230 LABORATORY MANUAL OF HUMAN ANATOMY 



the occipital artery (A. occipitalis) to be seen in the triangle? 

 Dissect out the portion of the accessory nerve (N. accessorius) 

 belonging to the triangle. Note its exact point of emergence 

 from the substance of the M. sternocleidomastoideus. Find the 

 rami nmsculares of the plexus cervicalis which run across the 

 triangle to disappear beneath the anterior margin of the M. 

 trapezius. How are they related in position to the N. accesso- 

 rius? Do you find any large cutaneous nerve of the cervix 

 (N. cutaneus cervicis) I 



Broad Muscles of the Back (First Layer). (Vide Fig. 5, p. 49.) 



(a) Trapezius muscle (M. trapezius). After its surface has been thor- 

 oughly cleaned, in conjunction with the dissector of the upper ex- 

 tremity, the exact origin and insertion of the M. trapezius are to 

 be established. Test the effect of contraction of its various por- 

 tions, fixing alternately the head and the shoulder girdle. With the 

 dissector of the upper limb, reflect the muscle and dissect the 

 structures on its deep surface. Divide the attachment to the linea 

 nuchae superior and protuberantia oecipitalis externa and cut 

 through it about one centimetre from the cervical vertebral spines 

 and five centimetres from the thoracic spines, taking care that the 

 knife does not injure the underlying structures. Turn the muscle 

 lateralward and allow the dissector of the upper limb to work out 

 the distribution of the N. accessorius and the nerves from the 

 cervical plexus going to the M. trapezius. Determine the origin 

 and distribution of the A. cervicalis superficialis. In your cadaver 

 does it come from the truncus thyreocervicalis or from the A. trans- 

 versa colli? 



(ft) Broadest muscle of back (M. latissimus dor si}. The dissector of the 

 arm has worked this muscle out and the dissector of the head should 

 review his knowledge of it. 



(c) Elevator muscle of scapula (M. levator scapulae] (0. T. levator 

 anguli scapulae) and neighboring vessels and nerves. Ex- 

 amine the M. levator scapulae and the nerves on its surface 

 (from the plexus cervicalis). Lift the muscle carefully and 

 find beneath it 



(ca) Descending branch (ramus descendens] of the A. transversa 



colli, often called the " posterior scapular artery." 



(cb) Dorsal nerve of scapula (N. dorsalis scapulae] (0. T. nerve 



to the rhomboids). Note the twigs from this nerve to the 

 M. levator scapulae. 



Establish the exact origin and insertion of the muscle 

 and test its action. 



Expose the scapular portion of the inferior belly (venter 

 inferior] of the M. omohyoideus, and note the position of the 

 A. transversa scapulae (0. T. suprascapular, or transversalis 

 humeri) and the suprascapular nerve (N. suprascapularis) and 



