The superficial structures on the back of the body are studied with the upper limb because the shoulder girdle is attached posteriorly with the axial skeleton by a number of muscles. These muscles are called posterior axio-appendicular muscles. They play an important role in the movements of the scapula. Further removal of the scapula in malignant disease (e.g., fibrosarcoma) requires detailed knowledge of the muscles, nerves, and vessels on the back.
SURFACE LANDMARKS
- Scapula (shoulder blade) is the most important surface landmark on the back. It is placed at a tangent on the posterolateral aspect of the rib cage. Vertically, it extends from 2nd to 7th rib. Although it is thickly covered by the muscles, still most of its outline can be felt in the living individual:
(a) Acromion process can be easily felt at the top of the shoulder.
(b) Crest of the spine of the scapula, runs medially and slightly downwards from the acromion to the medial border of the scapula, hence it can be easily palpated by finger drawn along it.
© Medial border can be traced upwards to the superior angle and downwards to the inferior angle. The superior angle of the scapula lies opposite the spine of T2 vertebra, the root of the spine lies at the level of T3 vertebra and the inferior angle of the scapula lies at the level of T7 vertebra.
SPECIAL FEATURE
The scapula is freely mobile as about 15 muscles are attached to its processes and fossae. The two scapulae are drawn apart when the arms are folded across the chest. The medial borders of the two scapulae are close to the midline when shoulders are drawn back.
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Eighth rib is palpable, immediately inferior to the inferior angle of the scapula. The lower ribs can be counted from it.
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Twelfth rib can be palpated if it projects beyond the lateral margin of the erector spinae muscle, about 3 cm above the iliac crest.
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Iliac crest is felt as a curved bony ridge below the waist. When traced forwards and backwards, it ends as anterior and posterior superior iliac spines, respectively. The posterior superior iliac spine may be felt in shallow dimple of skin above the buttock, about 5 cm from the median line.
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Sacrum—the back of sacrum lies between the right and left dimples (vide supra) and its spines can be palpated in the median plane.
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Coccyx is a slightly movable bone and may be felt deep between the buttocks in the natal cleft.
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Spines of vertebrae lie in the median furrow of the back and may be felt. The spine of 7th cervical vertebra (vertebra prominens) is readily felt at the root of the neck at the lower end of nuchal furrow. The approximate levels of other spines are given in Table.
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External occipital protuberance and superior nuchal lines—the external occipital protuberance is a bony projection felt in the midline on the back of the head. The curved bony ridge extending laterally on each side nuchal line. These bony features demarcate the junction between the head and neck posteriorly.
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Nuchal groove furrow is the median furrow, which extends from external occipital protuberance to the spine of C7 vertebra.
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Ligamentum nuchae is the median fibrous partition on the back of neck, which extends from external occipital protuberance to the spine of C7 vertebra and separates the short cervical spines from the skin.
CUTANEOUS NERVES
The cutaneous nerves on the back are derived from the posterior rami of the spinal nerves. Each primary ramus divides into medial and lateral branches:
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Up to T6, the cutaneous innervation is provided by medial branches, which emerge close to the median plane.
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Below T6, the cutaneous innervation is provided by lateral branches, which emerge in line with the lateral edge of the erector spinae muscle.
The cutaneous branches of upper three lumbar nerves emerge a short distance above the iliac crest and turn down over it to supply the skin of the gluteal region. The posterior rami of C1, C7, C8, L4, and L5 do not give any cutaneous branches.
CUTANEOUS ARTERIES
The arteries which accompany the cutaneous nerves on the back of body in the thoracic and lumbar regions are the dorsal branches of the posterior intercostal and lumbar arteries, respectively.