The serratus anterior muscle is a broad, flat muscle that forms the medial wall of the axilla. Its dual blood supply and unique organization into separate slips that originate from each of the first nine ribs make it ideal for segmental transfer of only the lower three to four slips, thereby preserving muscle function (Fig. 155.2). The lateral thoracic artery vascularizes the upper four or five slips, whereas the lower portion is consistently supplied by a large branch of the thoracodorsal artery. This vessel enters the serratus in its posterior third and gives off segmental branches to each of the lower five slips (Fig. 155.3), each branch accompanied by a corresponding vein and a twig of the long thoracic nerve. The most inferior three slips are particularly suitable for use as a free muscle transfer and will have a long pedicle if the serratus branch is dissected back to include the subscapular-thoracodorsal axis (Fig. 155.4).

The serratus anterior stabilizes the scapula by its broad insertion along the medial and inferior border. It is innervated by the long thoracic nerve that arises from C5, C6, and C7 and travels inferiorly along the serratus fascia, joining with the serratus branch of the thoracodorsal artery at about the level of the sixth slip (Fig. 155.3). Loss of the long thoracic nerve will cause winging of the scapula, a complication that can be avoided by separating the discrete fascicles of the long thoracic nerve to only the lower three slips. The proximal serratus innervation must be preserved to maintain the muscle as a functional unit. With magnification, the long thoracic nerve can be split into fascicular groups to the level of the axillary vein, but this tedious exercise probably puts the proximal nerve at unnecessary risk of injury. Therefore, it is better to use a shorter nerve segment and to interpose a nerve graft at the recipient site if a long nerve pedicle is needed. Removal of the lowest three slips of the serratus will not result in winging of the scapula as long as the proximal muscle retains its innervation.