Spinal nerve roots and rami, with axillary clinical landmarks
THE CONTEXT
Spinal nerves unite sensory and motor roots before dividing into mixed rami. The passage also describes lymphatic drainage across the umbilical level and arterial compression in the axilla.
Lymphatic and arterial landmarks
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Lymph above the umbilicus drains into axillary nodes, while lymph below it drains into the inguinal group.
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To control distal limb bleeding during injuries, operations or amputations, the artery can be compressed against the humerus in the lower part of the lateral axillary wall.
Formation of a spinal nerve
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Each spinal nerve forms by union of a dorsal root and a ventral root.
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The dorsal root is sensory, contains a spinal or dorsal root ganglion, and enters the dorsal horn and posterior funiculus of the spinal cord.
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The ventral root is motor and arises from anterior horn cells of the spinal cord.
Rami and plexus formation
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The spinal nerve divides into a short dorsal ramus and a long ventral ramus. Both contain motor and sensory fibres.
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The rami obtain sympathetic fibres through the grey ramus communicans.
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Only ventral primary rami form plexuses. The brachial plexus is formed by the ventral rami of C5–C8 and T1.