(a, b) Transverse section of the spinal canal at the C1 and C4 level. The cervical spinal canal is enlarged laterally because of the large cervical nerve roots that supply the upper limbs. The denticulate ligaments are evident on the lateral aspects of the cord, and a thick dorsal arachnoid septum is also present

(a, b) Transverse section of the spinal canal at the C5 and C7 level. The spinal canal becomes rounder at the lower cervical level (C7). The denticulate ligament, the dorsal arachnoid septum, and the dorsolateral arachnoid septum cover the dorsal and ventral roots

(a) Intrathecal structures anchoring the spinal cord and spinal roots. Several narrow, fibrous or membranous structures in the subarachnoid space cover and anchor the spinal cord, spinal roots, and nerves. These include the denticulate ligament (DL), the dorsal arachnoid septum (DAS), the dorsolateral arachnoid septum (DLAS), and the filum terminale (FT). The DL, referred to as an intrathecal ligament, is a representative intrathecal structure that anchors the spinal cord and roots [7]. The DL forms tent like structures on both sides of the spinal cord between the ventral and dorsal spinal roots and stabilizes the spinal cord. Each DL extends from the craniovertebral junction to T12, and features 18–20 triangular extensions that attach to the dura at their apices [8]. The DAS is also present in the midline, but it is less conspicuous than the denticulate ligament [9]. It is not a continuous sheet, but rather a series of broad leaflets derived from the intermediate leptomeningeal layer [9]. The DLAS arises as a reflection of the intermediate layer [9]. It is smaller than the DAS and is occasionally present at the cervical and thoracic level. This intermediate layer spreads over the surface of the cord and is connected to blood vessels, nerve roots, and pia mater by fine trabeculae. Thin membranes cross the DAS vertically at regular intervals, from the upper to the lower thoracic level. We have termed these membranes the dorsolateral transverse arachnoid septa (DLTAS). They divide the bilateral thoracic subarachnoid columns into small segmental chambers. (b, c) Pathological forms of spinal ligaments. Ossification of the posterior longitudinal ligament (OPLL) is a hyperostotic condition of the spine associated with spinal canal stenosis and cervical myelopathy. (b) MRI of a spine with OPLL. (c) Schema showing four types of OPLL [6]

(a, b) Transverse section of the spinal canal at the T3 level

(a, b) Transverse section of the spinal canal at the T4 level

(a, b) Sagittal section of the spinal canal in the upper thoracic region (T1–T3). The black arrows indicate the dorsal arachnoid septum and the dorsolateral transverse arachnoid septum. The dorsolateral transverse arachnoid septum is an intermediate layer of leptomeninges that form a discontinuous series of dorsal ligaments attaching the spinal cord to the arachnoid mater

Oblique section of the thoracic spinal canal from T2 to T3 (a) and T4 to T7 (b). The dorsal arachnoid septum and several dorsolateral (posterolateral) transverse arachnoid septa of the thin-layer type are exposed. The thin layers are concentrated in the dorsal region of the spinal canal

(a) Sagittal view of ligamenta flava at the thoracic level. The red circles indicate regions of epidural fat, and the yellow arrows indicate a ligamentum flavum. (b) Epidural fat at the T5 to T7 level