On each side, latissimus dorsi is a muscle that acts primarily upon the glenohumeral joint. It has a very wide origin:
The fibres converge as they pass laterally. The most inferior fibres form a rounded inferior border to the muscle that passes superiorly. The superior fibres pass horizontally over the inferior angle of the scapula. As the fibres pass around the inferior border of teres major, they twist through 180 degrees before inserting as a flat tendon into the floor of the intertubercular groove of the humerus. Twisting results in the most inferior dorsal fibres inserting most superiorly into the intertubercular groove.
Latissimus dorsi is supplied by the thoracodorsal nerve, a branch of the posterior cord of the brachial plexus (C6, C7, C8). The actions of latissimus dorsi include extension of the flexed arm at the glenohumeral joint. Also, it adducts and medially rotates the arm.
It forms part of the posterior axillary wall and is one boundary of the triangle of auscultation.
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