Which statement correctly describes the formation of the posterior cord of the brachial plexus?
- A It is formed by the anterior divisions of the superior and middle trunks only
- B It is formed by the anterior division of the inferior trunk alone
- C It is formed by the union of the posterior divisions of all three trunks ✓
- D It is formed by the union of all three anterior divisions
Correct answer: C. It is formed by the union of the posterior divisions of all three trunks
Explanation
Each of the three trunks splits into an anterior and a posterior division behind the clavicle. The three posterior divisions unite to form the posterior cord, which explains why it contains fibres from every root, B5 to T1, and supplies all the extensor musculature through the axillary and radial nerves plus the subscapular and thoracodorsal nerves. The three anterior divisions form the lateral and medial cords instead.
Reference: Gray's Anatomy for Students, 4th ed.
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