The progress of physiologic chemistry after the middle of the last century contributed indisputable proof of the untenability of the theory of milk secretion which prevailed prior to 1850. At that time it was assumed that the function of the mammary gland, as well as that of other secretory structures, involved essentially a filtering out from the blood of those substances that constitute the respective secretions. For certain components, such as the inorganic salts and water, an explanation of this sort seemed plausible. But as milk contains lactose, a sugar that is not present in the circulating fluid which perfuses the mammary gland, it became evident that the old theory of filtration must be given up.
To account for the appearance of specific products like lactose and casein in milk, when they are found nowhere else in the body, other secretory hypotheses were formulated. According to the oldest of these,