Breasts are remarkable organs. In women, they have evolved over millions of years for the function of lactation (milk production) and to serve as a secondary sexual characteristic.

Each breast has 15 to 20 sections, called lobes, which are arranged like the petals of a daisy. Each lobe has many smaller lobules, which end in dozens of tiny bulbs that can produce milk. The lobes, lobules, and bulbs are all linked by thin tubes called ducts. Fat (or adipose tissue) fills the spaces between lobules and ducts. These ducts lead to the nipple in the center of a dark area of skin called the areola. There are no muscles in the breast, but muscles lie under each breast and cover the ribs.

Each breast also contains blood vessels and vessels that carry lymph, a complex fluid that has important immune functions. The lymph vessels lead to small bean-shaped organs called lymph nodes, clusters of which are found under the arm, above the collarbone, and in the chest, as well as in many other parts of the body.

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