Medial pectoral n.

The medial pectoral nerve (medial anterior thoracic) arises from the medial cord of the brachial plexus (from the eighth cervical and first thoracic spinal levels).

It passes behind the first part of the axillary artery and curves forward between the axillary artery and vein. Anterior to the axillary artery, the medial pectoral nerve unites with a filament from the lateral pectoral nerve.

It then enters the deep surface of the pectoralis minor m., where it divides into a number of branches that supply the pectoralis minor m.  Two or three branches pierce the pectoralis minor and end in the pectoralis major m., supplying part of the pectoralis major as well.

A mnemonic to differentiate the medial pectoral nerve, which pierces both pectoral muscles, from the lateral pectoral nerve, which pierces only pectoralis major, is "Lateral Less, Medial More".