one yet, make sure you don’t miss it.

In this episode, I’ll show you how to draw the forms of the rib cage step by step.

I always like to start my drawings with the biggest shapes first. In this case it will

be the egg-like shape. It’s thinner at the top where the neck attaches, and thickest

about ⅓ from the bottom. Try not to draw the entire shape with one stroke. Construct

the shape with many segments, analyzing the angle of each line as you go. Constructing

a shape from many angles like this allows you to visualize the shape and spend more

time making sure it’s the correct shape. Rather than doing it in one stroke hoping

your hand lands in the right spot.

You can see how with this shape I established the angle of the top plane, front plane, the

angle between the corners of the rib cage, and the curve of the bottom and back. I’m

being specific about each section of the shape while maintaining the tilt and the width to

height relationship of the overall shape.

The top plane of the rib cage tilts forward, so keep that in mind as you try to visualize

the planes.

Find the bottom of the sternum. Remember, it’s about half way between the pit of the

neck and the bottom of the rib cage.

The cartilage that defines the thoracic arch starts from the bottom of the sternum. So,

from there I’ll extend the thoracic arch shape, while considering the way perspective

would affect that shape.

I like to relate one side to the other and make sure that the angle of that line follows

the angle of the front plane as if this was a box. I’ll do a simplified version of the

rib cage after this to show you what I mean.

This side of the bottom plane is covered up, but I’ll ghost it in anyway so I can visualize

these forms better.

Indicate the edges of the sternum, curving over the front plane.

A little notch in here for the xiphoid process

And we need an indication for the edge between the front plane and side plane. This is like

the edge on the box.

Everything on the front or back plane will be at this angle, like a box in perspective.

Let me show you what I mean when I talk about the rib cage being a box.

We’ll think of the front of the rib cage as this flat plane that’s rotated slightly

to the left and downward.

The widest part of the rib cage is ⅓ from the bottom, so from there I’ll extend a

plane inward. Still part of the front plane just getting narrower.

From that, we can complete the box with a side plane and top plane.

So, we have the big simplified volume of the rib cage, and we can just draw the shape of

the thoracic arch right on it. Start by finding the bottom of the sternum and then some major

angles downward toward the corners.

And since the thoracic arch is an opening, we can see the bottom edge on the far side.

It will be parallel to the other bottom edge.

Let’s clean up this back side.

Ok, so that’s the front of the rib cage. The major structure from the back is very

similar, but the wedge shape at the spine is a bit tricky, so I want to show you how

to do that.

Start the major shape in the same way I do for the front. Constructing that shape in

parts.

For this bottom edge of the back plane, I’m observing the angle here on his back and trying

to imagine what it would look like of it was a box. And then it transitions to the angle

of the side plane.

Find the edge between the side plane and back plane. At this point it should look like a

simple 3 dimensional form. Notice that even though he is leaning back and toward us a

bit, we’re not seeing the top plane of the rib cage. That’s because the top plane tilts

forward, so in most back poses the top plane will be hidden behind the back plane.

The 12th rib actually connects to the spine up here and then angles downward. So, I like

to define that edge. It’s the same edge that we ghosted in from the front view in

the previous drawing. It’s a continuation of the thoracic arch as it curves around the

bottom corner of the ribcage and comes back up to connect to the spine.

Ok, this is the part that I wanted to go over. Remember in the previous video I mentioned

that the back plane has a concave wedge that holds the spine. To cut this wedge shape out,

we can think of 3 planes. One center plane, and 2 side planes. The center plane just follows

the curvature of the thoracic vertebrae.

I want to point out that this S curve is highly exaggerated by the trapezius muscles. The

spine will curve a little bit, but the curve we’re seeing here is the muscle pushing

in past the center line.

Thoracic spine rotation does affect the rib cage, but not nearly as much as this pose

might suggest. Ok, let’s get back to that wedge.

On each side of this center plane, we’ll have a inward facing side plane.

This might just look like 4 vertical lines to you, but try to imagine the cross contour

of this surface. This plane angles like this. And this plane angles like this. This is part

of the back plane. And this is also part of the back plane.

If a trail of ants marched across this surface, that ant trail would be the cross contour

line. Around the side plane, across the back plane, down the wall of the wedge, across

the center, then back up the other wall of the wedge, and across the rest of the back

plane.

Keep practicing drawing the forms until you have them memorized. If you’re struggling

with perspective, go back and rewatch my video on structure. Practice drawing boxes in perspective.

Once you can easily draw a box, then you can start making that box more complicated by

cutting out wedges.

I hope you guys enjoyed this lesson! If you want to see more drawing examples go to proko.com/anatomy.

I have 8 more examples, which serve as the answers to the assignment from the last lesson.

You’ll also find 3d models of the rib cage and the simplified rib cage, that you can

rotate and draw from any angle. proko.com/anatomy

If you’d like to win a free membership to the premium anatomy course, go to facebook.com/prokotv

and share the post for this video. I’ll choose a random winner next Friday. Good luck!

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