Figure 2.

Digital reconstruction of human cryomicrotome axial images (using Skandha, copyright by the Department of Biologic Structures, University of Washington, Seattle, Washington), demonstrating the normal anatomy of the spinal canal and intervertebral foramen. A. Looking caudally from the plane of the 3^rd lumbar vertebral pedicles, the medial apertures of the left L3/4 and L4/5 are apparent where they branch off from the vertebral canal. Bone is white/blue, fat is red, ligamentum flavum is yellow, the venous system is dark blue, and the disc and posterior longitudinal ligament are brown. Empty spaces would be occupied by the thecal sac in the spinal canal and the nerve root sleeve in the intervertebral foramen. Abundant fat is present in the “axilla” area between the main dural sac and the diverging nerve root sleeve. B. A more oblique view of the same digitized specimen, with the fat removed to show the structures that may impinge on nerve tissue. The dorsal root ganglion and ventral root (shown in dotted orange outline) are in the rostral end of the foramen clear of the disc, but may be compressed if loss of disc height decreases the rostro-caudal dimension of the foramen. The most common location of disc herniation is parasagittal (arrow), which most often affects the nerve roots that are heading for their exit at the next lower foramen.