Figure 1.

Sequential cryomicrotome sections of the adult human intervertebral foramen between the third and fourth lumbar vertebrae, demonstrating the structure of the proximal spinal nerve. The specimen was injected with orange ink by an epidural approach prior to freezing and sectioning. The orange dot (arrowhead) is an ink-filled drill hole used for guiding alignment. Medial is left in the image, anterior is up; the scale bar is common to all panels. A. At the level of the pedicle, the ventral root (vr) and dorsal root (dr) have diverged from the thecal sac in a common dural sleeve and are apposed to the medial aspect of the pedicle. The ventral primary rami (vpr) of the next rostral level are seen entering the substance of the psoas muscle. B. Somewhat more caudal, at the level of the intervertebral foramen, the ventral root and dorsal root ganglion (drg) are well outside the spinal canal, where injectate has followed. A ramus communicans (rc) is traveling posteriorly from the paravertebral sympathetic chain (off the anterior edge of the image). C. At a more caudal level, even with the rostral margin of the intervertebral disc, the dorsal root ganglion has split into fascicles that merge with similar components from the ventral root and ramus communicans to form a small plexus, composing the initial fascicles of the spinal nerve (snf). D. At a somewhat more caudal level, even with the rostral end of the intervertebral foramen, the fascicles merge into the ventral and dorsal primary rami of the spinal nerve (vpr, dpr).