The posterolateral boundaries of Sibson fascia are the vertebral column, first rib, levator scapulae muscle, and scalenus medius
muscle; the medial boundary is formed by the superior mediastinal structures; and the anterior boundary is formed by the scalenus anterior and sternocleidomastoid muscles.
The scalenus anterior muscle pulls the lateral portion of the first rib proximally and posteriorly and the scalenus medius
muscle pulls the medial portion of the first rib proximally and medially.
The dorsal scapular nerve (DSN) is a motor nerve that arises mainly from the C5 spinal nerve root and travels within the scalenus medius muscle.
The nerve may also be entrapped within the scalenus medius muscle.
It is bounded superiorly by the clavicle, inferiorly by the first rib, anteromedially by the costoclavicular ligament, and posterolaterally by the scalenus medius muscle and the long thoracic nerve.
Some cover the end of the rib at the transverse process of the vertebra with scalenus medius muscle.
The muscular floor of the triangle comprises (from superior to inferior): splenius capitis, levator scapulae, and scalenus medius and posterior (Bruce et al 1967, Drake et al 2005, Grant and Basmajian 1965, Last 1978, Moore and Dalley 2006, Wood Jones 1953, Woodburne and Burkel 1988, Zuckerman 1961) (see Figure 1).
The muscle was wedged between the carotid sheath antero-medially; and the brachial plexus and scalenus medius postero-laterally as shown in Figure 2.
The plexus is found at the level of the first four cervical vertebrae deep to the sternocleidomastoid, in the layer superficial to the scalenus medius
and levator scapulae (5).
In one subject, a vein was identified between the scalenus anterior and scalenus medius
muscles amongst the trunks (see Figure 5).