Serratus anterior muscle
The serratus anterior muscle is a fan-shaped muscle at the lateral wall of the thorax. Its main part lies deep under the scapula and the pectoral muscles. It is easy to palpate between the pectoralis major and latissimus dorsi muscles.
Origins and Insertions
The serratus anterior muscle originates at the 1st to 9th rib and inserts at the ventral surface of the medial border of the scapula. Due to its course it has a “serrated” or “saw-toothed” appearance. The muscle is further divided into three parts:
- Superior part: 1st to 2nd rib → superior angle of the scapula
- Intermediate part: 2nd to 3rd rib → medial border of the scapula
- Inferior part: 4th to 9th rib → medial border and inferior angle of the scapula. This part is the most prominent and powerful one.
The innervation is supplied by the long thoracic nerve (C5-7), a branch of the brachial plexus.
The contraction of the entire serratus anterior leads to a ventrolateral movement of the scapula along the ribs. Due to the pull of the inferior part at the lower scapula, the shoulder joint is shifted superiorly. This shifting now enables to lift the arm above 90° (elevation).
In contrast the superior part depresses the scapula and thus acts antagonistically. Another function of the serratus anterior is the active stabilization of the scapula within the shoulder. Finally in a fixed scapula the muscle lifts the ribs and acts as an accessory inspiratory muscle.
A damage of the long thoracic nerve can lead to a functional loss of the serratus anterior. Common causes are operations in the axilla (e.g. lymph node removal), a compression due to carrying heavy loads (e.g. backpack) and trauma.
Typical symptoms include trouble elevating the arm and a generally unstable shoulder. Another classic sign is the medial “wing-like” tilting of the scapula (scapula alata).