Scalenes are 3 (sometimes 4) muscles on either side of the neck. The 3 common bellies are name for their position; scalenus anterior, scalenus medius, scalenus posterior. The anomaly is called scalenus minimus.
They are lateral cervical muscles that connect the cervical vertebrae to the upper thoracic cage. They are long muscles with fibers of uneven length.
Scalenus anterior lies almost completely under the clavicular head of sternocleidomastoid in the pedicle groove. It passes anterior to the cervical nerves and subclavian artery but behind the subclavian vein.
Scalenus posterior lies mostly under the anterior border of the trapezius. It passes posterior to the cervical nerves and subclavian artery.
Scalenus medius lies in the posterior cervical triangle between the other two scaolenes.
They vary in their attachments and size a great deal of the time (about 40-71% of the reviewed studies).
Scalenus anterior originates from a flat tendon on the first rib just under the clavicular SCM, medial to the thoracic bundle. It inserts along the anterior tubercles on the transverse process of C3-C6. The upper attachment on C3 is usually a more prominent tubercle than the other cervical vertebrae and easily distinguished with palpation.
There are reports of it extending to attach to the second rib as well.
Dissection information on the scalenus minimus is highly variable. It is reported from 7-71% of the time and varies in its attachments.
Scalenus minimus is the most variable of the scalene muscles. It originates from the fascia of the thoracic pleura and sometimes the first rib. It inserts on the posterior tubercle of C7, sometimes C6. It passes posterior to the subclavian artery.
Scalenus medius originates from a flat tendon on the 1st rib just lateral to the thoracic neurovascular bundle. It inserts on the posterior tubercles of the transverse processes of C2-C7. This traps C7-T1 between origin and insertion.
Scalenus posterior originates from a flat tendon on lateral aspect of the 2nd rib. It inserts on the posterior tubercles of the C4-C6 or C7.
This traps T1 andT2 between the bones of origin and insertion.
Here’s a document from Neuromuscular Assessment on the structure of scalene muscles:SO-Scalenes
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