The serratus anterior is a fan-shaped muscle on the lateral wall of the thorax. It is a powerful protractor of the scapula and prevents the medial border of the scapula from winging out.
It has three sections. The sections vary in their configuration, even from one side to another, on the same cadaver. The second rib may have a slip that inserts with the upper section at the superior angle of the scapula, or a slip that inserts along the medial border with the intermediate section, or both. The lower section may have slips that continue down as low as the 12th rib or stop as high as the 6th rib.
Origin – the anterior aspect of the first and, usually, second rib
Insertion – the superior angle of the scapula, blending with the fibers of the levator scapula
Function – protraction and downward rotation of the scapula
These sections tend to be wider with fibers that angle more upward. They attach to a longer section of the ribs and a wider section of the scapula than the slips in the superior or inferior bellies.
Origin – the anterior aspect of the ribs 2-4 or 5
Insertion – the medial border of the scapula
Function – protraction and slight elevation of the scapula
Origin – the anterior aspect of lower ribs. lower slips usually continue to the 8th rib in men and the 9th rib in women. There may be slips that attach as low as the 12th rib. One study revealed that they vary from one side to the other in the same cadaver 30% of the time.
Insertion – the inferior angle of the scapula
Function – protraction, depression and upward rotation of the scapula
Here’s a section from Neuromuscular Assessment, before the illustrations were available:SO-serratus-anterior-1
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