The mammary glands are typical glands of external secretion. They make the milk and pour it out of the breasts through little canals into the mouth of the suckling. Yet evidence forces us to conclude that they are also glands of internal secretion, that their internal secretion substitutes to a certain extent for the loss of that of the placenta but not quite. 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. We know that injection of corpus luteum will cause an hypertrophy of the breasts. The same effect is produced regularly during the menstrual period, with a consciousness of swelling of the breasts. Their atrophy at the menopause coincides with the shrinkage of the ovaries that takes place at that period. Activity of the breasts parallels indeed more or less the activity of the corpus luteum. With the prolonged activity of the corpus luteum during pregnancy, prolonged stimulation of the breasts occurs. The secretion of the post-pituitary would now cause the change from the internal cell secretion to milk. But it is inhibited from so doing by the placenta. When the placenta is removed, after labor, the post-pituitary can act, and a free flow of milk is established. However, to counterbalance this, and to prevent the post-pituitary from overacting, the breasts secrete a hormone with an action like that of placenta, but not so strong, which tends to inhibit the ovary. So is put off the imposition of a pregnancy upon a period of lactation, obviously bad for mother, infant, and embryo. We have here an exquisite sample of the checks and compensations which make for a self-balancing of the whole endocrine system. CRITICAL AGES The Dangerous Age is a phrase coined by a Scandinavian writer as a more dramatic euphemism for the time of life when sex function ceases, the climacteric. As a matter of fact, the age of adolescence is just as much of a dangerous age as the age of deliquescence. The only difference between them is that the dangers of the one have been hushed up, the dangers of the other well boomed and advertised. Both are dangerous to the individual, because both are periods of instability and readjustment of the cells, particularly the brain cells, to a deranged endocrine system and blood chemistry.