5. Vertebral Column.

Human beings belong to a vast group animals, the vertebrates. In simple terms we say that vertebrates are animals with a backbone. This statement barely touches the surface of the issue. Vertebrates are animals with a bony internal skeleton. Besides, all vertebrates have a fundamental common body plan. The central nervous system is closer to the back than it is to the belly, the digestive tube in the middle and the heart is ventral. The body is made of many segments (slices) built to a common plan, but specialised in different regions of the body. A “coelomic cavity” with its own special plan is seen in the trunk region and has a characteristic relationship with the organs in the trunk. This is by no means a complete list of vertebrate characteristics. Moreover, some of these features may be shared by other animal groups in a different manner. Such a study is beyond the scope of this unit.

Vertebrates belong to an even wider group of animals, chordates. It may be difficult to imagine that we human beings are in fact related to some the earlier chordates! However, we do share, at least during embryonic development, an important anatomical structure with all chordates. This structure is the notochord.

The notochord is the first stiff, internal support that appeared during the evolutionary story. As we have seen in early embryology, it also defines the axis of the body. The vertebral column evolved around the notochord and the neural tube, and we see a reflection of this fact during our embryonic development. The notochord disappears during further embryonic development, but leaves behind remnants recognisable in adult life.

One reason we study the vertebral column at this stage is that it gives us insight into the segmental nature of the body. The segmental plan is also reflected in the pattern of the spinal nerves which emerge from the vertebral column. At the same time we also recognise that the vertebral column itself is a marvellous functional mechanism, with its joints and muscles.

The human vertebral column : Overview.

Anterior Posterior Lateral The vertebral column is bony – it is strong, yet it is flexible, as it comprises a number of smaller bones. Its successively C lower parts bear the weight of the trunk above and the column transmits the weight of the body to the lower limbs. It contains and protects the spinal cord, a delicate part of the central nervous system. The thoracic part supports the cage of ribs, which in turn is a part of the mechanism of breathing. The flexibility of the column allows some degree of T movement, brought about by muscles.

Originally comprising 33 bones, in the fully developed state the vertebral column has 29 bones. Seven of these are in the neck (cervical vertebrae : cervix = neck), twelve in the thorax (thoracic) and five the in the small of the back (the lumbar L region). Below the lumbar region, the sacrum in the posterior part of the pelvis is formed by the fusion of five vertebrae; the coccyx (the ‘tail’) contains four fused vertebrae.

The vertebrae of each region are designated by the initial S letter of the region and the number of a particular vertebra in the region. Thus cervical vertebrae are numbered C1 to C7 from above downwards, thoracic vertebrae T1 to T12, lumbar Co Fig. 1. Three views of the vertebral column. vertebrae from L1 to L5. Though the sacrum is a fused mass, boundaries of individual vertebrae are visible on its anterior surface as horizontal lines and the individual ‘pieces’ are numbered S1 to S5. The vertebrae of the coccyx (coccygeal vertebrae) are not numbered individually. The abbreviation used is ‘Co’, in order to avoid confusion with the ‘C’ of cervical vertebrae. Since the tail is rudimentary in humans, the coccygeal region does not amount to much, either anatomically or functionally. Nevertheless, an injured coccyx can be a source of great pain, or at least discomfort! Variation in numbers of vertebrae is possible among individuals. This is more common in the lumbar and the sacral region. 1 Curvatures of the vertebral column. The vertebral column in an articulated skeleton shows curvatures in a lateral view. In the neck the curvature is convex forwards. The thoracic part of the column is concave forwards. The lumbar region again has an anterior convexity and the sacrum is concave forwards. These alternating curvatures are said to contribute to the ‘springy’ nature of the column.

In the uterus the foetus is curled up and the entire column is concave forwards. An anterior concavity is therefore described as a primary curvature – the first curvature of the column.

After birth some parts change their curvatures. These are known as secondary curvatures. At birth a child cannot hold the head steady. By the age of three months the muscles at the back of the neck become stronger, the baby can hold the head. The cervical column gradually becomes convex. When the child sits, stands and walks, the lumbar column similarly attains a forward convexity. An anterior convexity is a secondary curvature. The entire column thus shows alternating primary and secondary curvatures.

A generalised vertebra.

Vertebrae in different regions have characteristic features. These features are variations of a common plan. This common plan reflects the functions of the column as a whole. The common plan is best studied in a thoracic vertebra, though thoracic vertebrae do have their special features we ignore for the time being. A generalised vertebra is illustrated in fig. 2.

Each vertebra has a weight-bearing part, the ‘body’(1), in the form of a block of bone. A curved arch (2) joins the body to form a ring around the spinal cord. Because it protects the spinal cord, the arch is called the neural arch. The cavity enclosed between the body and the arch is the vertebral foramen. In a single vertebra it is like a foramen, but in the entire column, all such foramina form a canal for the spinal cord.

Fig. 2. A generalised vertebra. A : Parts numbered. B : Parts named. C : Two neighbouring vertebrae. Note the notches forming the intervertebral foramen (asterisk).

The neural arch has bony outgrowths or ‘processes’. The transverse processes (3), as the name suggests, spread transversely. The single process at the posterior end (4) is the spine. The small portion between the transverse process and the body is the pedicle, and the flat part between the transverse process and the spine is the lamina. Running up and down from the arch are columns of bone for forming synovial joints with the neighbouring vertebrae. These are the articular processes (‘5’ and ‘6’). Each of these has a flat area which the actual joint surface – this is the articular ‘facet’. The superior and inferior notches are related to the pedicle. When two or more vertebrae are seen as they are arranged in the body, these notches form a foramen through which a nerve passes. This foramen is the intervertebral foramen.

The intervertebral foramen must not be confused with the vertebral canal! The canal houses the spinal cord. In each vertebra, the canal is bounded by the body and the neural arch. The intervertebral foramina are between two vertebrae. They are formed mainly by the notches of adjacent vertebrae, open laterally and allow passage of nerves from the spinal cord.

2 An important note on the use of the term ‘spine’. As we have described above, the spine of a vertebra is a pointed process on the posterior side of a vertebra. This usage comes from the Latin word spina, meaning a thorn. The entire vertebral column is often referred to as the spinal column. This seems logical, as the spines of vertebrae are often seen forming a prominent feature down the midline of the back. Unfortunately, this term ‘spinal column’ tends to be abbreviated as ‘the spine’ when referring to the entire vertebral column. This can lead to confusion. It is safer to reserve the term spine for the spinous process of a vertebra.

Regional characteristics.

 Each region of the vertebral column has special features – some related to their functions, some developmental. o The cervical column allows movements of the head. It also gives attachments to strong muscles which balance the head. o The thoracic column has ribs attached to it. Ribs have movable joints with thoracic vertebrae, as movements of ribs are required for respiration. o The lumbar column bears great weight and has strong muscles and ligaments.

 Further, each region has some vertebrae which do not conform to the characteristic features. o This is explained by two reasons. One, there is a gradual transition between regions; therefore “borderline” vertebrae have overlapping features. Second, some vertebrae are specialised for specific functions. Vertebrae sharing the characteristic features are said to be typical for a region, others are atypical vertebrae of the region.

Be careful with the use of the words “typical” and “atypical”. The phrase “a typical vertebra” can sound like “atypical vertebra”. In the first instance, ‘a’ is an indefinite article. The second phrase, if it is to be grammatically correct, should be “an atypical vertebra”.

Vertebral bodies have to bear progressively greater weight in the craniocaudal direction; and show a gradual increase from the top to the bottom of the column. The upper part of the sacrum transmits the weight of the trunk to the lower limbs, and the sacrum and the coccyx show a gradual decrease in size. (Note : anatomical features of vertebrae are best studied in the lab!)

Cervical vertebrae. There are seven vertebrae in the neck. Of these, C3 to C6 are typical vertebrae. C1, C2 and C7 are atypical. The most striking feature of all cervical vertebrae is a foramen (hole) in the transverse process. This is called foramen transversarium. The features of a typical cervical vertebra are shown in Fig. 3. A typical cervical vertebra. fig. 5.3. Notice the transverse Left : superior view; right : lateral view. process (rectangle TP) with the foramen (F). The transverse process also shows two “tubercles”, anterior and posterior, labelled A and B. The spine is split into two parts a (bifid). The vertebral canal is large and triangular. The body of a typical cervical vertebra shows ridgelike projections (‘lips’) – on the superior surface they are lateral (the dotted line); on the inferior surface they are anterior and posterior (labelled ‘x’ in the picture on the right). These projections form special joints, the details of which are beyond the scope of this unit. In the lateral view, note that the articular processes (A-p) form a pillar, and that the articular facets on each of them (S-a-f and I-a-f) are obliquely placed.

3 The first two cervical vertebrae have features that account for the mechanism of movements of the head. The first vertebra “holds” the skull on its superior articular facets. This draws the analogy between this vertebra and the mythological figure of Atlas holding the globe of the earth. The vertebra is called atlas for this reason. The second vertebra forms the axis around which the head, with the atlas, rotates from side to side. The second vertebra is therefore gets the specific name “axis”. The atlas (C1). This vertebra Fig. 4. Atlas appears to have lost a part of the body (at ‘X’) – it only has an anterior arch. It does not have a spine, just a small tubercle (Y). Its superior articular facet is deep and long for a joint with the skull. Like the typical vertebrae, it does have a transverse foramen (*). The axis (C2). The ‘missing’ Fig. 5. Axis part of the body of atlas is seen as a tooth-like process called the dens or odontoid process – the terms mean ‘tooth’ and ‘tooth-like’ respectively. Its spine is stout, and the superior articular facets are large.

Correlate the picture of the axis with that of the atlas and note : The dens fits in the space just behind the anterior arch of the atlas. The anterior arch of the atlas has a smooth area for the joint with the dens. The superior articular facet of the axis (‘Y’ in fig 5) matches the inferior facet of the atlas (‘I-a-f’ in fig. 4).

The thickness of the spine of the axis must be explained by the attachment of strong muscles and ligaments. We shall discuss these very soon.

Vertebra prominens (C7). This vertebra has a long, stout (but not bifid) spine. The tip of the spine is usually visible as a projection at the lower end of the neck in the living. This is why it is called the prominent vertebra.

The foramina transversaria of cervical vertebrae transmit blood vessels called vertebral vessels. (The seventh carries only the vein). The vertebral arteries enter the skull after passing through the foramina of the atlas and supply blood to a large part of the brain.

Thoracic vertebrae. Thoracic vertebrae have ribs attached Superior view. Lateral view. to them. Ribs form joints with the bodies and the transverse processes. Thus there are facets on these parts (Fig 6, ‘X’, ‘Y’ and ‘Z’). Each body thus has two ribs attached to it on either side. (We shall see the details of these attachments when we study the thorax).

Fig. 6. Typical thoracic vertebra.

 The spine of a typical thoracic vertebra is long, pointed and has a downward slope.  The transverse processes have a backward slant.  The vertebral canal is small and almost circular.

4 The dotted line in fig. 6 indicates the curve along which the articular facets lie. In the lab, confirm that the facets are flat. The orientation of the facets facilitates axial rotation in the thoracic region.

Atypical thoracic vertebrae. (Identification or details of these is not core material). The first thoracic vertebra looks quite similar to the seventh cervical, and has only one facet on either side of the body. At the lower end the vertebrae tend to be similar to lumbar vertebrae – especially the 12th, which also has a single facet or the 12th rib. T10 and T11 also have some variable atypical features.

Lumbar vertebrae. Vertebrae L1 to L4 are typical lumbar vertebrae. In fig 7, observe the characteristic features :  A stout, quadrangular spine  A triangular canal  Slender transverse processes  Concave superior facets and reciprocally convex inferior ones. (Features best seen in the lab). Needless to say, they have large bodies.

Superior view Inferior view Lateral view

Fig. 7. Typical lumbar vertebra

The fifth lumbar vertebra is somewhat atypical – it has very large and conical transverse processes and a spine that is not truly quadrangular. These features are best seen in the lab. The significance of the stout transverse process will be appreciated when we study the abdominal wall.

The sacrum. The sacrum (fig. 8) is formed by the fusion of five vertebrae. It serves multiple purposes. As a part of the vertebral column, it sends out the sacral spinal nerves. The weight of the upper body is distributed by the sacrum to the lower limbs via the hip bone. The sacrum also forms a part of the wall of the pelvis.

All parts of the sacral vertebrae are fused to form continuous features. On the anterior surface of the sacrum the lines of fusion of the bodies of five vertebrae are seen as horizontal marks (arrows).

Fig. 8. Sacrum

Body Ala

The transverse process of the first piece is rather large and forms a mass called ala (= wing). The spines and the articular processes fuse to form ridges on the posterior side (see the asterisks and the dotted line). At the lower end, the arch is deficient to an extent, forming a large opening called the sacral hiatus. On the lateral side, the upper part of the sacrum has a large articular surface (outlined) for the hip bone. This surface, due to its shape like that of an ear, is called the auricular surface. The sacrum will be studied in greater detail with the pelvis. The coccyx hardly has any notable features of our interest.

5 Joints of the vertebral column.

Vertebrae are held together at joints, as they must be, to form a functional column which supports weight and yet allows some movement of the trunk. The canal of the column contains the delicate spinal cord; and excessive movement between neighbouring vertebrae is not desirable. The joints of the column are further strengthened by ligaments, bands of dense connective tissue. The joint system of the column is optimal for all purposes, though sometimes it is the subject of problems of varying severity.

We shall undertake a detailed study of the organisation of joints along with the limbs. At this stage, do not worry about the details of joint structure and their classification. Understand the broad principles as applicable to the vertebral column :  Very little movement is possible between neighbouring vertebrae.  The bodies of vertebrae are the main weight-bearing parts, and need a strong bond. They also need some kind of cushion to buffer some of the forces acting on them.  The column is a long, flexible structure, and needs strong connective tissue to hold it together. These are ligaments of the vertebral column.  Movements of the column effectively move the trunk – it can bend forwards, backwards or sideways; it can also be twisted to some extent.

The joints and ligaments are illustrated in fig. 9. Joints between bodies. The bodies of adjacent vertebrae are connected by a disc of white fibrocartilage (‘D’). The disc is called the intervertebral disc. Such a joint where bones are connected by white fibrocartilage, is called a symphysis. Between the disc and bone is a thin layer of hyaline cartilage, not shown in this figure. We shall study this as a joint in a later chapter, when we shall also see more examples of such joints. At this stage we focus on some special features of the intervertebral disc, described on the next page.

Joints between facets. The articular processes form synovial joints. The facets on the processes are covered by hyaline cartilage. The joint has a fibrous capsule and synovial membrane. Very little movement is Fig. 9. Joints and major possible at these joints, but it is essential for ligaments of the vertebral making adjustments when the vertebrae move. column. These joints are often called ‘z-joints’. The ‘z’comes from zygapophysis, a term used for the articular processes. You can ignore this word if you think it is a tongue-twister!)

Ligaments. Besides these joints, a number of ligaments connect the vertebrae. These are important in the stability of the vertebral column. Some ligaments connect adjacent vertebrae while some span over several vertebrae. Longitudinal ligaments. Two sets of strong ligaments are attached to the bodies of the vertebrae, running along the anterior and posterior surfaces of the bodies. These are the anterior and posterior longitudinal ligaments. Bundles of fibres of these ligaments run over several vertebrae, forming a continuous band along the entire length of the column. Interspinous ligaments. These ligaments connect the spines of adjacent vertebrae. They are attached to the entire lengths of the spines except the tip. (Interspinous = between spines). Supraspinous ligaments. Each ligamentous band spans a few vertebrae, attached to the tips. (Supraspinous = ‘above’ the spines). In quadrupeds they are literally ‘above’ the spines, in humans they are the most posterior ligaments of the column. In the neck region the ligaments form a very thick band They have a special name here : ligamentum nuchae (ligament of the nape of the neck). The ligament can be seen easily in a lamb neck chop as a thick, pale band on either side of the tip of the spine.

6 Ligamenta flava. (Singular – ligamentum flavum; flavum = yellow). As the name suggests, the ligament has a yellowish colour in the fresh state. This is due to the high elastic fibre content in this connective tissue. These ligaments connect laminae of adjacent vertebrae. (Not shown in fig 9).

In addition, there are ligaments between transverse processes (intertransverse ligaments); and short ligaments characteristic of certain region are also seen. (Not shown in fig 9).

Understand the key points about ligaments :

 These ligaments are bands of dense regular fibrous tissue.  With the help of fig 9, note that : o Anterior and posterior longitudinal ligaments connect bodies of vertebrae. o Supraspinous and interspinous ligaments connect spines of vertebrae. . Interspinous ligaments, as the name suggests, are between spines. This means that they are short, confined to the gap between two neighbouring spines. . Supraspinous ligaments are attached to tips of spines – they are longer, each spanning several spines, effectively making a continuous long structure.

Structure and function of the intervertebral disc. The intervertebral disc has roughly the same shape as the bodies of the vertebrae it connects (fig. 10). Anterior longitudinal ligament On the periphery (outer side) it is more fibrous – dense fibrous tissue. Deeper inside it is more cartilaginous. The white fibrocartilage gradually changes to dense Dense CT Annulus connective tissue. This combination is the annulus fibrosus fibrosus (= ‘fibrous ring’). The dense fibrous tissue merges with the ligaments between vertebral bodies. WFC

The centre of the disc is made of a jellylike material, the nucleus pulposus (= ‘pulp-like centre’). The nucleus Nucleus pulposus is a remnant of the notochord around which the pulposus vertebral body develops.

The annulus fibrosus is slightly deformable, due to its collagen content. This allows a very small degree of Posterior longitudinal ligament movement between the bodies. The collagen fibres Fig. 10. An intervertebral disc. effectively resists excessive movement. The white fibrocartilage also resists compression. Some degree of twisting is also permissible. However, the collagen fibres run in both clockwise and anti-clockwise directions, in spirals from one vertebra to the next. This spiral arrangement helps to resist excessive twisting forces. The nucleus pulposus has a high water content. This adds to the ability of the disc to resist compression.

Overall, the disc acts as a cushion, allows some movement and resists excessive movement, all at the same time. One aspect of deformation, as occurs in forward bending of the trunk, is illustrated in fig. 11. Notice that on the anterior side the disc is compressed, while on the posterior side it is stretched. Also notice in both figures 10 and 11 that the nucleus pulposus is closer to the posterior side of the disc. Excessive load on the vertebral column while bending forwards, as in lifting a very heavy weight while bending, can cause rupture of the posterior part of the annulus Fig. 11. Deformation of a disc. and the nucleus pulposus may ‘spill out’. This can compress spinal nerves or even the spinal cord, with serious consequences.

7 The motion segment.

This is an important concept in the movements of the vertebral column. As stated earlier, movement between adjacent vertebrae is necessarily limited. However, such small movements are added up over the length of the column, giving the column a significant range of movement. The motion segment is the movement between two neighbouring vertebrae. This is made possible largely by the intervertebral disc.

In these movements the facet joints can slide against each other.

A motion segment therefore has two components – the anterior component is movement between the bodies; the posterior component is movement between the articular facets.

Movements of the vertebral column.

The vertebral column can be bent forwards. This is called forward flexion or just flexion. The opposite movement (straightening the flexed column or bending over backwards) is called extension. It can also be bent to the right or the left side – these movements are called lateral flexion, right or left. Two vertebral bodies can also be rotated against each other. In this case the disc is twisted. Recall that the disc has spiral collagen bundles running in opposite directions. Depending on the direction of the twist, there is always one set of fibres that limits this movement. The extent of rotational movement is also limited by the orientation of the facets. The facets are best suited for rotation in the thoracic region because they are on an arc of a circle. The cervical facets limit the movement to some extent. The movements of the cervical facets are in fact rather complex. The lumber facets are kind of ‘locked in’ due to their concave and convex surfaces and do not permit much rotational movement.

Movements of the head.

The joints between the atlas and the axis (atlantoaxial joint) and the atlas and the skull (atlanto-occipital) play an important role in movements of the head. The atlanto-occipital joint. The curvature of the facets of the atlas and the occipital bone of the skull permit only a flexion-extension movement. One may think of this joint as the ‘the nodding joint’. Because of the curvature and orientation of the articular facets, the head cannot rotate sideways on the atlas. The atlantoaxial joint. There are three joint between the atlas and the axis. Here we consider the midline joint between the two vertebrae. (The other two joints are between the articular processes.) The anterior arch of the atlas and its transverse ligament (the green band in fig. 12) form a ring in which the dens of the axis rotates. Or, one can think of the axis as fixed, and say that the atlas rotates around the dens. In this movement, the head moves with the atlas. In fig 12, the red arrows indicate movement when the head turns to the right; the blue arrows indicate movement to the left. The articular areas for the occipital bone Fig. 12 The atlantoaxial joint . of the skull are marked ‘O’.

More on the movements of the vertebral column.

Movements which we might perceive as those of the vertebral column in fact have other components as well. We take two examples. 1. When we turn to look back, the vertebral column undergoes axial rotation no doubt, but this is not enough. The head also turns at the atlanto-axial joint. This is further supplemented by the movements of the pelvis at the hip joints. 2. Stooping to touch one’s own toes with the fingertips involves flexion of the vertebral column. However, this movement alone is not enough – the hip joints most move as well. Bending (flexion) at the hip joints is limited by the stretching of the muscles in the back of the thigh. These muscles, called hamstrings, can be trained to relax to achieve adequate flexion of the hip joints. (A more detailed and accurate explanation requires an understanding of the functioning of the muscles of the back of the thigh, not core material for this unit).

8 Muscles of the vertebral column.

The column that bears and transmits such weight needs powerful muscles to stabilise and move it. The muscles are numerous and form many groups. We are concerned more with concepts and their general plan. The general principles are :  The vertebral column has muscles on its anterior and posterior sides. The anterior muscles are essentially flexors of the column, the posterior ones are extensors. At this stage we focus on the posterior muscles.  As the muscles are paired (right and left), unilateral (one-sided) action of the major muscles in general brings about lateral flexion to that side.  The posterior muscles are a complex group. There are long and short muscles, superficial and deep muscles. It should be easy to imagine that the superficial muscles are longer, the deep muscles are shorter. (The fine print details are optional).

The posterior muscles are arranged in two large groups.

Fig 13 shows the two major groups of posterior muscles diagrammatically. These are described below as groups A and B. Group A muscles are superficial. They are shown on the right side. The deep muscles comprise group B, shown on the left side.

Erector spinae group. (Group A in fig. 13). This group comprises long bundles of muscle fibres running longitudinally up the back from the sacrum and ilium (a part of the hip bone).

The thick mass splits into three major muscles.  The most lateral muscle is called the iliocostalis. Understand the name : it runs from the ilium to the ribs. A part of this muscle reaches further up to the neck.  The middle part has the longest bundles – therefore called the longissimus. It runs the whole length of the column attaching to the transverse processes of all the vertebrae and to the occipital bone.  The medial portion runs along the spinous processes – thus called spinalis.

Transverso-spinalis group. (Group B in fig. 13). This group is composed of oblique fibres running between the transverse and spinous processes.

 The deepest part, rotatores (best seen in the thoracic region) spans only 1 or 2 segments. Because each one of them is short and between transverse and spinous processes, they cause rotation of the column.  The Multifidus is best developed in the lumbar region. The name multifidus indicates that it is split into smaller parts.  The Semispinalis (runs over half the spinal column) is well developed in the upper Fig. 13. half of the spine and ends by attaching to the occipital bone. Major muscle groups.

The deepest posterior muscles are very short bundles between adjacent spines and adjacent transverse processes, thus called interspinous and intertransverse muscles. These are not shown in the figure.

Fig 14. shows these muscles in a transverse section. Note the superficial group extending laterally to the ribs. The deep groups are between spines and transverse processes.

The anterior muscles are best seen in the cervical and lumbar regions. In general they are flexors and lateral flexors of the column. Their details are not necessary here. In the region of the atlas and the axis there are special short muscles for the movements of the head. However, conceptually, the movements of this region are more important than the details of these deep, short muscles. Fig. 14. Posterior vertebral muscles in cross section. 9 The vertebral canal.

This section requires an understanding of the spinal cord and its coverings. This is largely recapitulation from ANHB 1102. If you have not done ANHB 1102, you will need to read chapter 6 for clarifications. In ANHB 2212, the contents of chapter 6 are explained in Week 3.

The canal of the vertebral column contains the spinal cord, its coverings (meninges) and the nerves emerging from the spinal cord.

Developmentally the spinal cord is as long as the vertebral column. It is also built on a segmental plan, though there are no anatomical features on the surface of the spinal cord to mark the segments. A segmental spinal nerve emerges through each intervertebral foramen. We consider the portion of the spinal cord that gives rise to a pair of spinal nerves (right and left) as a spinal segment.

After birth the vertebral column grows more than the spinal cord. In the adult the spinal cord ends, on the average, at a level between the 1st and the 2nd lumbar vertebrae. The lower spinal nerves exiting through foramina below this level form a bundle from the their levels of origin from the spinal cord and their foramina of exit. This bundle is called the cauda equina (horse’s tail). The spinal cord is covered by three meninges. The innermost is a delicate membrane that clings to the spinal cord – the pia mater. The outer covering is the dura mater. Between the two is the arachnoid mater, stretched like a spider’s web between the dura and the pia.

The dura mater in the vertebral canal is a loose tube, separated from bone by a potential space called the epidural space. This space allows the tube, along with the enclosed spinal cord, to slide when the vertebral column moves. Deep to the arachnoid is a space (subarachnoid space) filled with cerebrospinal fluid (CSF).

Since the spinal cord ends between L1 and L2, below this level the tube contains only the cauda equina with the nerve roots loosely floating in CSF. Biochemical and microscopic examination of the CSF gives valuable clues to the diagnosis of nervous system diseases or injuries. A few drops of CSF can be withdrawn by a needle passed between vertebral spines (L3 – L4 gap being the most suitable). This procedure is called lumbar puncture. In epidural anaesthesia, an injection of a local anaesthetic in the epidural space seeps through the dura and blocks nerve impulses along the spinal roots.

A typical spinal nerve. (Fig. 15). A spinal nerve typically has two roots emerging from the spinal cord – the ventral root is motor, the dorsal root is sensory. The dorsal root has a small swelling, the dorsal root ganglion (explained along with Dorsal ramus nervous tissue). The two roots unite to form the mixed spinal nerve. These structures are located within the vertebral canal. The spinal nerve exits the canal through the intervertebral foramen. Ventral Fig. 15. Vertebral canal (left), its contents magnified (right). ramus

Immediately after its exit, the nerve divides into its first major branches. These are called the primary rami (= first branches; singular – ramus = branch). One goes to the posterior side to supply the muscles posterior to the vertebral column and a band of the skin about a hand’s width from the midline – the posterior or dorsal primary ramus. The other supplies all the rest of the muscle and the skin of that segment – this is the ventral primary ramus. Note that both the primary rami are mixed nerves, motor and sensory. For brevity and convenience, the word ‘primary’ may be omitted.

These concepts are developed further along with the study of nervous tissue and the body wall.

10 The development of the vertebral column.

Recapitulation of early embryology. We have described the three-layered embryo. Recall the notochord which defines the axis of the body and establishes bilateral symmetry. The mesoderm within the embryo forms three regions on either side of the notochord and the neural tube. The column of mesoderm immediately neighbouring the axis is the paraxial mesoderm. We now consider the paraxial mesoderm further.

Fig. 16. Transverse section of the trilaminar embryo.

Ect : ectoderm, N-p : neural plate, Nc : notochord, P : paraxial mesoderm, I : intermediate mesoderm, L : lateral plate mesoderm, End : endoderm.

Paraxial mesoderm forms somites. Nc

Fig. 17. P : paraxial mesoderm, one somite. Note that it has split into two parts. Nc : Notochord. Note that the neural tube (unlabelled) has ‘sunk in’ between the ectoderm and the notochord. Ignore the other parts of the mesoderm!

Fig.18. The sclerotomes now form vertebrae. Note that the body envelopes the notochord. The neural arch encloses the neural tube (spinal cord here).

At this stage we ignore the other regions of the mesoderm (intermediate and lateral plate). We shall discuss the myotome along with the body wall.

The paraxial mesoderm is segmented to form somites. Each somite differentiates into a sclerotome and a dermomyotome. The root ‘sclero-’ means ‘hard’; ‘tome’ is a cut. The sclerotome gives rise to a part of a vertebra. In ‘dermomyotome’, ‘dermo’ refers to the dermis of the skin; ‘myo’ refers to muscle tissue.

Thus, in a segment (slice) of the body :  The dermomyotome gives rise to the segmental dermis and muscle.  A segment of the spinal cord sends a segmental nerve to this mass.  The nerve is therefore ‘central’ to a segment.

The sclerotome develops in a slightly complex manner, but the complexity disappears when we correlate development with the final picture. Let us do it through the following description, somewhat simplified for easy understanding.

11 Fig 19 shows two segmental sclerotomes, numbered 1 and 2 and a part of a third sclerotome. Notice that :

 Each sclerotome has two parts – A and B in sclerotome 1, C and D in sclerotome 2.  In each segment there is a central lighter area.  B and C (parts of sclerotomes 1 and 2) unite to form one vertebra. (Similarly, D and E form another, and so on!)  The lighter area becomes the intervertebral disc.  The disc is in the centre of a segment, but is now seen between vertebrae.  The spinal nerve of the segment (yellow circle) is in line with the disc.  The anatomical relationship between the disc and the nerve is shown on the extreme right.

Fig. 19.

Simplified developmental explanation of the relationship between vertebrae, intervertebral discs and segmental spinal nerves.

One would expect the blood vessels to follow a similar course. But major blood vessels are intersegmental. You need not worry about them!

The interested student may refer to a textbook of embryology. As a core point, the concept that a vertebra is made of parts of two segments is important.

The part of the notochord surrounded by the disc is seen as the nucleus pulposus of the intervertebral disc. The part surrounded by the body of the vertebra disappears. Occasionally, notochordal cells may persist within a vertebral body. Rarely, they may give rise to notochordal tumours within the vertebral body, weakening the bone.

The main bony components that emerge from this arrangement are the centrum and the neural arch. Each of these develops as symmetrical right and left halves. The two halves of each part unite to form a single structure. Finally the centrum and the arch unite. A small part of the arch joins the centrum to form the body of the fully formed vertebra (fig 20). The body also includes a growth mechanism called the annular (ringlike) epiphysis. We shall elaborate on the concept of epiphysis with the study of bone (Week 13). Fig 20. Topographical and developmental parts of a vertebra..

The region of the transverse process deserves special mention. In the basic plan of a vertebra the transverse process has two developmental parts. The ventral (anterior) part is called the costal element. This is the rib- forming part (costa = rib). The dorsal (posterior) part is called the transverse element.

Theoretically therefore, all vertebrae have the potential to form a rib and a transverse process. Indeed, some vertebrate animals do have ribs attached to most vertebrae. The evolutionary history and comparative anatomy of ribs in vertebrates as a group is quite complex, and beyond the scope of this unit. In mammals including humans only the thoracic vertebrae have ribs; and the ribs form movable joints with vertebrae. The nature of these joints will be studied with the body wall. In the other regions, the costal element becomes an integral part of the transverse process.

12 In the cervical region the costal element forms the on the anterior and lateral sides of the foramen transversarium. This means that most of the cervical transverse process in fact comprises the costal element (‘C’ in fig. 21). This part has two little swellings (tubercles) for muscle attachments. It is interesting to know (but not core knowledge in this unit) that two of these muscles are attached to real ribs in the thorax. A small part, forming the posterior Fig. 21. Cervical vertebra : costal and transverse elements. boundary of the foramen transversarium. is the transverse element.

In the lumbar region the transverse process is almost entirely formed by the costal element, the true transverse element is represented only by a small tubercle on the posterior side of the process. (You do not need to identify these features in a lumbar vertebra). In the sacrum again, the transverse process is almost entirely formed by the costal element.

One is tempted to conclude that in the thoracic region, the main function of the transverse element is to provide strong support to the rib by forming an extra joint. Correlate this with anatomy of the joints between ribs and vertebrae when you study the thoracic wall.

The phylogenetic perspective.

A brief look at the comparative structure of the vertebral column reveals some interesting facts and functional correlations. Vertebrates are divided into five ‘classes’ – fishes, amphibians, reptiles, birds and mammals. We must bear in mind that within these classes there is a great deal of variation. The examples cited here are illustrative; easy to understand through day-to-day life observations.

Fishes have bodies which possess great buoyancy. They live in water and do not have to counter the force of gravity to a great extent. The vertebral column does not have to anchor limbs as a primary means of locomotion. They have great ability to turn their entire bodies towards food. They do not have to turn their heads; their laterally placed eyes give a wide field of vision. They have no ‘neck’. The entire vertebral column has an almost uniform structure – there are vertebrae of the trunk and vertebrae of the tail.

Amphibians (= double life; in water and on land) have limbs. Though this class shows a lot of variation, we shall take the common example of a frog or a toad. On land, they use their hind limbs to provide the major force for locomotion. The hind limbs need a solid anchor, and they do have a specialised sacral region. The neck barely exists, and they have but one vertebra in this region. This single vertebra has a special joint with the head, giving the head a limited degree of mobility.

Reptiles can be considered as true land animals with limbs. Once again beware of exceptions: some, like snakes, do not have limbs. Positioning of the head is facilitated by a distinct neck, and the stronger limbs are helped by a strong sacral region and the pelvis that goes with it.

Birds are a special case. A rigid trunk is helpful for them. In flight this helps with aerodynamics, walking on land on two legs needs a good balance. Many of the trunk vertebrae are fused in birds. The neck vertebrae are well developed and numerous – some birds have as many as twenty of them.

Mammals have reached a degree of tremendous regional specialisation. The cervical column allows a great degree of movement of the head. Surprisingly, with very few exceptions, all mammals have seven vertebrae in the neck – be it a giraffe or a whale. The atlas and axis have elaborate joints forming a complex with the occipital bone of the skull. This mechanism allows great mobility of the head. At the other end the sacrum forms an integral part of the pelvis. The entire assembly anchors the hind limb. Between the two ends is the column of the trunk. With the anchoring of the ribs, the thoracic column has its own distinctive characteristics. Let us also remember that the thoracic column allows axial rotation. The lumbar column allows a good deal of flexion in all directions. The overall number of all the trunk vertebrae is distributed between the thoracic and the lumbar column. Jumpers and runners have more lumbar vertebrae than “swingers” who have longer thoracic columns. 13 Variations in the number of vertebrae within a species usually occurs in the trunk. Though rare in humans, one does find a missing vertebra or an extra vertebra in this region. Separation of the first sacral vertebra or union of the fifth lumbar vertebra with the sacrum are also known variations. This alters the number of lumbar and sacral vertebrae in these individuals.

Humans are habitual bipedal animals. This means that we use bipedal posture and locomotion as a rule. (Certain other similar animals are occasional bipedals). This make the human vertebral column unique in that it directly bears the forces of gravity. A quadruped’s vertebral column is more like a bridge across two vertical limbs. Whether the human column has achieved perfection in this respect or not is a highly debatable question.

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