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/THE BACK/SKIN

Section 5 – THE BACK

CHAPTER 42 – The back

Most clinical disorders of the back present as low back pain with or without associated lower limb pain, so historically most attention has been paid to the anatomy of the lower (lumbosacral) back. In this Section, the term ‘the back’ will include the whole of the posterior aspect of the trunk and of the neck. The whole of this region has great clinical importance but its anatomy has often been neglected. Recent understanding of the detailed topography of the bony and soft-tissue elements of the lower back owes much to the work of Bogduk (2005).

The soft tissues of the back of the trunk and neck include the skin and subcutaneous fat, the underlying fascial layers and the musculature. The deep, ‘true’ or epaxial muscles lie within compartments in their own fascial ‘skeleton’. The bony framework to which the muscles and fasciae attach includes not only elements of the axial skeleton, i.e. the vertebral column and occiput, but also elements of the pectoral and pelvic girdles as well as the ribs. The occiput is described below, the scapula on page 793, the ribs on page 918 and the pelvis on page 1352.

SKIN

The skin of the back of the trunk is thick and highly protective, but has low discriminatory sensation. The superficial fascia is thick and fatty in most areas of the back. Its attachment to the deeper fascial layers is strong in the midline, especially in the neck, but becomes weaker more laterally. The skin of the back of the neck is thicker than that of the front of the neck, but thinner than that of the back of the trunk. The quantity, texture and distribution of hair vary with sex, race and the individual, though well-defined hair tracts have been delineated (Fig. 42.1).

  

Fig. 42.1  Hair tracts on the dorsal surface of the body.
(Redrawn by permission from Wood Jones F (ed) 1949 Buchanan’s Manual of Anatomy, 8th edn. London: Baillière Tindall and Cox.)

Lines of skin tension run horizontally in the cervical and lumbosacral regions but form segments of two adjacent circles in the thoracic region (Fig. 42.2).

  

Fig. 42.2  Lines of skin tension on the dorsum of the trunk and head.
(From Kraissl CL, Plast Reconstruct Surg 8: 1–28, 1951. By permission from Lippincott Williams and Wilkins.)

CUTANEOUS INNERVATION AND DERMATOMES

The skin of the back of the neck and trunk is innervated by the dorsal (posterior primary) rami of the spinal nerves (see Fig. 43.6 and pp. 755–756 where dorsal rami are covered in detail). In the cervical and upper thoracic regions (down to T6) skin is supplied by the medial branches of these rami, while in the lower thoracic, lumbar and sacral regions it is supplied by the lateral branches. The total area supplied by these dorsal rami is shown in Fig. 43.6. The spinal nerves involved include C2 to C5, T2 to L3, S2 to S4, and Co1. The pattern of their dermatomes is shown in Fig. 42.3. There is about half a segment of overlap between these cutaneous ‘strips’: the strips supplied by the dorsal rami do not correspond exactly to those served by ventral rami, and differ slightly in both width and position.

  

Fig. 42.3  Dermatomes on the dorsal surface of the body. The small diagram shows the regular arrangement of dermatomes in the upper and lower limbs of the embryo.
(Adapted with permission from Moffat DB 1993 Lecture Notes on Anatomy, 2nd edn. Oxford: Blackwell Scientific.)

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