it around.

Hey there, I'm Stan Prokopenko, you're watching Proko.

We're going to start our study of the skeleton with the spine. The spine is the connection

between the 3 major masses: the head, ribcage and pelvis. And it's wedged between 2 butt

cheeks.

When constructing the figure, it's common to start with these 3 masses, before adding

the limbs. Remember how in the figure drawing course we used the bean and Robo bean to quickly

establish the torso? The bean was a simple way of establishing the gesture of the torso

using simple shapes. The robo bean added more structure to the bean to describe its orientation

in space.

Melissa: I can't even see the spine. Why do we study it?

Here's the deal with the spine. It places the rib cage, the pelvis, and the skull wherever

they happen to be, and they can't go anywhere the spine doesn't let them. They inherit the

spine's limitations. If we want to construct a torso in any pose, we need to understand

the spine. Let's do it.

Big Structure of the Spine

The spine is strong enough to support the weight of the upper body, yet flexible enough

to move. It's composed of 24 individual vertebrae - hard bones that give the spine its strength.

The vertebrae have flexible cartilaginous discs between them, that allow the spine to

move as a single line. Each plane moves only a little, but they add up to a lot. Like string

of beads. Every little movement contributes to a graceful curve.

There are 4 sections to the spine. The cervical section of the neck consists of 7 vertebrae.

The Thoracic section of the ribcage has 12 vertebrae, one for each rib. The Lumbar section

of the lower back has 5 vertebrae. The fourth section is technically considered as 2 separate

sections, but I'm going to combine them - the sacrum and coccyx, which is the tailbone.

The sections give the spine a 4-arch curve. If the spine were a straight line, it would

be strong, but rigid. This 4 arch curve gives better flexibility for shock absorption and

aids in balance. And it's the framework for the posture of the body.

The cervical curve is the least curvy - it's almost a straight line. But, it does have

a very subtle forward curve. The thoracic curve is longest, and more curvy than the

cervical section. It curves backward and aligns with the shape of the ribcage. The lumbar

section curves forward and is even more curvey. The Sacral curve is the most curvey of all

the sections. So, as you can see the curves get progressively curvier as they go down

the spine.

Common Structure of the Vertebrae

The vertebrae of each section have slightly different structures, some for strength, some

for flexibility. However all the vertebrae, share the same common components.

Each has a thick disk-like Body, which connects to the neighboring vertebra with a squishy

little pillow, forming the main joint of the spine.

On the back of the body is a u-shaped Arch, creating a hole through which the spinal cord

runs. This locks the fragile spinal cord inside and provides protection.

On this arch are a few processes; little spikes that stick out like the needles on a porcupine.

A Spinous Process points out posteriorly. The subcutaneous tip is the only part of the

vertebrae that makes an appearance on the surface body.

The shape and angle of the spinous process changes as we move down the spine. Cervical

spinous process fans out like a lobster tail. Thoracic is a long spike. Lumbar is like the

blade of an axe. But these shapes are not observable on the surface. We can only observe

that the thoracic are pointy and the lumbar are longer. The first 6 cervical aren't visible

at all. Those are deeper in the neck, cover by the nuchal ligament. The first visible

vertebra is 7th Cervical, which is considered a major landmark of the body. This is the

most pronounced and clearly visible vertebra along the spine, seen right in the the middle

of this diamond shape between the trapezius muscles. Also, the middle vertebrae of the

thoracic section are usually not visible, even during forward lean, when the back muscles

are stretched. But, this varies. Sometimes you'll see all the thoracic and lumbar vertebrae.

Motion

What the spine does, affects the entire torso.

The thoracic section leans back, and the sacral leans forward. Since the rib cage and pelvis

attach to the spine, they inherit this lean. So this, is the default position. Not this.

From there each section has it's own range of motion.

The lumbar section has the largest and strongest vertebrae of the spine. It takes on the responsibility

of holding all the weight of the upper body. It also takes care of lateral bending, flexion

and extension. Especially flexion. When you bend down to touch your toes, most of that

bending happens at the lumbar region.

The lumbar region is able to bending side to side, mostly at the top 3 lumbar vertebrae

because the bottom two are connected to the sacrum by ligaments.

Rotation is restricted in the lumbar region..

The thoracic vertebrae are not as large and strong as the lumbar, so you'd think they

have more flexibility. But, you'd be wrong. The interlocking structure of the vertebrae

and the fact that they are attached to the ribcage, keep the thoracic section relatively

still. Flexion, extension and lateral bending are very minimal. However, the thoracic section

is able to rotate. Rotation is the main motion of the thoracic section.

The cervical spine is the thinnest and most delicate, so this allows for more flexibility

in the neck. Rotation along all 3 axes is possible. Flexion, extension, lateral bending

and rotation.

The first 2 vertebrae of the cervical section are unique. The Atlas and Axis. The axis has

a vertical cylindrical process inserting into atlas. Can you guess what kind of joint that

creates? [pause] You guessed it! A pivot joint. As we'll see in a few minutes, this allows

the head to rotate left and right.

50% of cervical rotation is at the joint where the atlas meets the axis. 50% of cervical

flexion and extension is at the joint where the altas meets the skull. So, the head can

rotate side to side and up and down without much help from the rest of the neck, but the

head can't bend laterally. The neck does that. Instead of drawing this, you would draw this.

Let's review. The cervical section is somewhat separate from the rest of the spine. It moves

the head around and has a lot of freedom to move in all directions. The thoracic and lumbar

sections are more limited and have to work together. Thoracic takes care of most rotation

and lumbar takes care of flexion, extension and lateral bending.

Drawing the spine

Ok all that information is great and all.. But how do we actually draw this stuff. How

does this apply to drawing the figure? I'll show you in the next video. Keep your eyes

out for that.

That's it, thanks for watching! If you're posting your drawings, use hashtag #proko

and don't forget to follow me on Facebook and Instagram. Also check out the Anatomy

for Artists group on Facebook at facebook.com/groups/anatomy4artists! If you like this video, share it with your

friends, and if you want to be updated about new videos click here to subscribe to the

Proko newsletter.