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The regional anatomy of the breast is illustrated in FIGURES 1 and 2. The principal blood supply to the breast comes from the medial perforating branches of the internal mammary artery and vein after they transverse the pectoralis major muscle and its anterior investing fascia. The medial aspect of the breast has lymphatic drainage into the internal mammary chain of lymph nodes within the chest, but this is quite variable. Most of the lymphatics from the breast drain to the axillary lymph node basin. The most proximal node or nodes may be located in atypical locations such as within the breast in the axillary tail of the upper/outer quadrant or very low on the lateral chest wall. Identification of these nodes using radionuclide tags and blue dye localization techniques is one of the additional benefits of a sentinel lymph node dissection. Axillary lymph nodes have been grouped into three levels defined by their anatomic relationship to the pectoralis minor muscle (FIGURE 2). Level I nodes are defined as those lateral to the edge of the pectoralis minor muscle. This area includes the external mammary, subscapular, and lateral axillary nodes. Level II nodes are behind or posterior to the muscle and are commonly defined as the central axillary lymph nodes. Level III nodes are located medial or superior to the pectoralis minor muscle. This group includes the subclavicular or apical lymph nodes. They reside in the apex of the axillary space behind the clavicle and deep to the axillary vein. In general, level I and II nodes are removed in axillary lymph node dissections. The overall boundaries of this standard axillary lymph node dissection are the chest wall (serratus anterior muscle) medially, axillary vein superiorly, subscapularis muscle plus thoracodorsal and long thoracic nerves posteriorly, and latissimus dorsi laterally.

The axillary vein is the major structure defining the superior border of the surgical dissection. The axillary artery (posterior and pulsatile) and the brachial plexus (superior and solid) are palpable but not exposed. Common regional findings are dual axillary veins or a very large, long thoracic vein running longitudinally along the lateral chest. After the axillary vein is exposed by the surgeon, a key landmark aids in finding the thoracodorsal nerve, which is deep on the subscapularis muscle. A pair of subscapular veins is identified (FIGURE 1). The more superficial one is divided, revealing the deep subscapular vein and the adjacent subscapular artery, which may be mistaken for the thoracodorsal nerve. This nerve, however, is posterior to the axillary vein and medial to the deep subscapular vein. It tends to angle toward the deep subscapular vein, whereas the subscapular artery is more parallel. A gentle mechanical stimulation of this nerve will result in muscle contraction.

Also running parallel to the axillary vein and rising perpendicularly from between the ribs on the chest wall are the sensory intercostal brachial skin nerves. One or ...

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