Pectoralis Minor

Introduction

Introduction to Pectoralis Minor

The pectoralis minor is a thin, triangular muscle located in the anterior thoracic wall, situated entirely deep to the larger pectoralis major muscle. Along with the pectoralis major, subclavius, and serratus anterior, it forms the musculature of the pectoral region. While modest in size compared to its superficial counterpart, the pectoralis minor plays an indispensable structural role. It serves as a key anatomical landmark in the axilla, dividing the axillary artery and brachial plexus into three distinct parts, and forms an essential component of the clavipectoral fascia complex.

Study

Anatomical Attachments and Relations

The pectoralis minor muscle originates from the anterior surfaces and superior borders of the third, fourth, and fifth ribs near their costal cartilages, as well as the deep fascia overlying the adjacent intercostal spaces. The muscle fibers run superolaterally, converging into a flat, narrow tendon that inserts into the medial border and superior surface of the coracoid process of the scapula. It is completely enveloped by the clavipectoral fascia, which extends superiorly to enclose the subclavius muscle and attaches to the clavicle.

Neurovascular Supply

The pectoralis minor is innervated exclusively by the medial pectoral nerve (root values C8, T1), which arises from the medial cord of the brachial plexus. Crucially, the medial pectoral nerve pierces through the substance of the pectoralis minor to reach and supply the deep surface of the pectoralis major muscle. In contrast, the lateral pectoral nerve (C5–C7) primarily passes superior to the muscle to supply the pectoralis major directly. Arterial supply is delivered predominantly by the pectoral branch of the thoracoacromial artery (a branch of the second part of the axillary artery), with additional contributions from the lateral thoracic artery.

Functional Biomechanics

The primary action of the pectoralis minor is the stabilization of the scapula by drawing it anteriorly and inferiorly against the thoracic wall (protraction and depression). When the scapula is fixed (for instance, during deep respiration or mechanical load), the pectoralis minor acts as an accessory muscle of inspiration, elevating the third through fifth ribs to expand the thoracic volume. It also assists in downward rotation of the glenoid cavity of the scapula.

Clinical Anatomy and Surgical Relations

The pectoralis minor is the principal anatomical landmark used to compartmentalize the axillary artery and axillary lymph nodes. The first part of the axillary artery lies proximal/medial to the muscle; the second part lies directly deep (posterior) to the muscle; and the third part extends distal/lateral to its lateral border. Axillary lymph nodes are classified similarly: Level I nodes are lateral to the muscle, Level II nodes are located deep to the muscle, and Level III nodes are medial/superior to it. Hypertonicity or tightening of this muscle can compress the cords of the brachial plexus and the axillary vessels against the coracoid process, a condition known as Pectoralis Minor Syndrome (a form of thoracic outlet syndrome).

Summary

Summary of Key Concepts

The pectoralis minor is a deep, triangular muscle of the anterior chest wall. It originates from ribs 3–5 and inserts on the coracoid process of the scapula. Innervated by the medial pectoral nerve, it protracts, depresses, and downwardly rotates the scapula, while serving as an accessory muscle of respiration. Clinically, it is the defining structural marker for the three segments of the axillary artery and the staging levels of axillary lymph node dissections during oncological breast surgeries.