40 THE UPPER EXTREMITY 



line of the angles of the ribs and the lateral margin of the 

 sacrospinalis muscle. In every case the cutaneous branches 

 derived from the thoracic nerves turn laterally in the super- 

 ficial fascia, and may be traced for a varying distance in this 

 direction. 



It is important to note that the area of skin supplied by 

 each of these cutaneous nerves is placed at a lower level 

 than the origin of the posterior branches from which it arises. 



In the lumbar region three cutaneous nerves reach the 

 surface by piercing the lumbar fascia at the lateral margin 

 of the sacrospinalis muscle, a short distance above the 

 ilium. They are the terminal twigs of the lateral branches of 

 the posterior rami of the' three upper lumbar spinal nerves ; 

 and they differ from those above, inasmuch as they turn 

 downwards over the crest of the ilium to supply the skin of 

 the gluteal region (Fig. 17). 



The cutaneous arteries which accompany these nerves come 

 from the posterior branches of the intercostal and lumbar 

 arteries. 



Muscles connecting the Limb to the Dorsal Aspect of the 

 Trunk. These are five in number, and are arranged in two 

 strata. Two form the superficial stratum, viz. the trapezius 

 and the latissimus dorsi. Both are broad, flat muscles which 

 cover the greater part of the dorsal aspect of the trunk, from 

 the occiput above to the ilium below. The trapezius lies 

 over the back of the neck and the thorax. The latissimus 

 dorsi is placed lower down. The deeper stratum of muscles, 

 composed of the levator scapulae and the two rhomboid 

 muscles, is placed under cover of the trapezius. 



Dissection. The trapezius should now be cleaned. This muscle belongs 

 only in part to the dissector of the upper extremity. The portion of it 

 which lies above the prominent spine of the seventh cervical vertebra is the 

 property of the dissector of the head and neck, and must be dissected by 

 him. The two dissectors should work in conjunction with each other ; and 

 when the entire muscle is exposed, each should give the other an opportunity 

 of studying it in its entirety. 



In cleaning the trapezius the limb must be placed in such a position as 

 will render the fibres of the muscle tense. If the dissection is being made 

 on the right side, the arm must be placed close to the trunk, and drawn 

 downwards, whilst the scapula is dragged well forwards over the end of 

 the block which supports the thorax. A transverse cut is now to be made 

 through the superficial and deep fasciae, from the seventh cervical spine 

 laterally. This incision will be found to coincide with the direction of 

 the fibres of the muscle at this level. From this point work gradually 

 downwards, raising both fasciae in a continuous layer from the surface of 



