Women’s breasts are made up of fat, nipple, glands (alveoli) and a network of ducts through which milk can pass from the glands to the nipples.
Each breast contains between 15 and 20 sections called lobes, each of which is composed of many smaller structures known as glands or alveoli. These glands produce milk. A system of small tubes known as ducts transports milk from the glands to a big central duct that has multiple openings in the nipple. A central duct opens into the nipple from each lobe.
A band of muscle surrounds each gland. This band can contract (squeeze), forcing the milk out of the glands, into the ducts and through to pools that lie beneath the areola, the pink/brown circle that surrounds the nipple. Eventually, a sucking baby extracts the milk by pressing and pumping it out from these pools through the nipple.
The spaces around the lobes and ducts are filled with fatty tissue and ligaments. The size of a non-lactating breast is largely determined by the amount of fat it contains.
Underneath the breasts there is fibrous tissue and muscle. The pectoral muscle passes underneath the breast and connects the chest and the arm. Lying further below the pectoral muscle are the ribs which are connected by intercostal muscles, which raise and lower the rib cage when breathing in and out.
Deep beyond the ribs is the pleural lining, a thin, moist membrane that lines the chest cavity.