The sacrum is wedge shaped and consists of five vertebrae fused together. It lies distal to the fifth lumbar vertebra and is connected distally, at its apex, to the coccyx. Dorsally, the sacrum has a convex surface, in the middle of which the median sacral crest stands out (Fig. 40.4a).

The crest is produced by the fusion of the rudimentary spinous processes of the upper third or fourth sacral vertebrae. Normally, the arch of the fifth and occasionally also of the fourth sacral vertebra is absent, so that there is a sacral hiatus at this point. The hiatus is bounded by the sacral horn as a remnant of the caudal articular process, and it is used as a passage by the five small sacral nerves and by the coccygeal nerves. Between the median sacral crest and the lateral sacral crest lie the four sacral openings (the posterior sacral foramina), through which the dorsal branches of the sacral spinal nerves emerge. The anterior view shows a concave aspect. Alongside the transverse lines (fused vertebrae), there are large anterior openings (the anterior pelvic sacral foramina), through which the primary anterior parts of the sacral nerves emerge (Fig. 40.4b).

The vertebrae are supported from the axis to the cranial sacrum by intervertebral disks and by various ligaments (Fig. 40.5a, b). The intervertebral disks lie between neighboring vertebrae and function as fixed connecting elements and pressure-absorbing buffers. The disks are at their thinnest in the area of T3–T7 and thickest in the lumbar area.

The anterior longitudinal ligament (Figs. 40.6 and 40.7) is attached at the anterior edge of the vertebral bodies and intervertebral disks and is at its thickest in the thoracic area. The posterior longitudinal ligament (Figs. 40.6 and 40.7) is wider cranially than it is caudally, and it lies behind the vertebral bodies in the medullary canal. The supraspinous ligaments extend as far as the sacrum along the tips of the spinous processes, with which they are connected, and continue cranially in the nuchal ligament and caudally in the interspinous ligament. They become thicker from cranial to caudal. The interspinous ligaments connect the roots and tips of the spinous processes.

The intertransverse ligaments serve to connect the transverse processes (Fig. 40.5b).

The ligamentum flavum largely consists of yellow, elastic fibers and it connects the neighboring laminae (Figs. 40.6 and 40.7). It is at its thinnest in the midline (small fissure spaces exist for the veins running from the internal vertebral venous plexus to the external vertebral venous plexus), and its thickness increases laterally. The size and shape of the ligamentum flavum vary at the various levels of the spine. Caudally, for example, it is thicker than in the cranial direction.

There are usually only 22 genuine ligamenta flava, as they do not develop between the atlas and occiput or between the atlas and the axis. The mean length of the ligamentum flavum is 0.84 cm in the cervical spine, 1.03 cm in the thoracic spine, and 1.61 cm in the lumbar spine. The mean width is 2.30 cm in the cervical region, 2.0 cm in the thoracic region, and 3.22 cm in the lumbar region. The ligamenta flava have a mean thickness of 1.37 mm in the cervical spine, 1.75 mm in the thoracic spine, and 2.55 mm in the lumbar spine. According to Yong-Hing et al. (1976), the human ligamentum flavum consists of 50–80 % elastin and 20–50 % collagen [9].

The stability of the iliolumbosacral region is ensured by lumbosacral and sacroiliac connections that transfer the entire weight of the trunk via the hip bones to the lower extremities. These ligamentous connections serve to connect the vertebrae with one another and to stabilize the sacrum.

Clinically important ligaments: interspinous, supraspinous, iliolumbar, interosseous sacroiliac, sacrospinous, and sacrotuberous ligaments (Fig. 40.8a, b).

The mean length of the spinal cord from the foramen magnum downward is 45.9 cm in men and 41.5 cm in women [8, 9]. The conus medullaris continues in the threadlike median filum terminale as far as the posterior side of the coccyx (Figs. 40.7, 40.9a–c, and 40.13).

The cauda equina is generally regarded as consisting of the thick, horsetail-shaped fascicle of nerve fibers in the lower part of the dural sac, containing the paired rootlets of the lowest thoracic and entire lumbar and coccygeal medulla. All of the motor and sensory fibers of the lumbosacral plexus, pudendal nerve, and coccygeal nerve course inside the lumbosacral subarachnoid space. A constant increase in the thickness of the root fascicle is seen between the L1 and L3 segments. The terminal filum of the spinal cord is approximately 153 mm long (range 123–178 mm) (Figs. 40.9b, 40.11, 40.13, and 40.14).

The dura mater and arachnoid, and consequently the subarachnoid space as well, extend downward as far as the level of the second sacral vertebra.

The spinal cord is surrounded and protected by the meninges (the dura mater, arachnoid mater, and pia mater) and by cerebrospinal fluid, epidural fatty tissue, and veins (Figs. 40.10 and 40.11).

The meninges form a connected, unified organ with important protective functions for the brain and spinal cord; they provide mechanical, immunological, and thermal protection and are also important for metabolism in the CNS. The mechanical function of the meninges consists of providing fixation for the brain in the cranium and for the spinal cord in the vertebral canal, as well as in forming a highly adaptable fluid mantle that functions like a water bed. The fixation is provided by the dura mater.

The dura mater of the spinal cord, a fibroelastic membrane, extends as far as the second sacral vertebra, where it ends in a blind sac. It encloses the anterior and posterior spinal nerve roots.

It surrounds the anterior and posterior spinal nerve roots. It consists of collagenous fibers and a few elastic fibers; it is 0.1–0.5 mm thick and is usually thinner anteriorly than it is posteriorly. The dural sac is often enlarged in the area between L4 and S1 and also between T12 and L1 (Spischarny’s terminal cystoma). The thickness of the spinal dura mater declines from medial to lateral. The dura mater is often thinner ventrally than dorsally, and it sends off leaves and fibers into the epidural space. It shows notable foliation near the intervertebral foramina, where it passes on the one hand into the epineurium of the peripheral nerves and into the capsule of the spinal ganglia and on the other hand radiates into the cavernous body of the vascular plexus characterizing this area. One of the major characteristics of the dura mater is its extremely marked vascularization. Particularly in the boundary zone with the arachnoid, numerous capillaries and venules are fenestrated, promoting the exchange of various metabolites and cells between the blood and dura. The vascularization increases strongly in the lateral areas, particularly in the area of the dural infundibulum. The dural vessels are connected with the epidural vascular plexus and thus with the internal and external vertebral vein plexus as well. There are numerous nerve fibers in close proximity to the dural vessels [6, 12]. In terms of size, C fibers predominate. In terms of function, the dural nerves consist of sympathetic, parasympathetic, and sensory fibers. A whole range of neurotransmitters and neuropeptides have been identified in the dural nerves, such as acetylcholine, serotonin, substance P, calcitonin gene-related peptide (CGRP), neuropeptide Y, vasoactive intestinal polypeptide (VIP), etc. [3, 7]. In addition to vasoactive nerves, local factors also play a role in the regulation of dural perfusion, such as adenosine, which has been found abundantly in the dura [4]. Via a multilayered cell cluster known as the subdural nerve papilla (neurothele) [1, 5], the arachnoid membrane is directly connected with the innermost lamellae of the dura. A “subdural fissure”—often present in postmortem and only arising during life in specific circumstances—must be regarded as an artifact.

Between the dura mater and the arachnoid, there is a space, the subdural space, in which a small amount of lymph-like fluid is located.

The arachnoid mater (Figs. 40.10 and 40.11), a nonvascularized membrane, also ends at the level of the second sacral vertebra. Between the arachnoid mater and the pia mater lies the subarachnoid space, which is filled with cerebrospinal fluid (see the section “Cerebrospinal fluid”, below). The expansion forces present here reduce the weight of the brain “floating” in the cerebrospinal fluid to just under 50 g.

This protects the brain from harmful acceleration forces.

The same applies to the spinal cord. There is a substantial amount of exchange between the cranial and spinal cerebrospinal fluid. Magnetic resonance studies have confirmed Du Boulay’s older view that there are fluid waves synchronous with cardiac systole that move the cerebrospinal fluid from the basal cisterns into the cervical subarachnoid space [10]; respiration-synchronous fluid movements are superimposed on these [11]. The spinal subarachnoid space, with the extremely elastic spinal dura and adaptable arachnoid, makes it possible to absorb these rhythmic fluid waves (compliance). Artificial losses of cerebrospinal fluid undoubtedly have severe effects on the system’s natural mechanism and reduce the water-bed function of the exterior subarachnoid space. The subarachnoid space is important for the thermoregulation of the CNS, as it acts as a temperature buffer [13] both against excessive external temperatures and also against functional or pathological increases in temperature in the CNS. The normal basic temperature of the cerebrospinal fluid is approximately 37 °C. It is important to note that the cerebrospinal fluid does not represent “standing water” but is instead constantly in movement and continually being freshly produced, mainly in the choroid plexus. It is reabsorbed over a wide surface, not only via the arachnoid granulations or villi in the area of the nerve exit sites but also diffusely through the arachnoid into the dural vascular system and via the pia into the vessels of the pial vascular network [12]. The choroid plexus is not only a production organ but also a site for reabsorption of cerebrospinal fluid.