Spinal dura	Cranial dura
Single layered and consists of meningeal layer only	Double layered and consists of an inner meningeal layer and outer endosteal layer
Does not form folds	Forms folds, viz. falx cerebri, falx cerebelli, tentorium cerebelli and diaphragma sellae
Epidural space present	Epidural space absent

Arachnoid Mater

Arachnoid mater is a thin delicate transparent avascular membrane which invests the spinal cord loosely. Above it is continuous with the arachnoid mater of the brain and below it extends up to the lower border of the second sacral vertebra (S2).

Pia Mater

Pia mater is a thin highly vascular membrane that closely invests the spinal cord and continues below the spinal cord as a thin thread-like prolongation, the filum terminale.

The subarachnoid space between the pia mater and the arachnoid mater is filled with cerebrospinal fluid (CSF). Above it is continuous with subarachnoid space around the brain. Distal to the termination of spinal cord, the sub-arachnoid space around the filum terminale, becomes roomy, forming a pool of CSF called lumbar cistern. The lumbar puncture is done at this site to take out the CSF (Fig. 7.3).

Fig. 7.3 Sagittal section through lumbosacral region showing conus medullaris, filum terminale (internum and externum), lower end of subarachnoid space and site of lumbar puncture.

N.B. The spinal subarachnoid space is commonly termed ‘spinal intrathecal space’ by the clinicians.

Clinical Correlation

Lumbar puncture

Lumbar puncture is done to withdraw cerebrospinal fluid for various diagnostic and therapeutic purposes. The puncture should be done well below the termination of the cord, i.e. lower border of LI.

A horizontal line joining the highest points of the iliac crests passes through the spine of the fourth lumbar vertebra. Therefore, the interspinous spaces immediately above and below this landmark can be used with safety. The interspinous space between L3 and L4 is the most preferred site (Fig. 7.3). Because in this region subarachnoid space is more roomy and contains only filum terminale and roots of lumbar, sacral and coccygeal nerves forming the cauda equina. During this procedure, the spine must be fully flexed with patient either lying on the side or seated. So, that the interspinous spaces are opened up to their maximum extent and lower end of spinal cord is slightly raised. The needle is passed inwards and somewhat crani-ally exactly in the midline. The supraspinous and interspinous ligaments are traversed; and then dura mater is penetrated, the latter with a distinct feel of ‘give way’. Occasionally, the root pain is experienced if the roots of cauda equina are impinged upon, but usually, they float clear of the needle.

Processes of the pia mater (also called special parts of the pia mater)

The lowest tooth is forked, and the posterior root of the first lumbar nerve lies on the outer prong of the fork (Fig. 7.5). In the lower region of the spinal cord, it is the surgeon’s guide to the first lumbar nerve and gives him a nerve root of known number from which he can determine the position of whatever nerve roots he is in search of.

Fig. 7.5 Posterior view of a part of spinal cord showing forked lowest tooth of ligamentum denticulatum.

External Features of the Spinal Cord

The external features of the spinal cord are (Fig. 7.6):

Fig. 7.6 External features of the spinal cord: (A) on anterior aspect, and (B) on posterior aspect. Also note the important levels (C1, L1, S2, etc.) in relation to the spinal cord.

1. Fissures and sulci.

2. Attachment of spinal nerves.

3. Enlargements.

4. Cauda equina.

Fissures and Sulci

The anterior aspect of the spinal cord (Fig. 7.6A) presents anterior median fissure, and two anterolateral sulci while the posterior aspect (Fig. 7.6B) presents: posteromedian sul-cus, two posterolateral and two posterointermediate sulci.

The anterior median fissure is deep and extends along the entire length of the cord. The anterior spinal artery runs in it.

The posterior median sulcus is a faint longitudinal groove. From its floor, a septum of neuroglial tissue (posterior median septum) extends into the substance of the cord to a variable extent.

The surface of the cord is divided into two symmetrical halves by an anterior median fissure and a posterior median sulcus.

Each half of the cord is further subdivided into posterior, lateral and anterior regions by anterolateral and pos-terolateral sulci. Through anterolateral sulcus emerge the ventral root fibres and through posterolateral sulcus enter the dorsal root fibres of the spinal nerves.

Attachment of Spinal Nerves

Thirty-one pairs of spinal nerves emerge from the sides of the cord.

Of these, 8 are cervical, 12 are thoracic, 5 are lumbar, 5 are sacral, and 1 is coccygeal.

The cervical nerves leave the vertebral canal above the corresponding vertebrae with the exception of eighth, which emerges between seventh cervical and first thoracic vertebrae. The remainder spinal nerves emerge below the corresponding vertebrae. Each spinal nerve is attached to the cord by two roots, anterior motor root and posterior sensory root. The posterior root has a ganglion, the posterior root ganglion consisting of pseudounipolar cells. Each of the two roots is made up of number of rootlets that arise from the spinal cord over a certain length (Fig. 7.7). The rootlets of posterior root enter the dorsolateral aspect of the cord along the posterolateral sulcus whereas the rootlets of anterior root emerge from the anterolateral sulcus/anterolateral aspect of cord opposite to the anterior grey column.

Fig. 7.7 A section of spinal cord showing attachment of nerve roots on one side.

Spinal segments

Like the vertebral column, the spinal cord is also segmented though the segments are not visible externally. The part of spinal cord to which a pair of spinal nerves (right and left) is attached is known as spinal segment (Fig. 7.8).

Fig. 7.8 Schematic diagram to represent the spinal segments.

The number of spinal segments corresponds to the number of vertebrae in thoracic, lumbar and sacral regions, but in cervical region, one segment is more than the number of vertebrae, whereas in coccygeal region there is only one segment for four coccygeal vertebrae.

Thus, the spinal cord is made up of 31 spinal segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and one coccy-geal. Since, the length of spinal cord (45 cm) is smaller than the length of vertebral column (65 cm), the spinal segments are, short and crowded, especially in the lower part of the cord. Thus, the spinal and vertebral segments (spines) do not lie at the same level. The spinal segments as a rule always lie above their numerically corresponding vertebral spines. In the lower part of the spinal cord, they lie well above their corresponding vertebrae (Fig. 7.9). For example, the lumbar segments are related to the thoracic vertebrae.

Fig. 7.9 The spinal segments and their corresponding vertebrae levels.

Approximate vertebral levels matched to spinal cord segments are presented in Table 7.2.

Table 7.2

Approximate vertebral levels of the spinal cord segments

These levels, although not strictly accurate, give a reliable assessment of the level of cord compression following an injury or disease of the surrounding vertebrae. The approximate relationship helps in understanding the consequences of spinal injuries and clinical findings of motor and sensory levels. They also help in planning for surgical approach for diagnostic and therapeutic purposes.

Enlargements

The spinal cord is roughly cylindrical in shape with an average diameter of about 1.25 cm. However, opposite to the attachments of the nerve roots contributing to the formation of brachial and lumbosacral plexuses, the spinal cord presents definite fusiform swellings called cervical and lumbar enlargements respectively (Fig. 7.6A, B).

These enlargements are produced due to the presence of large number of large motor neurons in these regions to supply the musculature of the upper and lower limbs and associated girdles.

The cervical enlargement extends from C5 to T1 spinal segments whereas lumbar enlargement extends from L2 to S3 spinal segments.

The vertebral levels of these enlargements are quite different from those of spinal segments, viz. the cervical enlargement lies opposite C3 to T1 vertebrae and lumbar enlargement lies opposite T9 to L1 vertebra.

The two enlargements are compared in Table 7.3.

Table 7.3

Comparison between cervical and lumbar spinal enlargements

	Cervical enlargement	Lumbar enlargement
Extent	From C5 to T1 spinal segments	From L2 to S3 spinal segments
Vertebral level	From C3 to T1 vertebrae	From T9 to L1 vertebrae
Widest circumference	About 38 mm at the level of C6 segment	About 35 mm at the level of S1 segment
Innervation	Muscles of upper limbs and their associated girdles	Muscles of the lower limbs and their associated girdles

Cauda Equina

Since the cord is shorter than the vertebral column, length and obliquity of spinal nerve roots increase progressively from above downwards, so that spinal nerves may emerge through their respective intervertebral foramina (Fig. 7.10A). As a result the nerve roots of lumbar, sacral and coccygeal nerves from the caudal part of the cord takes more or less a vertical course and form a bunch of nerve fibres around the filum terminale called cauda equina because of its fancied resemblance to the tail of a horse (cauda – tail; equina – horse). The cauda equina consists of the roots of the lower four pairs of lumbar, five pairs of sacral and one pair of coccygeal nerves (Fig. 7.10B).