The breast is mainly composed of glands, ducts, fatty tissue and fibrous tissue. Its internal structure is like a small tree growing upside down. The breast gland consists of 15 to 20 lobes, each of which is divided into several lobules, each of which is composed of 10 to 100 vesicles. These vesicles are arranged closely around the ducts, and the openings of the vesicles are connected to the ducts. The ducts converge to form interlobular ducts, and the ducts converge to form a duct for the entire lobe, also known as the milk duct. There are 15 to 20 milk ducts, which are arranged in a radial pattern with the nipple as the center, converging on the areola and opening at the nipple, called the milk ducts. The milk ducts are narrower at the nipple and then expand into a pot belly called the sinus of the milk duct, which has the role of storing milk. The opening of the ducts is lined with squamous epithelial cells and the narrowing is lined with migrating epithelium, while the ducts below the pot belly are lined with double-layered columnar epithelium or single-layered columnar epithelium.