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Next, cut the sternocostal head of pectoralis major muscle near its attachment to the sternum and the costal cartilage of the 6th rib and reflect it laterally. Remove the clavipectoral fascia, the fascia which extends from the clavicle and envelopes the underlying pectoralis minor muscle. Upon reflection of the pectoralis major muscle, identify, on its deep surface, the lateral pectoral nerve [upper nerve displaced by forceps], medial pectoral nerve [lower nerve displaced by forceps], and accompanying vessels.

Links and References:
Grant's: 6.14 and 6.15
Netter (1ed.): 174, 175, and 403 (2ed.): 174, 175, and 399
Rohen and Yokochi: 362, 384, and 385