500 



HUMAN ANATOMY. 



Nerve-Supply. — The external branch of the spinal accessory and the second 

 and third cervical nerves. 



Action. — The two muscles of opposite sides, acting together, will draw the 

 head forward, thus bending the neck. Acting singly, each muscle will tend to draw 

 the head towards its own side and at the same time to rotate it towards the opposite 

 side. 



Relations. — Superficially the muscle is covered by the platysma, and is crossed 

 obliquely by the external jugular vein and in varying directions by the superficial 



branches of the cervical plexus. It covers. 

 Fig. 511. above, the upper part of the posterior belly of 



the digastric, the splenius capitis, the levator 

 scapulae and the scaleni, and below it crosses 

 the omq-hyoid and covers the lower attach- 

 ments of the sterno-hyoid and sterno-thyroid. 

 It also covers the common carotid artery and 

 the lower portions of the external and internal 

 carotids, the facial and internal jugular veins, 

 the cervical plexus, and the lateral lobe of the 

 thyroid gland. 





Cleido- 

 occipital 



nio-occipital 



mastoid, su- 

 :ial and deep 



mastoid 



Variations.— Considerable variation exists in the 

 amount of fusion of the two heads, their complete 

 distinctness being of so frequent occurrence as to be 

 regarded as normal by some authors. But, in ad- 

 dition to these two portions, the muscle presents fre- 

 quently a separation into other parts, and compara- 

 ti\e anatomy re\-eals a primary constitution of the 

 muscle from' at least five distinct portions, any one 

 or more of which may appear as distinct bundles 

 (Fig. 511). These portions are arranged in two 

 layers, the superficial one consisting of a superficial 

 sterno-mastoid, a sterno-occipital, and a cleido- 

 occipital portion, while the deep one is formed by 

 a deep sterno-mastoid and a cleido-mastoid portion, 

 the names applied indicating the attachments of the 

 various bundles. 



Occasionally the lower portion of the muscle 

 is traversed by a tendinous intersection, a peculiarity of interest in connection with the formation 

 of the muscle by the fusion of portions derived from different myotomes. 



Cieido-occipi- 

 tal, lower 

 part turned 

 downward 



Quadricipital type of sterno-mastoid, showing the 

 components of the muscle. {After Maubrac.) 



2. Trapezius (Figs. 512, 559). 



Attachments. — The trapezius is the most superficial muscle upon the dorsal 

 surface of the body, and is a triangular sheet whose base corresponds with the mid- 

 dorsal line. The two muscles of opposite sides being thus placed base to base, form 

 a rhomboidal sheet which covers the nape of the neck and the upper part of the back 

 and shoulders, resembling somewhat a monk's cowl, whence the name cncidlaris 

 sometimes applied to the' muscle. 



It arises above from the superior nuchal line and the external occipital pro- 

 tuberance, and thence along the ligamentum nuchae and the spinous processes of 

 the seventh cervical and all the thoracic vertebrae, together with the supraspinous 

 ligaments. The upper fibres pass downward and outward, the middle ones directly 

 outward, and the lower ones upward and outward, and are inserted, the upper ones 

 into the outer third of the posterior border of the clavicle, the middle ones into the 

 inner border and upper surface of the acromion process and the upper border of the 

 spine of the scapula, and the lower ones into a tubercle at the base of the scapular 

 spine. 



Throughout the greater part of its len^^th the origin of the muscle is by short 

 tendinous fibres intermingled with muscle-tissue, but from about the middle of the 

 ligamentum nuchse to the spinous process of the second thoracic vertebra it is entirely 

 tendinous. Furthermore, throughout the upper half of this portion of the origin the 

 tendinous fibres gradually increase in length and throughout its lower half they again 

 diminish, so that there is formed by the two muscles of opposite sides, in this region, 



