Interrelationships of constituents and partition of salts in milk samples from eight species.

Skim-milk samples from rat, rabbit, pig, sheep, goat, horse and man were analysed for lactose, casein and the total and ultrafilterable concentrations of the main salts. Results are compared with data for the cow. The ultrafiltrate concentrations of Ca and Mg were positively correlated with that of citrate and the colloidal concentrations of Ca, Mg and citrate were positively correlated with that of Pi, suggesting that common, general, principles determine the partition of salts in milks. Casein concentration in the skim-milks was inversely related to that of lactose in accordance with a recent theoretical treatment of the principal mechanism of milk secretion. It is postulated that much of the interspecific diversity in milk composition can be explained by adaptations in a single secretory mechanism.