The effect of salts on cell permeability as shown by studies of milk secretion—(continued)

Abstract

It has been shown by Peskett (1933) that in a group of cows of the same breed certain of the differences in milk composition which are associated with different animals at the same time, or with the same animal at different times, can be correlated to some extent with variations in blood composition. In these experiments evidence of correlation between the ratio of solids-not-fat to fat in milk and that of sodium to diffusible calcium in blood was obtained. It was suggested, too, that the variations in milk composition may occur as a result of alterations in the relative permeability of the membranes of the mammary cells to lipoids and non-lipoids, caused by changes in the proportions of those salts in the blood. In keeping with the approximate constancy in milk composition that is generally observed among cows of the same breed, the variability of the blood and milk values was small in these experiments which were confined to cows of the Dairy Shorthorn breed. With cows representative of a number of breeds, wider variations in milk composition are to be expected. In the following experiments we have extended our observations to include Ayrshire, British Friesian, and Guernsey cows in addition to Dairy Shotthorns. Of these, Guernseys and British Friesians occupy extreme positions as regards differences in milk composition, the former yielding milk very much richer in fat than the latter, while Ayrshires and Dairy Shorthorns are placed intermediately. The fat variations are associated with comparatively small differences in solids-not-far content. In studying these different breeds we hoped, therefore, to provide some explanation of the marked differences in milk composition occurring between them, whilst confirming the earlier work under a wider range of conditions.