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The breast is an organ that sits on top of the upper ribs and chest muscles. There is a left and right breast and each one has mainly glands, ducts, and fatty tissue. The amount of fatty tissue in the breast determines the size of each breast.

Key parts of breasts include the:

• Lobes and lobules
• Ducts and nipple
• Fat and connective tissue
• Lymph vessels and nodes

Each breast contains 15-20 lobes. Each lobe is composed of many lobules, at the end of which are sacs where milk is produced in response to hormonal signals.

Ducts are small canals that come out from the lobules and carry the milk to the nipple.

The nipple is the opening in the skin of the breast where the ducts come together and turn into larger ducts so the milk can leave the breast. The nipple is surrounded by slightly darker thicker skin called the areola.

The fat and connective tissue (stroma) surround the ducts and lobules and help keep them in place.

The breast has a rich supply of lymph vessels and lymph nodes. Most of the lymph vessels of the breast drain into:

• Lymph nodes under the arm (axillary nodes)
• Lymph nodes above (supraclavicular) and below (infraclavicular) the collar bone 
• Lymph nodes inside the chest near the breastbone (internal mammary lymph nodes)