BY H S. HALCRO WARDLAW. 821 



Gaines (1915) demonstrated, however, that pituitary extract exerts 

 its effect by stimulating the smooth musculature of the mammary 

 gland, and leads to an increased extrusion, but not to an increased 

 secretion of milk (see also Mackenzie, 1911), since the volume of 

 milk produced under its influence is no greater than the decrease 

 of the volume of the udder. 



The effect of the seasons of the year, on the mean composition of 

 cows' milk in Great Britain, is well shown by the monthly aver- 

 ages which are published, from year to year, by Droop Richmontl 

 (1887 onwards). The milk secreted during the winter-months has 

 slightly higher percentages of fat, and of solitls not fat, than the 

 milk secreted during the summer-months. The work of Lythgoe 

 (1914) has shown that a similar effect is exerted in North 

 America, and that the behaviour of the percentage of solids not 

 fat is due chiefly to variation of the percentage of protein. 



Seasons, in the sense of good and bad seasons, of course, have a 

 very marked effect on the milk-production of a cow, but this eifect 

 is an indirect one, and is due to variations in the quality and 

 quantity of the food-supply of the animals. 



The effect of the course of lactation on the composition of cows' 

 milk has been studied in some detail. The greatest changes occur, 

 as is well known, during the period immediately after calving. 

 Colostrum, the milk secreted during the first day or two of lacta- 

 tion, contains a much larger percentage of protein than ripe milk. 

 The extra protein, which is present in colostrum, has been shown 

 by Crowther and RaLstrick (1916) to be a globulin, identical with 

 tlie globulin of the blood-plasma of the cow. In ripe milk, the 

 amount of this globulin is very small (0'03%). 



After the first week of lactation, the composition of the milk 

 does not vary rapidly in any one direction, although it is subject 

 to daily fluctuations. No precise generalisation can be formulated 

 for the variation of composition during lactation. Recent work 

 shows, however, that, on the whole, the average percentages of fat, 

 protein and ash fall during the first three to six weeks of the post- 

 colostral period, vary about constant values during the greater 



