Latissimus Dorsi: Anatomy, Variations, and Clinical Significance
The latissimus dorsi is a powerful, fan-shaped muscle that covers a significant portion of the back. Characterized by its broad medial origin and narrow lateral insertion, it plays a critical role in upper body movement. Its unique structure allows it to bridge the lower back, the pelvis, and the arm, making it one of the most expansive muscles in the human body.
Anatomical Structure and Origin
The muscle originates from several key points across the posterior torso. It attaches to the spinous processes of the six lower thoracic vertebrae (located deep to the trapezius) and extends through the thoracolumbar fascia—a deep layer of connective tissue—to the spinous processes of the lumbar and sacral vertebrae. Additionally, it originates from the posterior third of the outer lip of the iliac crest (the top of the hip bone) and the last four ribs, where it interdigitates with the external oblique muscle.
The fibers of the latissimus dorsi travel upward and laterally, passing over the lower angle of the scapula (shoulder blade). The orientation of these fibers varies by region: the upper fibers are nearly horizontal, the middle fibers are oblique, and the lower fibers are vertical.
[ไม่มีภาพประกอบ]Insertion and the "Two Majors"
The muscle wraps around the lower border of the teres major and inserts into the floor of the intertubercular sulcus (also known as the bicipital groove) of the humerus via a tendon approximately 1.5 inches wide. A synovial bursa—a fluid-filled sac that reduces friction—is located between the tendons of the latissimus dorsi and the teres major.
An easy way to remember this insertion is the phrase "A Miss Between Two Majors." The latissimus dorsi (the "Miss") inserts into the floor of the groove, flanked by the teres major on the medial lip and the pectoralis major on the lateral lip.
Anatomical Variations
Human anatomy is rarely uniform, and the latissimus dorsi exhibits several common variations. The number of dorsal vertebrae to which the muscle attaches typically ranges from four to eight, and the extent of its attachment to the iliac crest can vary between individuals.
The Axillary Arch
Found in approximately 7% of the population, the axillary arch is a muscular slip measuring 7 to 10 cm in length and 5 to 15 mm in breadth. It springs from the upper edge of the latissimus dorsi and crosses the axilla (armpit) in front of the axillary vessels and nerves to join the pectoralis major, coracobrachialis, or the fascia of the biceps brachii. Because it crosses the axillary artery above the common site for surgical ligatures, it can potentially mislead surgeons. MRI data has correlated the presence of this arch with symptoms of neurological impingement.
Other Muscular Variants
- Latissimocondyloideus: Found in about 5% of humans, this is a muscular fibrous slip that passes from the latissimus dorsi tendon to the long head of the triceps brachii. It is considered a remnant of the dorsoepitrochlearis brachii found in apes.
- Scapular Connection: The relationship between the muscle and the scapula varies. In a study of 100 cadavers, 43% had substantial fibers originating from the scapula, 36% had a soft fibrous link, and 21% had little to no connection.
Clinical Triangles
The latissimus dorsi helps define two clinically important triangular spaces in the body.
Lumbar Triangle of Petit
This is a small triangular interval located where the lateral margin of the latissimus dorsi is separated from the external oblique muscle. Its base is formed by the iliac crest, and its floor is composed of the internal oblique muscle.
Triangle of Auscultation
Located behind the scapula, this triangle 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. When the arms are folded across the chest and the trunk is bent forward, the sixth and seventh ribs become subcutaneous here, making this an ideal spot for auscultation (listening to internal body sounds with a stethoscope).
Nerve Supply and Coordination
The latissimus dorsi is innervated by the thoracodorsal nerve (also known as the long subscapular nerve), which carries fibers from the sixth, seventh, and eighth cervical nerves. Interestingly, electromyography indicates that the muscle consists of six distinct groups of fibers that the central nervous system can coordinate independently.
Summary Table
| Feature | Details |
|---|---|
| Origin | Lower 6 thoracic vertebrae, lumbar/sacral vertebrae, iliac crest, and last 4 ribs |
| Insertion | Floor of the intertubercular sulcus (bicipital groove) of the humerus |
| Innervation | Thoracodorsal nerve (C6, C7, C8) |
| Key Landmarks | Lumbar triangle of Petit, Triangle of auscultation |
| Common Variant | Axillary arch (present in ~7% of population) |
Key Facts
- The latissimus dorsi is a fan-shaped muscle originating from the lower back, pelvis, and lower ribs.
- It inserts into the humerus, positioned between the pectoralis major and teres major.
- The triangle of auscultation allows for easier listening to lung sounds.
- The axillary arch is a rare muscular variant that can cause neurological impingement or complicate surgery.
- It is controlled by the thoracodorsal nerve and can be coordinated in six independent fiber groups.
Frequently Asked Questions
What is the "A Miss Between Two Majors" mnemonic?
It is a memory aid for the insertion of the latissimus dorsi. The "Miss" (latissimus dorsi) inserts into the floor of the intertubercular groove, while the two "Majors" (pectoralis major and teres major) insert on the lateral and medial lips of the same groove, respectively.
What is the clinical importance of the triangle of auscultation?
Because the muscle layers are thinner in this area, the sixth and seventh ribs become more accessible to the surface when the body is positioned correctly, making it the best location for a clinician to perform auscultation of the lungs.
How common is the axillary arch, and why does it matter?
The axillary arch occurs in about 7% of people. It is clinically significant because it crosses the axillary artery and nerves, which can lead to neurological impingement or potentially mislead a surgeon during procedures in the axilla.
Which nerve controls the latissimus dorsi?
The muscle is innervated by the thoracodorsal nerve, which originates from the sixth, seventh, and eighth cervical nerves (C6, C7, C8).
What is the latissimocondyloideus?
The latissimocondyloideus is a rare muscular slip (found in ~5% of humans) that connects the latissimus dorsi tendon to the long head of the triceps brachii, representing an evolutionary trait found in apes.