W. Taylor and A. D. Husband 123 



Vau der Laan(9) shows that a state of osmotic equihbrium exists 

 between the blood and the milk, and that this equahtv of osmotic pressure 

 persists even in diseases of the udder. As lactose is the substance which 

 is likely to play the most considerable part in determining the osmotic 

 pressure in the milk, the osmotic pressure in the blood is likely to be 

 expressed to a very great degree by the percentage in which this con- 

 stituent is present in the milk ; and as, normally, the osmotic pressure 

 in the blood must be considered to be a constant, with but slight varia- 

 tions from day to day, it is to be expected that an equal constancy 

 will prevail in the percentage in which the lactose is present in the milk. 

 It has been found that, while the percentage of lactose tends to vary 

 directly as the daily volume, at the height of lactation these variations 

 are normally so shght that an approximately constant percentage is 

 maintained. The conclusion to be drawn is not that the lactose is 

 elaborated till it reaches this practically constant percentage, but that 

 it is kept down to this constant level by the rate of secretion of the milk. 

 The suggested theory of secretion is, therefore, that the elaboration of 

 lactose produces a flow of milk by a process of osmosis. This would explain 

 why. at the height of lactation, the percentage of lactose varies but 

 slightly and tends to do so directly as the daily volume, at the same time 

 reducing the percentages of all the other constituents of the milk by 

 a process of dilution — hence their inverse percentage relationship to 

 volume. 



coxcLrsioxs. 



1. The percentage composition of the milk seems to be determined 

 by its rate of secretion. 



2. The percentages of protein, fat and ash vary inversely, and the 

 percentage of lactose varies directly, as the daily volume, the greatest 

 variation being .shown by the fat and the least by the inorganic elements. 



!5. There is an inverse relationship between the percentage of lactose 

 and the percentages of all the other constituents of the milk, this being 

 particidarly apparent in the case of the fat. 



4. Diet has no direct influence on the percentage composition of the 

 milk, except in the case of the non-protein' nitrogen which is not a product 

 of the mammary gland. Diet has, however, an indirect influence by 

 reason of its efiect on the daily volume. 



5. A high protein diet would appear to stimulate the rate of secretion 

 of the milk. 



