Rhomboid muscles

Origin and insertion

Rhomboid minor is a small, cylindrical muscle that consists of two layers; dorsal and ventral. They partially envelop the levator scapulae inferiorly. Rhomboid minor originates from the nuchal ligament and spinous processes of the seventh cervical (C7) and first thoracic (T1) vertebrae. The muscle extends obliquely in an inferolateral direction, to insert into the base of the medial end of the spine of scapula; the dorsal layer attaches dorsolateral and inferior to levator scapulae while the ventral layer inserts inferomedially to levator scapulae. At the insertion point of the ventral layer, the fasciae of rhomboid minor and serratus anterior muscles unite. In certain individuals, a thin muscle called rhomboid occipitalis can extend between rhomboid minor and the occipital bone.

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Rhomboid major is a broad quadrilateral muscle that originates from the spinous processes of the second to second to fifth thoracic vertebrae (T2-T5). The muscle extends obliquely in an inferolateral direction to insert into the medial border of scapula, between the inferior angle and root of the spine of scapula. The attachment takes place both on the dorsal and costal aspects of the medial border of scapula.

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Relations

The rhomboid muscles overlie the deep (intrinsic) muscles of the back and are covered by the trapezius muscle. The exception is a small triangular region over the inferior border of rhomboid major (triangle of auscultation). This area is bordered by the latissimus dorsi muscle inferiorly, trapezius muscle superomedially and rhomboid major superolaterally. It is an important clinical landmark that serves for lung auscultation from the back.

The rhomboids are generally completely separate muscles, but they can be fused in certain individuals. Rhomboid minor partially overlaps the superior portion of rhomboid major.

Innervation

Both rhomboid muscles are innervated by the dorsal scapular nerve (C4-C5), a branch of the brachial plexus.

Blood supply

Rhomboid minor and major receive arterial blood from three sources:

• Dorsal scapular artery and deep branch of transverse cervical artery. Both of these blood vessels stem from the thyrocervical trunk.
• Dorsal branch of upper five or six posterior intercostal arteries, branches of the thoracic aorta.

Function

The main action of the rhomboid muscles is scapular retraction around the scapulothoracic joint. Scapular retraction is a simultaneous sliding of the scapula superiorly and medially along the trunk.

This superomedial movement of the scapula rotates the glenoid cavity inferiorly, dropping the shoulder girdle. By opposing excessive scapular protraction, the rhomboids help to maintain a correct posture when sitting, standing and walking. 

In addition, the contraction of the rhomboids fixes and stabilizes the scapula into position. This provides an anchor point in space from which various muscles of the upper limb can act and a fulcrum around which the upper limb can move.

Take the following quiz to test your knowledge on the rhomboids, as well as other superficial muscles of the back!