The fifth lumbar ventral spinal nerve rootlets join to form a number of aggregated rootlet bundles. These in turn fuse to form the ventral root. Each rootlet is surrounded by a sheath which consists of a single fenestrated layer of cells and their attenuated cytoplasmic processes and which is open ended proximally. Immediately superficial to the spinal cord surface, rootlets are separated from one another by a labyrinth of inter-radicular spaces containing small blood vessels. Between adjacent rootlets the inter-radicular space tapers distally to an apex. The endoneurial space of the rootlet communicates with the subpial and inter-radicular spaces. Each aggregated rootlet bundle is surrounded by a multilayered sheath. Proximally, the outer layers of this sheath are continuous with the superficial layers of the pia mater. Both of these, as well as the root sheath with which the rootlet bundle sheaths are continuous distally, are complete and lack fenestrations. Accordingly, the endoneurial space, though continuous with the inter-radicular and subpial spaces, is isolated from the subarachnoid space.