The scalenus medius is the largest and most posterior scalene, forming the posterior wall of the interscalene triangle alongside the anterior scalene anteriorly. Its first rib insertion posterior to the subclavian artery groove places it behind the artery — the artery runs between the anterior scalene (anterior) and middle scalene (posterior) on the first rib. The long thoracic nerve and dorsal scapular nerve emerge through or between the middle scalene fibres.
| Origin | Posterior tubercles of C2 through C7 transverse processes (the largest scalene) |
|---|---|
| Insertion | Superior surface of the first rib posterior to the subclavian artery groove |
| Nerve Supply | Ventral rami of C3, C4, C5, C6 |
| Blood Supply | Ascending cervical artery |
| Actions | Ipsilateral lateral flexion; Elevates the first rib; The posterior wall of the interscalene triangle |
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The long thoracic nerve (to serratus anterior) and dorsal scapular nerve (to rhomboids and levator) penetrate the middle scalene muscle, making them vulnerable to scalene muscle spasm, traction injuries, and scalenectomy procedures.
The long thoracic nerve is susceptible to compression within the middle scalene fibres in thoracic outlet syndrome, producing serratus anterior weakness and medial scapular winging alongside the more typical brachial plexus symptoms. Intraoperative identification of both the long thoracic and dorsal scapular nerves through the middle scalene is essential during scalenectomy to avoid creating new neurological deficits.
The middle scalene is palpable posterior to the anterior scalene in the posterior cervical triangle, identified as the firm muscular mass between the sternocleidomastoid and the posterior cervical muscles.
Serratus anterior weakness from long thoracic nerve entrapment within the middle scalene fibres producing medial scapular winging in thoracic outlet syndrome.
