MUSCLES OP THE BACK. 173 



First Layer. 



Dissection. The muscles of this layer are to be dissected by 

 making an incision along the middle line of the back, from the 

 tubercle on the occipital bone to the coccyx. From the upper 

 point of this incision carry a second along the side of the neck, to 

 the middle of the clavicle. Inferiorly, an incision must be made 

 from the extremity of the sacrum, along the crest of the ilium, to- 

 about its middle. For convenience of dissection, a fourth may be 

 carried from the middle of the spine to the acromion process. The 

 integument and superficial fascia, together, are to be dissected off 

 the muscles, in the course of their fibres, over the whole of this 

 region. 



The trapezius muscle (trapezium, a quadrangle with unequal 

 sides) arises from the superior curved line or semicircular ridge, of 

 the occipital bone, from the ligamentum nuchse, supra-spinous 

 ligament, and spinous processes of the last cervical and all the 

 dorsal vertebra?. The fibres converge from these various points, 

 and are inserted into the scapular third of the clavicle, the acromion 

 process, and the whole length of the upper border of the spine of the 

 scapula. The inferior fibres become tendinous near to the scapula, 

 and glide over the triangular surface at the posterior extremity of 

 its spine % upon a bursa mucosa. When the trapezius is dissected 

 on both sides, the two muscles resemble a trapezium, or diamond- 

 shaped quadrangle, on the posterior part of the shoulders : hence 

 the muscle was formerly named cucullaris (cucullus, a monk's cowl.) 

 The cervical and upper part of the dorsal portion of the muscle is 

 tendinous at its origin, and forms, with the muscle of the opposite 

 side, a kind of tendinous ellipse. 



Relations. By its superficial surface with the integument and 

 superficial fascia, to which it is closely adherent by its cervical por- 

 tion, loosely by its dorsal portion. By its deep surface, from above 

 downwards, with the complexus, splenius, levator anguli scapula?, 

 supra-spinatus, a small portion of the serratus posticus superior, 

 rhomboideus minor, rhomboideus major, intervertebral aponeurosis 

 which separates it from the erector spinas, and with the latissimus 

 dorsi. The anterior border of the cervical portion of this muscle 

 forms the posterior boundary of the posterior triangle of the neck. 

 The clavicular insertion of the muscle sometimes advances to the 

 middle of the clavicle, or as far as the outer border of the sterno- 

 mastoid, and occasionally it has been seen to overlap the latter. 

 This is a point of much importance to be borne in mind in the 

 operation for ligature of the subclavian artery. The spinal accessory 

 nerve passes beneath the anterior border, near to the clavicle, pre- 

 viously to its distribution to the muscle. 



The ligamentum nucha? is a thin cellulo-fibrous layer extended 

 from the tubercle and spine of the occipital bone, to the spinous 

 process of the seventh cervical vertebra, where it is continuous 



