The Vertebral Column: Have Some Backbone

One of my favorite parts of the school year is the end of the Spring semester, when I get to collect the wonderful and varied assortment of final projects from my Advanced Anatomy students. These assignments are so much fun to collect because there is such a wide variety; the students are given the freedom to make just about anything they want, as long as their creation demonstrates some of the material they'd learned in class during the semester.

Many students this year chose projects that reflected their major— illustration students did anatomical illustrations, painting students did figure paintings with labeled surface landmarks, photography students shot photos of models demonstrating surface anatomy, and sculpture students like Izzy Carranza (below) made anatomical sculptures.

Izzy Carranza's spine sculpture from the Spring 2013 Advanced Anatomy class.

This lovely life size model of the human spine was crafted by Izzy in the sculpture lab during the last few weeks of school. Because it was too large and awkward to bring back and forth to class every day, Izzy would instead stop by at the beginning of class to check in, then bring me to the sculpture room to talk about his progress. So I got to see it develop in sort of time-lapse fashion. It began as a simple cylindrical slab of clay over a curved armature and slowly took shape into twenty four accurate—and even labeled—vertebrae.

So... how about a spine post?

The human spine is a 4-arched column of 24 vertebrae, plus two more bones, the sacrum and the coccyx. The arches of the spine curve anteriorly and posteriorly, so the curves can only be seen from a lateral view. The two posterior curves of the spine form the backs of two separate bony cavities. The two anterior curves balance out the posterior curves for an overall vertical orientation. The curves of the spine also serve to give it sort of a spring-like quality--the ability to contract and expand when pressure is placed on and released from it. These curves, along with the spine's great number of small bones, contribute to its flexibility, both anteriorly and posteriorly.

The image below shows these curves and the body cavities that two of them form.

The human vertebral column curves anteriorly and posteriorly, so the curves can only be seen from a lateral view. The two posterior curves form the back of two body cavities— the thoracic cavity and the pelvic cavity.  There are seven cervical vertebrae, 12 thoracic vertebrae,  five lumbar vertebrae, and a sacrum and coccyx.

The above image also shows the different areas of the spine and the different types of vertebrae. From the top down, the first section of the spine is the cervical spine, in which we have seven cervical vertebrae. Cervical means of the neck, and these are, of course, the vertebrae in the neck. The cervical vertebrae are numbered from the top down, so the most superior is referred to a the first cervical vertebra, or C1. Then the next is C2, all the way down to C7.

The next section is the thoracic spine, which is given its name because of its relationship to the thorax-- the rib cage and the structures inside it. There are twelve thoracic vertebrae vertebrae in this section, and they are referred to as T1, T2, all the way down to T12.

The next section is the lumbar spine, which forms the small anterior curve on the lower back. There are five lumbar vertebrae in this section, and they are referred to as L1, L2, all the way down to L5. On a side note, some people have a sixth lumbar vertebra, but this is not very common.

The last section of the spine is made up of two bones, the sacrum and the coccyx. These are both solid bones in the ossified skeleton, but they begin in the cartilaginous infant skeleton as more vertebrae! The sacrum, although a solid bone, starts out as five sacral vertebrae in the infant skeleton, and the coccyx, or tailbone, starts out as four coccygeal vertebrae at that time. When you hear people say that infants have more bones than adults, this is what they're talking about.

One more element that makes the spine so flexible is the presence of intervertebral discs— disks of cartilage that lie among the vertebrae. (Inter- means between.) These discs not only allow a greater range of motion among the vertebrae, but they also add cushioning and act as shock absorbers. There are discs among all vertebrae except between C1 and C2. There is no disc here because this articulation is a bit different than those along the rest of the spine. The articulation between C1 and C2 is the point at which our side-to-side head movements occur (as in shaking your head "no") and that movement requires that there is no disc. There is, however, a disc between the last vertebra (L5) and the sacrum. We can see the discs rendered in blue in the close up lumbar spine image below.

This image shows a close up anterior view of the spine with cartilaginous intervertebral discs (in blue) among the vertebrae. Notice there is also a disc between L5 and the sacrum.

As figure artists, we uses the spine's general shape and curvature as a starting point when rendering teh back. The anterior/posterior curves are apparent in just about any body position. But there is one other aspect of the spine the concerns us. One the posterior side of each vertebra, we can find a long spike, or a spinous process. This is the one feature of the vertebrae that we can see and feel on the surface of the body. They are easy to spot running down the posterior midline of the torso.

While every vertebra has a spinous process, not every one can be seen on the body's surface. The spinous processes of the first six vertebra (C1 through C6) are relatively short, and they obscured by soft tissue on the posterior neck— the nuchal ligament, to be specific. But the next spinous process, that of C7, is quite suddenly longer than those above it. As such, it is given the name vertebra prominens, which just means prominent vertebra. The image below shows this.

The spinous process of C7 is quite suddenly much longer than those superior to it. As such, it's the first we can see on the human body's surface. We can often see the next few below it as well, but never  those above it, as they are obscured by the nuchal ligament.

C7 is easy to spot because it's almost always the first spinous process we can see when going from the top down on the posterior torso. It's easy to orient youself on the human spine if you can find C7 first.  The spinous process below that would be that of T1, then T2, etc. We may not be able to see every spinous process along the spine, but we will almost always see (or at least feel) that C7 and we'll never see anything up above that. Whether or not the spinous processes of the thoracic or lumbar vertebrae show depends on several variables— the amount of soft tissue, the position of the body, the lighting, etc.

A student brought this lovely photo by Eugene Suo-Me to class one day after we'd finished the spine unit. It beautifully demonstrates the appearance of the spinous processes on the body's surface. Please check out more of Eugene's beautiful photography here.

This beautiful photograph by Eugene Suo-Me shows several spinous processes. Those of C1 through C6 are obscured by the nuchal ligament. But after that, C7 shows up quite suddenly and prominently, hence its name vertebrae prominens. After that, we can usually see the spinous processes of the first few thoracic vertebrae, if not more.

Finally, here is an example of a body position in which we can see lumbar vertebrae. In this pastel nude study by Degas, the figure is bending forward, which makes the soft tissue on the lower posterior torso stretch out over the bony spine. As such the spinous processes in this area (the lumbar region) are more pronounced. I think we can see the spinous process of C7 in this image as well.

The spinous processes of some of the lumbar vertebrae, as well as that of C7 can be seen in Degas' pastel nude.

Well that's it for today. I do have quite a few more wonderful advanced anatomy projects to show here, so one of those will most likely be next. Until next time.