The structure indicated is the pectoralis minor muscle.

The pectoralis minor muscle lies deep to the much larger, powerful pectoralis major muscle.

The pectoralis minor muscle is covered anteriorly by the clavipectoral fascia. This muscle divides the axillary artery into three parts. Part 1 lies proximally to the pectoralis minor, part 2 is the part of the axillary artery covered by the pectoralis minor, and the third part lies distal to the pectoralis minor.

The pectoralis minor inserts onto the coracoid process of the scapula. There are three muscles which attach to the coracoid process:

1. Short head of biceps brachii
2. Coracobrachialis
3. Pectoralis minor

Origin: anteromedial surfaces of ribs 3,4,5

Insertion: coracoid process of scapula

Action: depresses shoulder, protracts scapula

Innervation: medial pectoral nerve