The rhomboid (latin : Rhomboidei ; french: Muscles Rhomboïdes) named from their shape, which is rhomboidal, or like a parallelogram - are sometimes looked upon as a single muscle, but may be usually separated into the following : Rhomboid minor muscle and Rhomboid major muscle.

The rhomboid minor muscle

The rhomboid minor, the lesser and upper of the two, is a four-sided sheet, forming an elongated parallelogram.

Origin

The lower part of the nuchal ligament (ligamentum nuchae), the spines of the seventh cervical and first thoracic vertebrae, and the supraspinous ligament between them.

Insertion

The vertebral border of the scapula opposite its spine.

Structure

Its origin and insertion are by short tendinous fibers, between which its fleshy fibers run parallel to one another, downwards and outwards.

Nerve-supply

From the brachial plexus by a branch of the fifth cervical nerve, which enters its deep aspect near the upper border a short distance above its asertion.

Action and relations

For action and relations, see the account of the following muscle.

The rhomboid major muscle

The rhomboid major - the lower and larger of the two muscles - is a broad rhomboidal sheet.

Origin

The spines of the four or five upper thoracic vertebrae, and the supra- spinous ligaments between them.

Insertion

The vertebral border of the scapula opposite the infraspinous fossa.

Structure

At the origin, of short tendinous fibers, succeeded by parallel fleshy bundles, which pass downwards and outwards to a narrow tendinous expansion ,which is feebly attached to the scapula over the upper three-fourths of its insertion, but with thick and strong fibers near the inferior angle of that bone,

Nerve-supply

The same as the preceding, and entering the upper part of the deep surface near its insertion.

Action

The rhomboids draw the scapula inwards and backwards towards he middle line, and at the same time upwards. They also rotate the scapula so Ls to depress the point of the shoulder. In this way they, together with the levator ^nguli scapulae, will help in drawing down the arm, after it has been elevated hrough the rotation of the scapula by the trapezius and serratus magnus.

Acting from the scapula, the rhomboidei will help the trapezius in drawing the middle line of the back towards that bone.

Relations of the two rhomboids

Superficially, the trapezius and, at the lower )art of the rhomboideus major, the deep fascia, and latissimus dorsi ; deeply, the lerratus posticus superior, splenius colli, the vertebral aponeurosis covering the ipper continuations of the erector spinae, the external intercostals, and the posterior capular vessels.

Variations

The rhomboid minor is frequently absent, and occasionally there is no romboid major. The fibers of the latter muscle may be inserted almost entirely into he lower angle of the scapula. Occasionally its lower fibers join those of the latissimus dorsi, and they have also been found continuous with a part of the teres major. An accessory band may join the rhomboids from the occipital bone (the occipito-scapularis).