Lymphatic drainage of the breast showing lymph node groups and levels. 1. Internal mammary artery and vein; 2. Substernal cross-drainage to contralateral internal mammary lymphatic chain; 3. Subclavius muscle and Halsted ligament; 4. Lateral pectoral nerve (from the lateral cord); 5. Pectoral branch from thoracoacromial vein; 6. Pectoralis minor muscle; 7. Pectoralis major muscle; 8. Lateral thoracic vein; 9. Medial pectoral nerve (from the medial cord); 10. Pectoralis minor muscle; 11. Median nerve; 12. Subscapular vein; 13. Thoracodorsal vein; A. Internal mammary lymph nodes; B. Apical lymph nodes; C. Interpectoral (Rotter) lymph nodes; D. Axillary vein lymph nodes; E. Central lymph nodes; F. Scapular lymph nodes; G. External mammary lymph nodes; Level I lymph nodes: lateral to lateral border of pectoralis minor muscle; Level II lymph nodes: behind pectoralis minor muscle; Level III lymph nodes: medial to medial border of pectoralis minor muscle.