The mammary glands are typical glands of external secretion.
"The Glands Regulating Personality"
Louis Berman, M.D.
What seems to happen in fact, is this: the corpus luteum secretion stimulates the dormant cells of the mammary glands, formed during puberty, but latent until the advent of pregnancy.
"The Glands Regulating Personality"
Louis Berman, M.D.
In heavy milkers, before and just after calving, it is the rule that the mammary gland is enlarged, hot, tense, and tender, and that a slight exudation or pasty swelling extends forward from the gland on the lower surface of the abdomen.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler