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