The latissimus dorsi (french : muscle grand dorsal) - named from its being the broadest of the back muscles - is a fan-shaped sheet forming a right-angled triangle, the right angle being contained between its upper and vertebral borders.

Origin

• The five or six lower thoracic spines, and the supraspinous ligaments ;
• the lower part of the vertebral aponeurosis (see account of Lumbar fascia), by which it is attached to the spines of all the lumbar and sacral vertebrae ;
• the posterior third of the outer lip of the crest of the ilium ;
• horizontal lines crossing the outer surface of the last three or four ribs external to their angles (these lines by their lower borders give origin to processes of the external oblique muscle, which thus interdigitates with the latissimus dorsi) ;
• the dorsal aspect of the inferior angle of the scapula.

Insertion

The bottom of the bicipital groove of the humerus, as far upwards as the lesser tuberosity.

Structure

Its first and third parts arise by short tendinous fibers. The origin of the second part from the vertebral aponeurosis is by fleshy fibers in a line which descends obliquely downwards and outwards from the last thoracic spine to the back of the crest of the ilium. Its origin from the ribs and scapula is muscular, he fleshy fibers are of nearly equal length, and they converge upon the tendon in such a way that those which arise from the ribs and crista ilii are inserted highest into the humerus, while those which spring from the thoracic spine are attached to the lower part of the bicipital groove. The broad sheet wraps round the side of the thorax, and is also folded upon itself, so that the anterior surface at the origin becomes the posterior at the insertion. A groove is thus formed, in which e the outer border of the scapula and the teres major. The tendon of the teres major is usually attached at the borders to that of the latissimus dorsi by strong connective tissue ; but a bursa intervenes between them near their insertion.

Nerve-supply

From the posterior cord of the brachial plexus by means of the long subscapular nerve. This is derived from the fifth cervical nerve, and enters the muscle upon its deep surface in the lower part of the axilla.

Action

It draws the humerus backwards, downwards, and inwards ; at the same time rotating it inwards. The movement of the arm in swimming is a good example of its action. When the arm is placed close to the side, it will draw their shoulder backwards and downwards.

Acting from the humerus as a fixed point, it is very important in climbing, as it, draws the pelvis and lower part of the trunk upwards and forwards towards the arms.

By its costal origin it will assist in forced inspiration when the arm is fixed.

Relations

Superficially, the Trapezius, fasciae, and integument, the The pectoralis major muscle, axillary vessels, and branches of the brachial plexus ; deeply, the rhomboid major, the vertebral aponeurosis, covering the upward continuations of the erector spinae muscle, the serratus posterior inferior (serratus posticus inferior), external intercostals, external oblique, Infraspinatus, Serratus anterior muscle (serratus magnus), and Teres major muscles.

Variations

It varies in the height of its origin from the spinal column, and also in the number of ribs from which it arises. From its axillary border slips may cross the axilla to the tendon of the pectoralis major, or may cross the great vessels and nerves to the coracoid process or the deep fascia at the upper part of the arm. A slip of fascia or muscle may be continued down from its tendon of insertion to the olecranon in association with the triceps.