Jan. 20, 1919 Variations and Mode of Secretion of Milk Solids 95 



mammary gland of a Holstein-Friesian cow showed that they were quite 

 small. Considered in the light of this small size, it is likely that obser- 

 vations on the distal end of a cell might be called by one observer the 

 destruction of this portion and by another the cell in its natural 

 shape. This explanation of the confusion in interpretation of obser- 

 vations in these cells is made further probable by the change of shape 

 which cells undergo at different stages of lactation. Thus, in the mam- 

 mary gland of a bitch when just emptied, Heidenhain says that the 

 cells were high and columnar and in another bitch where milk had not 

 been drawn for 48 hours the cells were flat. The weakness of the his- 

 tological evidence is obvious. To the final solution of the problem it 

 appears that other evidence beside the histological observations must 

 be presented. 



Some evidence from the physiological side has been presented in the 

 foregoing pages of the interaction of the four variables, age, quantity 

 of milk secreted, diurnal variations, and the effect of the content of one 

 solid on the relation of the other variable, butter fat, or solids-not-fat. 

 These variables give criteria to the efficacy of the three contending 

 hypotheses to explain the release of the milk solids. 



Consideration of the manner of metabolism and energy requirement for 

 most bodily functions seems to furnish the explanation of the slight 

 negative correlation of butter-fat concentration with the amount of milk 

 produced, where no such correlation exists with amount of milk pro- 

 duction and the other solids. Energy has been shown to be required for 

 most bodily functions. There does not seem to be any reason to suppose 

 that the mammary gland is any exception to this rule when milk is pro- 

 duced. Such energy requirement would of necessity be dependent on 

 the amount of production — that is, the high producer will require more 

 energy and to produce "this energy will consequently take a slightly 

 greater amount of fat that might have gone into the milk. The other 

 solids would not be required to furnish any of this energy and conse- 

 quently would show no effect of the amount of milk produced on their 

 concentration. The correlations obtained show the associations that 

 would be expected with this explanation. The conclusion seems justi- 

 fiable that the energy required in the production of milk causes a slight 

 reduction in the amount of fat present in the milk of high-producing 

 cows. 



The maintenance of the fat concentration of the milk throughout life 

 and the decline of the solids-not-fat apparently represent the normal 

 conditions going on throughout the whole body. It is common knowl- 

 edge that increase in age generally brings with it a relative decrease in 

 the protein upbuild of the body and an increase in the fat. This in- 

 creased metabolism of fat appears to extend itself to the mammary 

 gland, as well as to other parts of the body, being just great enough to 

 maintain the butter-fat concentration throughout life, whereas the 



