The trapezius (or cucullaris, as it has been called from its resemblance to a cowl = cucullus ; french : muscle trapèze) is named from τραπέζι, a table, on account of the four-sided figure formed by the muscles of the two sides. It is a fan-shaped sheet forming an obtuse-angled triangle, the long side of which corresponds with the spine.

 

Origin

  • The inner third of the superior nuchal line of the occipital bone, and the external occipital protuberance ;
  • the posterior border of the nuchal ligament (latin: ligamentum nuchae) ;
  • the spines of the seventh cervical vertebra, together with the supraspinous ligaments.

 

Insertion

  • The posterior border of the outer third of the clavicle and the adjacent part of its upper surface ;
  • the inner edge of the upper surface of the acromion process ;
  • the upper border of the spine of the scapula, and a small tubercle at its inner extremity.

Structure

The origin of the muscle is by short tendons intermingled with fleshy fibers, except at two places, where the tendinous fibers form a continuous sheet. The first of these is from the middle of the nuchal ligament to the second thoracic spine ; here a conspicuous oval aponeurosis is formed by the tendons of the two sides. The second is at the lower acute angle of the muscle where it arises from the lowest thoracic vertebra. The muscular fibers converge from the extensive origin, and just before their insertion the sheet of the muscle is folded upon itself to adapt it to the V-shaped process of the shoulder girdle, into which it is inserted by fleshy fibers, except at the inner extremity of the spine of the scapula, when a tendinous sheet plays over the triangular base of that process, before it is inserted into the tubercle at the inner end of the spine. Sometimes a bursa intervene between this sheet and the triangular surface of the scapular spine.

Nerve-supply

From the spinal accessory ; and from the deep cervical plexus by branches of the third and fourth cervical nerves, which, after communicating with the spinal accessory, enter with it the deep surface of the muscle a short distance above the clavicle.

Action


Its upper fibers draw upwards the outer end of the clavicle and the point of the shoulder ; and acting from below, they extend the head, flex the neck to the same side, and turn the face to the opposite side. Its middle fibers draw the scapula inwards towards the spine ; at the same time they produce a rotation of the scapula on the thorax, by which the point of the shoulder is raised. The lower fibers draw the scapula downwards and inwards, and at the same time rota it so as to raise the point of the shoulder.

Acting as a whole, the muscle draws the scapula towards the middle line of the back, and elevates the shoulder by the rotation it impresses upon the shoulder blade. By drawing the scapula backwards, it gives some help to the pectoral minor and other muscles Which elevate the ribs in forced inspiration. When takes its fixed point from the shoulder-blade and clavicle, as when the hand grasp firmly some immovable object, the muscle will draw the spines of the vertebrae towards the scapula.

The presence of the oval aponeurotic patch may be explained by the fact that the range of movement of the scapula in a horizontal direction backwards and inwards is more limited than when the inward movement is combined with an upward or downward direction. This limitation is due to the ligamentous attachments of the clavicle and shoulder blade.

Relations


Superficially, the integuments and subcutaneous nerves ; deeply, the splenius complex, serratus posterior superior, rhomboid, the vertebral aponeurosis covering the continuations upwards of the erector spine, the latissimus dorsi, levator scapulae, omohyoid, scalen medius and posterior the supraspinatus, and a small portion of the infraspinatus.

Variations


Occasionally the upper or the lower part of its origin may fail. The clavicular part of its insertion sometimes extends far forwards upon the clavicle. Frequently fibers pass from its anterior border to the inner end of the clavicle either in front or behind the sterno-mastoid, forming an arch under which run some of the superficial nerves. This arch may even extend to the sternum. A similar transverse band is occasionally found in the upper part of the posterior triangle, the transverse nuchal. Sometime a longitudinal band of fibers covers the spinal origin of the trapezius.

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