Pectoralis Major Muscle

Description

Deep to the skin and subcutaneous adipose tissue, the mammary gland parenchyma appears as lobules that drain via lactiferous ducts toward the nipple-areolar complex, with fibrous connective tissue septa (suspensory ligaments of Cooper) extending from the dermis to the gland. Posteriorly, the retromammary space separates the breast from the pectoralis major muscle, which forms the muscular bed across the anterior thoracic wall. Medially the gland approaches the parasternal region, while laterally the tissue tapers toward the axillary tail where lymphatic channels track toward the axilla. Seeing the pectoralis major in cross-section matters because it defines the posterior margin of breast tissue and the plane surgeons exploit during mastectomy, implant placement, and tissue expander positioning. Tumors that abut or breach the retromammary space and invade pectoralis major change staging and operative planning, and this relationship is also central to interpreting chest wall involvement on mammography, ultrasound, and breast MRI. Lymphatic anatomy at the lateral breast explains why sentinel lymph node mapping targets axillary drainage. A practical teaching landmark. Use this illustration for gross anatomy and breast imaging curricula to clarify glandular versus fatty composition, ductal convergence at the nipple, and the deep interface between breast and pectoralis major. It also fits surgical oncology and plastic surgery materials discussing modified radical mastectomy planes, subpectoral versus prepectoral reconstruction, and patterns of axillary nodal spread. Anatomical accuracy verified by SciePro's Medical Advisory Board.