Get to know your back

Legend has it that we owe part of our spinal anatomy, indirectly at least, to St. Thomas ê Becket, the 12th century archbishop who came to an abrupt end in the stone caverns of Canterbury Cathedral. The Chancellor turned churchman ran afoul of King Henry II, who was heard to remark how dearly he wished to be rid of “this turbulent priest.”

Four of Henry’s knights took him at his word, and the worthy cleric was put to the sword. For having allowed such a horror to transpire within their domain, the people of Kent were cursed to be born forever after with tails, a sign, presumably, of their having entered into a pact with the devil.

It’s not only a devil’s tail
In fact, all of us, descended or not from the people of Kent, have vestigial tails, in the fused and tapered bones of the coccyx (pronounced “cock six,”), the curved afterthought we know as a tailbone though apparently the coccyx (Latin for “cuckoo”) more closely resembled a jungle bird’s bill to the ancient anatomist who gave it its name.

Roots and legends aside, the coccyx is a miracle of engineering: It’s light, flexible and strong enough to bear most of the body’s weight, as ll as a hefty metaphorical burden: Everyone knows, for example, that cowards are spineless, while heroes show a lot of backbone. Idle labourers are exhorted to “put your backs into it!” yet we “turn our backs” on those for whom we feel nothing but scorn.

The wealth of allusions suggests the importance we accord our nether side as well we might. Without our spines, we’d be unable to stand or offer protection to our vital organs. We’d have nothing to which we could attach the ribs that encircle our chests, nothing to anchor the muscles that allow us to twist and turn and lift and pull, nothing to cradle and protect the nerves that run from our heads to our fingers and toes and every corporeal niche in between.

To say we’d be nothing without our spines isn’t much of an exaggeration: We’d be jellyfish.

Backs are more than the spine
Our backs, of course, aren’t only spine they’re complex topographies of bones and cartilage, discs, ligaments, muscles, blood vessels, nerves and soft tissue but the spine is the engineering marvel that brings it all together. The adult spine is an undulating column, a series of concave and convex curves composed of vertebrae stacked 33 (sometimes 34) high and separated by discs. The curves deftly cradle the body’s organs, but they also give the spine a tensile strength, and our posture and gait a balance, they wouldn’t possess were the spine perfectly straight.

The spine is divided into five sections, each composed of a specific number of vertebrae; for ease of reference, the three upper, mobile sections are numbered, top down, beginning with the seven cervical, or neck, vertebrae (C1 C7); below them are the 12 thoracic (or dorsal) vertebrae (T1 T12), each of which proudly bears one pair of ribs; followed by the five large bones of the lumbar, vertebrae (L1 L5) the site of most lower back pain.

The two bottom sections of the spine comprise the “false” vertebrae. The five bones of the sacrum fuse at about age 25, forming a single flat, wedge shaped bone that’s attached, via the sacroiliac (or SI) joints, to the fan shaped iliac bones of the pelvis. Hanging from the sacrum is our Kentish coccyx, with four or five vertebrae, depending on your genetic inheritance.

Vertebrae are solid, drum shaped cylinders with a roughly circular channel the spinal canal on their backs that’s formed by bony protrusions called pedicles and laminae. Each vertebra also bears seven spiky projections called processes on its back, which are linked by an intricate tapestry of ligaments and muscles.

What are those bumps?
One transverse process juts out on either side, while a third, called the spinous process, extends backward and down, like a hooked nose; it’s these processes you see and feel as bumps down the centre of someone’s back.

The other four of the seven processes on each vertebrae are in pairs, one (the superior articular processes) at the top of each vertebra, the other (the inferior articular processes) at the bottom. The inferior processes of one vertebra extend downward to meet the superior processes of the vertebra below to form facet joints, which are cushioned with oily smooth cartilage and sealed in a tough, self lubricating capsule that secures the vertebrae while allowing them to slide against each other as we move.

The vertebrae of the mobile spine are fairly similar from top to bottom, though lumbar vertebrae are larger and heavier than thoracic vertebrae, which are larger in turn than cervical vertebrae. The facet joints of the cervical vertebrae are also smaller and less stable than the thoracic and lumbar vertebrae, to permit greater rotation and movement in the neck.

Heads may turn
The two uppermost cervical vertebrae, known as the atlas (after the Titan of Greek myth who was condemned to support the world on his shoulders) and the axis, differ from the other vertebrae. The atlas isn’t solid like its cousins; it’s a kind of bony ring, with a depression on either side of its upper surface to cradle round protrusions on the underside of the occipital bone the base of the skull, our personal 12 to 16 lb. world. Unlike the other vertebrae, no disc separates the atlas from the axis; instead, the axis has a bony projection that fits through the ring in the atlas, allowing the atlas to rotate, and our heads to turn.

Next page: Muscles, disks, and nerves

The bones of the neck and skull are bound together with ligament (bands of tough connective tissue) and muscle. In fact, ligaments join all of the vertebrae in short bands between the processes and in two long bands extending the full length of the spine, fore (the anterior longitudinal ligament) and aft (the supraspinous ligament). Ligaments provide support and stability and help to prevent the spine from making movements beyond its appropriate range of motion.

Muscles are heavy liftersIt’s our muscles, of course, that allow us to make any motions at all; the muscles of the back are astonishing in their complexity. There are short muscles between processes and facet joints, several layers of medium length muscles connecting in some cases vertebrae several bones apart, and long muscles that run the length of the spine or connect the spine with various other parts of the body.

Those pesky disks
Separating the bones of the spine (except the atlas and axis) are the intervertebral discs, cartilaginous shock absorbers that keep the vertebrae from grinding against each other; they also provide the necessary “give” that allows vertebrae to tilt ever so slightly to form its characteristic curves.

For ease of reference, discs are numbered according to the vertebrae above and below them. Thus, the disc between the second and third lumbar vertebrae is called L2 L3, or simply L2 3; the disc between the bottom lumbar vertebra and the top sacral vertebra is L5 S1.

Each disc is joined to the vertebrae above and below it by a layer of cartilage; its exposed side, the annulus fibrosus (Latin for “fibrous ring”), is composed of several layers of extremely tough tissue that are often compared not unfavourably to the layers of a radial belted car tire.

Why we shrink
Inside the tough exterior is the gelatinous nucleus pulposus (“pulpy kernel”), a jelly like mass that’s mostly water. As we age, the sprightly nucleus thickens, becoming less gel like. With that, the spine’s shock absorbing capacity slowly diminishes, the discs flatten marginally, and we lose a bit of our former stature.

Discs account for about one fourth of the spine’s total height, though that changes through the course of the day as the average person shrinks slightly, because the weight of the body squeezes more water out of the discs than they take in. At night assuming one sleeps lying down the discs are replenished. The effects of gravity and weight on height are most apparent in their absence: Astronauts have been known to “grow” four inches or more after an extended tour in the weightlessness of space.

And then the nerves
That’s pretty much it, except for the spinal cord – the most complex part of your back. Descending the spine inside the protective confines of the spinal canal, it’s the second most important component of the central nervous system (after the brain). It’s encased in and protected by three layers of fibrous tissue, the tough outer dura mater, the middle arachnoid layer and the inner pia mater.

The spinal cord is a trunk line from the brain. It divides into 62 branches called nerve roots (31 on either side of the spine), which emerge through the intervertebral foramen, the spaces behind and between two vertebrae. Each nerve root then further divides into smaller tributaries, simply called nerves, which run to nearby facet joints, discs, muscles, and on to every part of the body.

As Judylaine Fine explains in The Ultimate Back Book, one of these nerves deserves further mention: “The nerve roots that emerge from between L4 and L5, as well as the four pairs of nerve roots below, merge together at hip level, beneath the gluteus maximus muscle [in the buttocks]. This combination forms the thickest nerve of the body: the sciatic nerve.” (The sciatic nerve then runs down the back of each thigh to the knee, where it divides into two branches that continue down the front and back of the leg to the foot.)

As Dr. Hall explains in TNBD, the spinal cord doesn’t make it to the bottom of the spinal canal; it actually ends “at the upper end of the lumbar region, just below your ribs. From there down to the pelvis, the canal is filled only with nerve roots, which have left the cord but have not yet left the spine. To the early anatomists, this tangle of nerves looked like the tail of a horse, hence its name, cauda equina, or horse’s tail.”

Now you’re ready to talk to your doctor with a good understanding of all the back parts that could be causing pain.