

 degree, have no direct, or only a very slight influence 

 on the milk secretion, apparently because this is con- 

 trolled by the sympathetic nerve centres. The various 

 conditions that have, or are believed to have, an influence 

 upon the chemical composition, and especially upon the 

 fat content of milk, are considered in the following para- 

 graphs. 



1. Breed peculiarities. Cows of different breeds 

 give milk of somewhat different composition. In gen- 

 eral, it is believed that breeds from the highlands and 



