For ten lunar months, not another ovum breaks through the covering of the ovary, and the uterus is left undisturbed.  The placental secretion plays a most important role as brake upon the post-pituitary, the most active of the feminizing uterus-disturbing endocrines.  Until at last something happens that puts the placenta out of commission in this function of restraint, and the long bottled up post-pituitary secretion explodes the crisis apparent as the process of labor.

A condition of self-poisoning often occurs in pregnancy, with symptoms orchestrating from mild notes like nausea and vomiting to the high keys of convulsions and insanities.  They represent what happens when an unbalanced endocrine system is attacked by the placenta.  Depending upon where in the internal secretion chain the weak point, the Achilles’ heel spot, will be found, the nature of the reaction will vary.  And even after labor, after the explosive crisis, so much of the reserve endocrine materials may be consumed, that an actual mania or a chronic weakness may come in its wake.

Yet the placental secretion must not be looked upon as something wholly evil in its potentialities.  Without enough of it to hold the uterus stimulating endocrines, particularly the post-pituitary, in check, still-birth results.  If there is enough, and not too much of it, the woman will not feel ill at all, or perhaps only transiently, but will be possessed of a curious feeling of drowsy content and passive, relaxed happiness.  Let there be relatively too much of it, too little of the other glands, and the grosser transfigurations and ailments of the child-bearing period follow.

THE MAMMARY GLANDS

Once pregnancy is terminated by labor, the placenta is expelled from the body as the after-birth.  The placenta removed, a new arrangement of the balance of power among the endocrines becomes necessary.  But a new-comer appears upon the scene to take up the function left vacant by the absent placenta.  This new-comer is the secretion of the activated breasts, the mammary glands.  They make for a persistence of the state of equilibrium among the endocrines attained during pregnancy.

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.