This muscle, covering such a large area of the back, is characterized by its very wide origin and its very narrow insertion. It is a derivative of the upper limb myotomes, and so is supplied by a branch of the brachial plexus.
The origin commences above, at the spine of the seventh thoracic vertebra, and extends downwards along the spinous processes and supraspinous ligaments of all the lumbar and sacral vertebrae. Fleshy in the thoracic portion, the origin becomes aponeurotic in the lumbar and sacral region, and fuses with the posterior layer of the lumbar fascia, by which layer it also arises from the central ridge on the posterior part of the crest of the ilium. Lateral to this it arises by flesh from the posterior third of the outer lip of the iliac crest. The upper border of the flat sheet of muscle runs horizontally, and is covered by the lower triangular part of trapezius, and flows over the inferior angle of the scapula, from which a few fibers arise to join the muscle. The lateral border of the muscle, thicker and more rounded than its thin upper border, runs vertically upwards, being reinforced by four slips from the lowest four ribs, whose fibers of origin interdigitate with those of the external oblique in this situation. This lateral border of latissimus dorsi forms a boundary of the lumbar triangle. The muscle converges towards the posterior axillary fold, of which it forms the lower border. The fibers, sweeping spirally around the lower border of teres major, are replaced by a flattened tendon about 2.5 cm broad which is inserted into the floor of the intertubercular groove. The surfaces of the muscle, anterior and posterior, are reversed at the insertion of the tendon as a result of the spiral turn around the teres major. This glistening white tendon contrasts with adjacent muscle and is a useful landmark in the lower posterior wall of the axilla.
Nerve supply of latissimus dorsi:
By the thoracodorsal nerve (C6, 7, 8) from the posterior cord of the brachial plexus. It is vulnerable in operations on the axilla, for in its course down the posterior wall it slopes forwards to enter the medial surface of the muscle just behind its anterior border. The latissimus dorsi is developed in the extensor compartment of the limb and migrates to its wide attachment on the trunk, taking its nerve with it.
Action of latissimus dorsi:
It extends the shoulder joint and medially rotates the humerus (e.g. folding the arms behind the back, or scratching the opposite scapula), but in combination with pectoralis major it is a powerful adductor. Especially used in restoring the upper limb from abduction above the shoulder, it is essentially the climbing muscle.
Its costal fibers of origin can assist in deep inspiration, elevating the lower four ribs towards the fixed humerus. But the remainder of the muscle, sweeping from the vertebral column around the convexity of the posterolateral chest wall, compresses the lower thorax and is an accessory muscle of expiration. These fibers sometimes become sore after prolonged attacks of severe coughing. In spinal injury the muscle may move the pelvis; it is the only muscle of the upper limb to have a pelvic attachment (via the lumbar fascia).