354 DISSECTION OF THE BACK. 



CUTANEOUS NERVES. The tegumentary nerves are derived from the 

 posterior primary branches of the spinal nerves, which subdivide amongst 

 the deep muscles into two pieces, inner and outer. Arteries accompany- 

 ing the greater number of the nerves, bifurcate like them, and furnish 

 cutaneous offsets. 



Cervical nerves. In the neck the nerves are derived from the inner of 

 the two pieces into which the posterior trunks bifurcate : they perforate 

 the trapezius, and supply the neck and the back of the head. They are 

 four in number, viz., one from each, except the first and the three last. 



The branch of the second nerve is named large occipital, and accom- 

 panies the occipital artery to the back of the head (p. 23). 



The branch of the third cervical nerve supplies a transverse offset to 

 the neck, and then ascends to the lower part of the head, where it is dis- 

 tributed near the middle line, uniting with the great occipital nerve 



* Dorsal nerves. These are obtained from both the inner and outer 

 pieces the upper six from the inner, and the lower six from the outer. 

 On the surface they are directed outwards in the integument over the 

 trapezius and latissimus dorsi muscles. 



The upper six perforate the trapezius near the spines of the vertebras ; 

 and the branch of the second, which is larger than the rest, reaches as 

 far as the scapula. The lower six pierce the latissimus dorsi mostly in a 

 line with the angles of the ribs ; they are oftentimes uncertain in number. 



* Lumbar nerves. In the loins the nerves are derived from the outer 

 pieces of the first three ; they perforate the latissimus dorsi muscle at the 

 outer border of the erector spinae, and crossing the iliac crest of the in- 

 nominate bone, are distributed in the integuments of the buttock. 



* FIRST LAYER OF MUSCLES (fig. 116). Two muscles, the trapezius 

 and the latissimus dorsi, are included in this layer. 



Dissection. The superficial fatty layer is to be taken from the trape- 

 zius and latissimus dorsi in the direction of the fibres of each, viz., from 

 the shoulder to the spinal column ; and the upper limb is to be carried 

 backwards or forwards according as it may be necessary to put on the 

 stretch different portions of the muscles. 



Some of the cutaneous nerves and vessels may be left, in order that 

 they may be traced afterwards through the muscles to their origin. 



* The TRAPEZIUS MUSCLE (fig. 116, A ) is triangular in shape, with 

 the base towards the spine, but the two have a trapezoid form. The 

 muscle has an extensive apotfeurotic origin along the middle line from 

 the spines of all the dorsal vertebrae and their supraspinous ligament ; from 

 the spinous process of the seventh cervical vertebra ; from the ligamentum 

 nuchae between the last point and the head ; and lastly from the inner third 

 of the superior transverse ridge of the occipital bone. From this origin the 

 fibres are directed outwards, converging to the shoulder, and are inserted 

 into the outer third of the clavicle, at its posterior aspect ; into the pos- 

 terior border of the acromion ; and into the upper edge of the spine of 

 the scapula as far as an inch from the root of that process, as well as into 

 a rough impression on the surface of the spine near the hinder part. 



The muscle is subcutaneous. At the outer side the lowest fleshy fibres 

 end in a small triangular tendon, which glides over the smooth surface at 

 the root of the spine of the scapula. The anterior border bounds behind 

 the posterior triangular space of the neck. By its insertion the trapezius 

 corresponds with the origin of the deltoid muscle. 



Action. If all the fibres of the muscle act, the scapula gliding on the 



