The lateral pectoral nerve (lateral anterior thoracic) arises from the lateral cord of the brachial plexus (from the fifth, sixth, and seventh cervical spinal levels).
It passes across the axillary artery and vein, pierces the clavipectoral fascia, and is distributed to the deep surface of the pectoralis major m. The lateral pectoral n. supplies motor innervation to much of the pectoralis major m.
Note that in the image above, the lateral pectoral n. is highlighted in blue and the pectoralis major m. is nearly invisible in order to visualize the distribution of the branches of the nerve as it supplies the pectoralis major m.
Note that in the image above, several branches of the lateral pectoral n. are seen exiting the space between the clavicle and the pectoralis minor to enter the posterior surface of the pectoralis major m. Also visible are branches of the medial pectoral n. piercing the pectoralis minor m. to enter the posterior surface of the pectoralis major m. On the chest wall, the medial pectoral n. is actually lateral to the lateral pectoral n.