TRAPEZIUS 349 



to the base of the temporal bone. It is incomplete and is more or less fused to the posterior 

 belly of the digastric muscle, the styloid process, and the muscles arising from this process. 

 Between the styloid process and the angle of the jaw this lamina is strengthened to form the 

 stylo-mandibular ligament. 



In the back, beyond the spine of the scapula, the fascia arising from the investing adherent 

 fascial sheath of the trapezius muscle is continued laterally across the fascia investing the infra- 

 spinatus muscle, and becomes fused with the most superficial layer of this fascia and more 

 distally with that of the latissimus dorsi muscle. Near this lateral line of fusion it is usually 

 closely adherent to the tela subcutanea. 



MUSCLES 



-The sterno-cleido-mastoideus (fig. 348). — Origin. — By a medial (sternal) head from the 

 front of the manubrium and by a lateral (clavicular) head from the upper border of the median 

 third of the clavicle. Between the two origins there intervenes a triangular area covered by 

 the external cervical fascia. Its insertion is — (1) on the anterior border and outer surface of 

 the mastoid process, and (2) on the lateral half of the superior nuchal line of the occipital bone. 



Structure. — The tendons are comparatively short, the longest being that on the anterior 

 surface of the sternal attachment. The fibre-bundles of the muscle take a nearly parallel 

 course from origin to insertion. Five fasciculi may be more or less clearly recognised. In a 

 superficial layer — (1) a superficial sterno-mastoid; (2) a sterno-occipital; and (3) a cleido- 

 occipital. In a deep laj'er — (4) a deep sterno-mastoid and (5) a cleido-mastoid. 



Nerve-supply. — (1) From the spinal accessory nerve, which gives it branches during its 

 course through the deep portion of the muscle, and (2) by branches from the anterior primary 

 divisions of the second and third (?) cervical nerves. These branches enter the deep surface 

 of the upper half of the muscle. 



Action. — To bend the head and neck toward the shoulder and rotate the head toward the 

 opposite side. When both muscles act, the neck is flexed toward the thorax and the chin is 

 raised; or, with fixed head, the sternum is raised, as in forced respiration. When the head is 

 bent back, the two muscles may further increase the hyperextension. 



Relations. — The muscle and its sheath are covered externally by the tela subcutanea, which 

 here contains the platysma and the external jugular vein, as well as the superficial branches of 

 the cervical plexus. Beneath the muscle lie the sterno-hyoid, sterno-thyreoid, omo-hyoid. 

 levator scapulae, scaleni, splenius, and digastric muscles, the cervical plexus, the common carotid 

 artery, internal jugular vein, and the vagus nerve. The spinal accessory nerve usually runs 

 through its deep cleido-mastoid portion. 



Variatio7is. — There is considerable variation in the extent of independence of the main 

 fasciculi of the muscle. In many of the lower animals the cleido-mastoid portion of the muscle 

 is quite distinct from the sterno-mastoid portion, and this condition is frequently found in 

 man. The cleido-occipital portion of the muscle is that most frequently absent (Wood found 

 it present in 37 out of 102 instances). The clavicular portion of the muscle varies greatly in 

 width. The sternal head has been seen to extend as far as the attachment of the fifth rib. 

 Slips from the muscle may pass to various neighbouring structures. The main fasciculi of the 

 muscle may be doubled. Sometimes one or more tendinous inscriptions cross a part or the 

 whole of the superficial layer of the muscle. 



The trapezius (fig. 355). — Origin. — By a flat aponeurosis from the superior nuchal Une and 

 external protuberance of the occipital bone, the ligamentum nuchse, and the vertebral spines 

 and supraspinous ligament from the seventh cervical to the twelfth thoracic vertebra. The 

 aponeuroses of the right and left muscles are continuous across the middle line. Between the 

 middle of the ligamentum nuchae and the second thoracic vertebra, the aponeuroses give rise 

 to an extensive quadrilateral tendinous area. At the distal extremity of the muscle they are 

 also well developed. 



Structure and Insertion. — The superior fibre-bundles pass obliquely downward, lateralward, 

 and forward to the postero-superior aspect of the lateral third of the clavicle; the middle fibre- 

 bundles, transversely to the medial edge of the acromion and the upper border of the spine of the 

 scapula; the lower fibre-bundles, obliquely upward and laterally to terminate through a flat, 

 triangular tendon on a tubercle at the medial end of the spine of the scapula. 



Nerve-supply. — The external branch of the spinal accessory nerve descends for a distance 

 near the superior border of the trapezius muscle and then along the ventral surface. Soon it 

 gives rise to ascending branches for the superior portion of the muscle and descending branches 

 for the middle and inferior portions. The main branches of distribution run about midway 

 between the origin and insertion of the fibre-bundles. The branches from the second (?), third 

 and fourth cervical nerves anastomose with the trunk of the spinal accessory, sometimes as it 

 passes along the margin of the muscle, at other times within the substance of the upper portion 

 of the muscle. 



Actio7i. — When the whole muscle contracts, it draws the scapula toward the spine and 

 turns it so that the inferior angle points laterally, the lateral angle upward. In addition the 

 upper portion draws the point of the shoulder upward, and with the scapula fixed extends the 

 head, bends the neck toward the same side, and turns the face to the opposite side. The lower 

 portion of the muscle tends to draw the scapula downward and inward and at the same time to 

 rotate the inferior angle of the scapula outward. 



Relations. — -It is covered merely by skin and fascia. It lies external to the seniispinalis, 

 splenii, rhomboidei, latissimus dorsi, levator scapute, supraspinatus, and a small portion of the 

 infraspinatus muscles. 



Variations. — The lower limit of attachment of the muscle may be as high as the fourth 

 thoracic vertebra. The right and left muscles are seldom symmetrical. The upper attach- 

 ment may not extend to the skull. The clavicular attachment may be much more extensive 



