Surface anatomy of the back .
The scapula is placed against the posterolateral wall of the thorax. It lies over the second to seventh ribs and extends into the posterior wall of the axilla. It is thickly covered with muscles, but most of its outline can be felt in the living subject. Find the acromion at the top of the shoulder. Draw your finger along the bony ridge (crest of the spine of the scapula) which runs medially and slightly downwards from the acromion to the medial border of the scapula . Trace the medial border to the inferior angle and, if possible, to the superior angle, palpating it through the muscles that cover it.
The scapula is held in position by muscles and the clavicle. It is very movable—the scapulae move apart when the arms are folded across the chest. When the shoulders are drawn back, the medial borders of the scapulae are brought close to each other and the posterior median line. The rib felt immediately inferior to the inferior angle of the scapula is usually the eighth rib and the lower ribs may be counted from it.
The twelfth rib is not palpable, unless it projects beyond the lateral margin of the back muscle—the erector spinae . The iliac crest is the curved bony ridge felt below the waist. Trace it forwards to the anterior superior iliac spine and backwards to the posterior superior iliac spine. The posterior superior iliac spine is felt in a shallow dimple in the skin above the buttock and about 5 cm from the median plane.
Between the left and right dimples is the back of the sacrum. Usually three sacral spines can be palpated in the median plane. The coccyx is the slightly mobile bone felt deep between the buttocks in the median plane. Feel the tips of the spines of the vertebrae in the median furrow of the back. These are the only parts of the vertebral column which are easily felt. It is difficult to identify individual spines directly, but the seventh cervical spine (vertebra prominens) is the uppermost spine which can be readily felt at the root of your neck.
Above the vertebra prominens, only the second cervical spine can be felt easily. It is about 5 cm below the external occipital protuberance which is on the lower part of the back of the head where the median furrow of the neck (nuchal groove) meets the skull. The short cervical spines (compare with C7) are separated from the skin by a median fibrous partition—the ligamentum nuchae.
The posterior edge of the ligamentum nuchae stretches from the external occipital protuberance to the seventh cervical spine. The superior nuchal line is a curved ridge on the occipital bone of the skull, extending laterally from a midline bony elevation—the external occipital protuberance .
Cutaneous nerves and arteries .
The cutaneous nerves of the back are branches of the dorsal rami of the spinal nerves. Each dorsal ramus divides into a medial and a lateral branch . Both of these enter and supply the erector spinae muscles. But only one continues through the erector spinae to supply the overlying skin.
In the cervical and upper six or seven thoracic segments, the medial branches from the dorsal rami form the cutaneous nerves. They pierce the deep fascia close to the median plane. Below this level, the lateral branches form the cutaneous nerves and emerge in line with the lateral edge of the erector spinae, piercing either the latissimus dorsi (upper nerves) or the dense deep fascia (thoracolumbar fascia) of the small of the back (lower nerves).
Each of these cutaneous nerves divides into a smaller medial and a larger lateral branch. In the thoracic and lumbar nerves, branches of the dorsal rami descend before entering the skin. Thus, the area of skin supplied by the dorsal ramus of each nerve lies at a lower level than that at which the spinal nerve emerges. This makes the dermatomes of the trunk more nearly horizontal than would be expected from the oblique course of the ventral rami.
The cutaneous branches of the dorsal rami of the upper three lumbar nerves pierce the deep fascia a short distance superior to the iliac crest and turn down to supply the skin of the gluteal region. The arteries which accompany the cutaneous nerves of the back arise from the dorsal branches of the posterior intercostal and lumbar arteries.
Muscles that attach the scapula to the trunk .
Two of these muscles—the pectoralis minor and the serratus anterior—have been seen already. Other muscles attaching the scapula to the trunk are the trapezius, rhomboid major, rhomboid minor and levator scapulae . The latissimus dorsi and pectoralis major are the only two muscles which attach the humerus to the trunk.
1. Trapezius Muscle .
The trapezius is a broad muscle placed superficially in the upper part of the back. The right and left trapezius together are shaped like a trapezium, which gives the muscle its name. It has an extensive origin from the external occipital protuberance and the superior nuchal line on the occiput, the ligamentum nuchae and seventh cervical vertebra in the neck, the spines of all 12 thoracic vertebrae and the supraspinous ligaments.
From this almost completely midline origin, the fibres run laterally and come together to be inserted into the lateral third of the clavicle, the acromion and the crest of the spine of the scapula . Nerve supply: cranial nerve XI—the spinal accessory nerve . Actions: elevation of the shoulder, and retraction and lateral rotation of the scapula.
2. Latissimus dorsi Muscle .
This broad sheet of muscle arises from the lower six thoracic spines and the supraspinous ligaments between them, deep to the trapezius; the thoracolumbar fascia ; posterior part of the iliac crest and lower three or four ribs. The fibres of the latissimus dorsi come together and wind around the inferior border of the teres major to reach its anterior surface and are inserted into the intertubercular groove of the humerus.
Nerve supply: thoracodorsal nerve . Actions: it is a powerful adductor of the humerus and a depressor of the shoulder. It is used to pull the arm down from its fully abducted position above the head, as in rope climbing. When the shoulder is flexed, it acts as an extensor of that joint. When the shoulder is fixed, it helps with retraction and medial rotation of the scapula.
3. Levator scapulae Muscle .
The levator scapulae takes origin from the transverse processes of cervical vertebrae 1 to 4 and is inserted into the medial border of the scapula between the superior angle and the root of the spinous process. Nerve supply: dorsal scapular nerve . Actions: elevation and medial rotation of the scapula.
4. Rhomboid minor Muscle .
The rhomboid minor arises from the lower part of the ligamentum nuchae and the spine of the first thoracic vertebra. It is inserted into the medial border of the scapula, at the root of the spine. Nerve supply: dorsal scapular nerve . Actions: retraction and elevation of the scapula.
5. Rhomboid major Muscle .
The rhomboid major arises from the spines of the second to fifth thoracic vertebrae. It is inserted into the medial border of the scapula, inferior to the root of the spine. Nerve supply: dorsal scapular nerve . Actions: retraction, medial rotation, and elevation of the scapula.
Spinal accessory nerve .
The accessory, or eleventh, cranial nerve consists of cranial and spinal parts. The spinal part arises from the cervical spinal medulla (C. 1 to 5), enters the skull, and re-emerges in the neck. It passes posteroinferiorly through the sternocleidomastoid to the deep surface of the trapezius supplying both muscles.
Dorsal scapular nerve .
This nerve passes posteroinferiorly in the lower part of the neck from the ventral ramus of the fifth cervical nerve. It runs deep to the lower part of the levator scapulae and the rhomboid muscles, and supplies them. It is accompanied by the deep branch of the transverse cervical artery .
Transverse cervical artery .
The transverse cervical artery is a branch of the thyrocervical trunk from the first part of the subclavian artery. It divides into a superficial branch of the transverse cervical artery which accompanies the spinal accessory nerve deep to the trapezius, and a deep branch of the transverse cervical artery (also known as the dorsal scapular artery) which accompanies the dorsal scapular nerve deep to the levator scapulae and the rhomboids . Both branches take part in the anastomosis around the scapula.
Movements of the scapula .
The scapula is able to slide freely over the chest wall because of the loose connective tissue deep to the serratus anterior. The scapular movements are protraction, retraction, elevation, depression and medial and lateral rotation. These movements are produced by the muscles which attach the scapula to the trunk and indirectly by the muscles passing from the trunk to the humerus when the shoulder joint is fixed. All these movements take place around the sternoclavicular joint with minor adjustments at the acromioclavicular joint.
1. Protraction .
This forward movement of the scapula on the chest wall is produced by the serratus anterior and is assisted by the pectoral muscles . During protraction, all eight digitations of the serratus anterior contract with the pectoralis minor and the sternocostal fibres of the pectoralis major. This movement is used in reaching forwards, punching, and pushing.
2. Retraction .
Retraction is the reverse of protraction. It draws the scapulae back towards the median plane and braces back the shoulders. It is produced by the contraction of the middle fibres of the trapezius which pass horizontally from the ligamentum nuchae, and the seventh cervical and upper thoracic spines to the acromion and the lateral part of the spine of the scapula, and also by the rhomboid muscles passing from a similar origin to the medial border of the scapula .
3. Elevation .
Elevation, as in shrugging the shoulders, is achieved by the simultaneous contraction of the levator scapulae and the upper fibres of the trapezius which descend from the skull and ligamentum nuchae to the clavicle and acromion .
4. Depression .
This movement is achieved by gravity and the contraction of the pectoralis minor, the lower fibres of the pectoralis major and trapezius, and the latissimus dorsi.
5. Lateral rotation .
Rotation takes place around a horizontal axis passing through the middle of the scapular spine and the sternoclavicular joint . In lateral rotation, the upper fibres of the trapezius raise the acromion and lateral part of the clavicle, while its lower fibres depress the medial end of the spine of the scapula. Together, they laterally rotate the scapula. The lower five digitations of the serratus anterior converge on the inferior angle of the scapula and play a powerful part in this movement by pulling that angle laterally and forwards.
Lateral rotation tilts the glenoid cavity upwards and is important in abduction of the upper limb above the horizontal. Normally, scapular and shoulder joint movements occur together, but, if the scapular movement is paralysed, abduction of the limb to the horizontal cannot be achieved because the weight of the limb forces the scapula into medial rotation.
6. Medial rotation .
This is the opposite movement to lateral rotation. Gravity plays a large part in this movement, as in depression of the scapula. In addition, combined contraction of the levator scapulae, rhomboids and latissimus dorsi produces an active movement which is assisted by the pectoral muscles .
When all the muscles attaching the scapula to the trunk are contracted, the scapula is fixed to form a stable base on which upper limb movements can take place. It is also used in transmitting forces from the trunk to the upper limbs, as in lifting heavy weights in the hands by straightening the flexed legs.
Triangle of Auscultation .
Triangle of auscultation is a small triangular interval bounded medially by the lateral border of the trapezius, laterally by the medial border of the scapula and inferiorly by the upper border of the latissimus dorsi. The floor of the triangle is formed by the 6th and 7th rib and 6th intercostal space (ICS), and the rhomboid major. This is the only part of the back which is not covered by big muscles.
Respiratory sounds of apex of lower lobe heard through a stethoscope are better heard over this triangle on each side. On the left side, the cardiac orifice of the stomach lies deep to the triangle and in days before X-rays were discovered, the sounds of swallowed liquids were auscultated over this triangle to confirm the oesophageal tumor .
Lumbar Triangle of Petit .
Lumbar triangle of Petit is another small triangle surrounded by muscles. It is bounded medially by the lateral border of the latissimus dorsi, laterally by the posterior border of the external oblique muscle of the abdomen, and inferiorly by the iliac crest (which forms the base). The occasional hernia at this site is called lumbar hernia . After completing the dissection of the back, the limb with clavicle and scapula is detached from the trunk.
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