SUPERFICIAL AND INTERMEDIATE BACK MUSCLES
The superficial back is a good place to hone your dissection skills, as many of the structures are large and easy to identify. Before beginning, be sure to note the superficial anatomy, because in the clinical setting those will serve as landmarks for identifying the underlying tissue, which you will be unable to dissect in your living patients. Use this dissection to get a feel for the usefulness of the scalpel and at what times it is a liability instead of an asset. It is best to use the scalpel to cut through the epidermis and dermis, and the fatty and membranous layers of the superficial fascia. The objective is to reach the natural plane between the membranous layer of the superficial fascia and the membranous deep investing fascia that wraps around and separates the muscles. Once in this plane, blunt dissection and a little force can separate the superficial layers and reveal the muscles of interest rather quickly and without destroying natural borders.
There are two distinct muscle groups within the back, and these are called “intrinsic” and “extrinsic” muscles of the back. An “intrinsic” structure is one that resides fully within that region, while an “extrinsic” structure has attachments outside the region. In the back, this distinction is a result of the embryologic origin of the structures. Intrinsic back muscles are ones derived from the epaxial mesoderm, the group of mesoderm that forms above the axis of the spinal cord (epi- above, axial- of the axis). Extrinsic muscles are formed from hypaxial mesoderm, from below the spinal cord. Extrinsic muscles of the back attach to the limbs or other structures, and are innervated by the anterior (or ventral) primary rami of the spinal nerves, because they developed ventrally to the spinal cord. Intrinsic muscles of the back act on the vertebral column itself and are innervated by posterior (or dorsal) primary rami, as they developed dorsal to the spinal cord. Understanding embryological development is an important way to reinforce anatomical principles.
Begin by placing the cadaver in the prone (face down) position.
Make note of the surface anatomy structures that will guide your clinical observations, including the external occipital protuberance, C7 spinous process, spine of scapula leading laterally to the acromion process; the iliac crest; and the skin dimples that overlie the posterior superior iliac spine.
Referring to the “Back Incisions Guide” below, make a midline vertical incision from the external occipital protuberance at the base of the skull down to the level of the iliac crest inferiorly. Continue by making bilateral horizontal incisions to the lateral mid-axillary boundary from the superior and inferior extents of your previous incision.
Note the fascial plane between the superficial and investing fascias and use it to begin reflecting laterally (from medial to lateral) the skin and superficial fascia from the investing fascia and musculature. ...