On each side, latissimus dorsi is a muscle that acts primarily upon the glenohumeral joint. It has a very wide origin:
- dorsal layer of thoracolumbar fascia; hence it is attached to the:
- spinous processes of the lower six thoracic vertebrae
- lumbar vertebrae
- sacral vertebrae
- supraspinous and interspinous ligaments between the vertebrae
- iliac crest: by the lumbar fascial aponeurosis dorsally and continous with a muscular insertion in the posterior third of the crest more laterally
- inferior angle of the scapula via fascia
- most inferior 3-4 ribs externally; the muscle fibres may interdigitate with external oblique
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.