Latissimus dorsi Anatomy

The Latissimus dorsi muscle is a large and triangular/fan-shaped muscle of the back with a very wide origin and but very narrow insertion. The Latissimus dorsi has upper, middle and lower parts whose fibers run in different directions. The upper part of this flat sheet of muscle runs horizontally and it is covered medially by the lower triangular part of the trapezius muscle and passes over the inferior angle of the scapula, from which a few fibers sometimes arise. The middle part of Latissimus dorsi runs obliquely upwards and outwards while the lower part of the muscle runs vertically upwards to be reinforced by four slips from the lowest four ribs where the fibers of origin interdigitate with those of the external oblique muscle. The Deltoid and Trapezius muscles serve as antagonist to the Latissimus dorsi.

A boundary of the lumbar triangle is formed by the lateral border of latissimus dorsi and the lower border of the posterior axillary fold is also formed by the same muscle. The Latissimus dorsi muscle coils spirally around the lower border of teres major with some intermingling of their fibers. Occasionally some muscle fibers from the edge of latissimus dorsi cross in front of the axillary vessels and nerves to blend with the tendon of pectoralis major forming a muscular axillary arch.

Latissimus dorsi Functions and Actions

1. The function of Latissimus dorsi is to extend, adduct and medially rotate the humerus. This muscle also raises the body toward arms during climbing.
2. In combination with the pectoralis major, the latissimus dorsi is a powerful adductor of the humerus, and plays a major role in downward rotation of the scapula.
3. Together with the pectoralis major, the latissimus dorsi raises the trunk to the arm, which occurs when performing chin-ups (hoisting oneself so the chin touches an overhead bar) or when climbing a tree. These movements are also used when chopping wood, paddling a canoe, and swimming (particularly during the crawl stroke).

Latissimus dorsi Origin and Insertion

Latissimus dorsi originates from the spines of the lower six thoracic vertebrae and the posterior layer of the lumbar fascia which also helps the attachment of this muscle to the lumbar and sacral vertebral spines and also to the posterior part of the crest of the ilium. Some muscular fibers also arise from the outer lip of the iliac crest.

The flattened, shiny, white tendon of Latissimus dorsi which measures about 3cm broad is inserted into the floor of the intertubercular sulcus. Because of the spiral turn around teres major the anterior and posterior surfaces of Latissimus dorsi are reversed at the tendon with the lowest fibers at the point of origin inserting as the highest at the humerus, while those that originate highest insert lowest.

Latissimus dorsi Nerve Supply

The Thoracodorsal nerve with C6, C7 and C8 fibers innervates the Latissimus dorsi, the dominant fibers being C6 and C7. Thoracodorsal nerve arises as a branch from the posterior cord of the brachial plexus and in its course in the axilla, it is vulnerable in operations on the axilla because slopes downwards and forwards to enter the medial surface of Latissimus dorsi muscle just behind its anterior border in front of the thoracodorsal vessels.

Blood Supply of Latissimus dorsi

The arterial blood supply is from the Thoracodorsal artery, which is a branch of the subscapular artery; there are additional small arterial branches from dorsal branches of posterior intercostal and lumbar arteries. The venous drainage is from the thoracodorsal vein, which drains the apical part of Latissimus dorsi.

Clinical Importance of Latissimus dorsi

1. Latissimus dorsi Tendon:This glistening white tendon of Latissimus dorsi contrasts with adjacent muscle and therefore serves as a useful landmark at the lateral margin of the posterior wall of the axilla during surgical dissection in the axilla.
2. Latissimus dorsi Flap:Latissimus dorsi flap can be formed during reconstructive breast surgery. A part of the muscle, sometimes with an overlying paddle of skin is used as a musculocutaneous flap it is rotated around to the front and used to create a mound simulating the breast. The thoracodorsal artery is an important source of blood supply to the flap.
3. Triangle of Auscultation:The Triangle of Auscultation is a gap located in the thick back musculature and is useful as a good place to examine posterior segments of the lungs with a stethoscope. This triangle of auscultation is formed by the superior horizontal border of the latissimus dorsi, the medial border of the scapula, and the inferolateral border of the trapezius. When the scapulae are drawn anteriorly by folding the arms across the chest and the trunk is flexed, the auscultatory triangle enlarges.
4. Importance of Latissimus dorsi in spinal injury:Latissimus dorsi muscle may move the pelvis and trunk in spinal injury because it is the only muscle of the upper limb to have a pelvic attachment (via the lumbar fascia).
5. Use of Latissimus dorsi in Cardiac surgeries:Latissimus dorsi can be used in cardiomyoplasty by being wrapped round the heart with its nerve supply intact and stimulated by a pulse generator so that it can act as a ‘biological assist device’.

Latissimus dorsi Test

To test the action of the Latissimus dorsi, the arm is abducted to a right angle and then adducted before being extended and medially rotated against resistance; the lateral part of the muscle below the posterior axillary fold can be seen and felt contracting. The contraction of the Latissimus dorsi can also be felt below the axillary fold when the patient coughs.

Latissimus dorsi Paralysis

When there is paralysis of the latissimus dorsi as a result of injury to the thoracodorsal nerve, the person cannot raise the trunk with the upper limbs, as occurs during climbing. Again, the person cannot use an axillary crutch because the shoulder is pushed superiorly by it.