The milk is produced deep to the surface in the milk-producing alveoli. These alveoli lead via series of branching ducts to a main milk duct which opens at the nipple. There are thus 15 openings at the nipple, each leading to its own milk producing alveoli.
The main milk duct has a dilatation just below the surface which is known as the lactiferous sinus in which some milk is stored.
Baby can take milk from this sinus after taking the nipple and surrounding tissue into its mouth and using a sucking champing action. But the baby cannot by this action obtain milk from the deeper milk-producing alveoli.
However, the suckling stimulates the sensory receptors present in the nipple and a nervous reflex via the hypothalamus releases oxytocin from the posterior pituitary gland.
The oxytocin contracts the myoepithelium surrounding the alveoli and forces the milk forwards towards the nipple.
This process is termed milk ejection without an adequate level of circulating oxytocin, the breast may be engorged with milk, but the baby cannot get an adequate milk supply.