The latissimus is a large fan-shaped muscle which covers a wide area of the back, being broad at its (medial) origin, narrow at its (lateral) insertion. It originates from the spinous processes of the six lower thoracic vertebrae (deep to the trapezius), and, through the thoracolumbar fascia, the spinous processes of the lumbar and sacral, and the posterior third of the outer lip of the iliac crest, and the last four ribs (interdigitating with external oblique muscle). Its fibers pass upward and laterally over the lower angle of the scapula, from which an extra slip is typically added to the muscle. Its upper fibers are nearly horizontal, middle, oblique, and lower, vertical: it then wraps around the lower border of the teres major and is inserted, ventrally to that muscle, by a tendon 1.5 inches wide into the floor of the intertubercular sulcus (sin. bicipital groove) of the humerus, a little higher than the teres major. A
synovial bursa lies between the tendons of insertion of the latissimus and teres major.
Variations The number of dorsal vertebrae, to which it is attached, varies from four to eight; the number of costal attachments varies; muscle fibers may or may not reach the crest of the
ilium. A
muscle slip, the
axillary arch, varying from 7 to 10 cm in length, and from 5 to 15 mm in breadth, occasionally springs from the upper edge of the latissimus dorsi about the middle of the posterior fold of the axilla, and crosses the axilla in front of the axillary vessels and nerves, to join the under surface of the tendon of the
pectoralis major, the
coracobrachialis, or the fascia over the
biceps brachii. This axillary arch crosses the
axillary artery, just above the spot usually selected for the application of a ligature, and may mislead a surgeon. It is present in about 7% of the population and may be easily recognized by the transverse direction of its fibers. Guy
et al. extensively described this muscular variant using MRI data and positively correlated its presence with symptoms of neurological impingement. A fibrous slip usually passes from the upper border of the tendon of the Latissimus dorsi, near its insertion, to the long head of the
triceps brachii. This is occasionally muscular, and is the representative of the dorsoepitrochlearis brachii of
apes. This muscular form is found in ~5% of humans and is sometimes termed the latissimocondyloideus. The latissimus dorsi crosses the inferior angle of the
scapula. A study found that, of 100 cadavers dissected: • 43% had "a substantial amount" of fibers in the latissimus dorsi originating from the scapula. • 36% had few or no muscular fibers, but a "soft fibrous link" between the scapula and the latissimus dorsi • 21% had little or no connecting tissue between the two structures.
Triangles • The lateral margin of the latissimus dorsi is separated below from the
obliquus externus abdominis by a small triangular interval, the
lumbar triangle of Petit, the base of which is formed by the
iliac crest, and its floor by the
obliquus internus abdominis. • Another triangle is situated behind the scapula. It is bounded above by the
trapezius, below by the latissimus dorsi, and laterally by the vertebral border of the scapula; the floor is partly formed by the
rhomboideus major. If the
scapula is drawn forward by folding the arms across the chest, and the trunk bent forward, parts of the sixth and seventh ribs and the interspace between them become subcutaneous and available for
auscultation. The space is therefore known as the
triangle of auscultation. • The latissimus dorsi can be remembered best for insertion as "A Miss Between Two Majors". As the latissimus dorsi inserts into the floor of the intertubercular groove of the
humerus it is surrounded by two major muscles. The teres major inserts medially on the medial lip of the intertubercular groove and the pectoralis major inserts laterally onto the lateral lip.
Nerve supply The latissimus dorsi is innervated by the sixth, seventh, and eighth
cervical nerves through the
thoracodorsal (long subscapular) nerve.
Electromyography suggests that it consists of six groups of muscle fibres that can be independently coordinated by the
central nervous system. ==Function==