DISSECTION OF THE BACK 51 



Dissection. When searching for the cutaneous nerves cut 

 boldly down through the superficial fascia, in the direction in 

 which the nerves run (Fig. 26), until the plane is reached at 

 which the superficial and deep fascia blend. It is there that the 

 main trunks are to be found, and in a well-injected subject the 

 cutaneous arteries will serve as guides. A more rapid way of 

 finding the cutaneous nerves in this region is to reflect the super- 

 ficial and deep fascia laterally, from the vertebral spines, in one 

 layer ; the nerves are then found as they issue from the muscles. 

 This plan, however, should be adopted only by the senior student. 



Nervi Cutanei et Vasa Cutanea (Cutaneous Nerves and 

 Vessels). The cutaneous nerves of the back are derived from 

 the posterior rami of the spinal nerves. As the posterior 

 rami pass backwards, they subdivide into medial and 

 lateral divisions. Both divisions supply twigs to the muscles 

 amongst which they lie ; but one or the other also contains 

 some sensory fibres which come to the surface, in the shape 

 of a cutaneous nerve, to supply the skin. 



In the thoracic region the upper six or seven cutaneous 

 nerves are the terminations of the medial divisions of the 

 posterior rami of the thoracic nerves. They become super- 

 ficial close to the vertebral spines, and are to be sought 

 for near the median plane. It is not uncommon to find one 

 or more of them piercing the trapezius, one or two inches 

 lateral to the line of emergence of the others. The branch 

 which comes from the second thoracic nerve is the largest of 

 the series ; and it may be traced laterally, across the spine of 

 the scapula, towards the shoulder. The lower five or six 

 cutaneous nerves in the thoracic region are the terminal twigs of 

 the lateral divisions of the posterior rami of the thoracic 

 nerves ; and, consequently, they must be looked for at a 

 short distance from the middle line of the back. The upper 

 nerves of this group reach the surface after piercing the 

 latissimus dorsi muscle on the line of the angles of the ribs. 

 The lower nerves of the group pierce the lumbo-dorsal fascia 

 at the lateral margin of the sacrospinalis muscle. In every 

 case the cutaneous branches derived from the thoracic nerves 

 turn laterally, in the superficial fascia, and may be traced for 

 a varying distance in that 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 ramus from which it arises. 



In the lumbar region three cutaneous nerves reach the 

 surface after piercing the lumbo-dorsal fascia at the lateral 



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