Breast anatomy. Artwork of a vertical cross-section through a female breast seen from the side. At lefht are the ribs are the pectoral muscles, which support the breast. The mammary gland (pink) is divided into many lobules. These contain cells that secrete milk to feed new-born babies and infants. The milk is carried to the nipple (far right) by milk, or lactiferous, ducts. The majority of the breast is composed of fatty (adipose) tissue.