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Fresh cadaveric lumbar spines of 20 adult large breed dogs were used to study the sixth and seventh lumbar spinal nerves along their course through their respective intervertebral foramen. The relationship between the periosteum lining the vertebral canal (endorhachis; peridural membrane) and the vessels inside the vertebral canal, and the relationship between the nerves and the wall of the intervertebral foramen and the extraspinal suspensory apparatus were investigated. Each intervertebral foramen contained a fibrous septum that divided it into two sub-compartments by connecting the fibrous capsule of the facet joints with the intervertebral disc and the adjoining vertebral body. The lumbar nerves and the main artery passed through the cranial sub-compartment and the main vein passed through the caudal sub-compartment. In all cases, there was a circumneural sleeve that connected the ventral branches of the lumbar nerves extraspinally with the fibrous capsule of the facet joints dorsally, the fibrous septum caudally, and the caudal vertebral notch and accessory process cranioventrally. The deep layer of the circumneural sleeve was formed by the periosteum lining the vertebral canal pouching laterally through the intervertebral foramen; the superficial (lateral) layer was formed by the deep sheet of the thoracolumbar fascia. The deep sheet of the thoracolumbar fascia continued cranially and caudally to the circumneural sleeve to attach it to the vertebral body and the intervertebral disc. Regional and individual differences were noted in the composition and length of the circumneural sleeve. The potential biomechanical and clinical roles of these variations are discussed.