The scapula affords attachment for many large and powerful muscles that play an important part in the varied movements, not alone of the shoulder but also of the entire upper extremity. Thus, the trapezius aids in elevating the scapula and approximating it to the spine; also in rotating the scapula, causing the acromion to ascend and the lower angle to move outward. The action of the trapezius is supplemented by that of the elevator muscle of the angle of the scapula and of the rhomboids, the latter of which rotate the scapula on its outer angle and cause the lower angle to move inward. The serratus magnus moves the scapula outward, forward and slightly upward. It tends to rotate the scapula on its inner angle and thus to elevate the acromion, but in this it is opposed by the elevator muscle of the angle of the scapula and the rhomboids.
ESHNER AA. THREE CASES OF PARALYSIS OF THE SERRATUS MAGNUS AND THE TRAPEZIUS— ALAR SCAPULA.. JAMA. 1902;XXXVIII(5):300–306. doi:10.1001/jama.1902.62480050014001e
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