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Anatomy of the Rib Cage

April 30, 201517 Comments

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Assignment: Draw the Rib Cage

Construct a Robo Skelly rib cage and the pelvis using the bucket method. Try to be as accurate as you can with them. Don’t just draw a generic rib cage shape in there. Look for clues from landmarks and muscle attachments that will tell you exactly where the rib cage is. Post your work in the Anatomy for Artists Facebook group.

joint assignment

Download Assignment Photos

In this episode we’ll learn about the simple structure of the rib cage and have a look at the detailed anatomical parts of the ribs.

Simplified Structure

The rib cage is often simplified as an oval shape. For a gesture drawing, that’s good enough. But for an anatomy study, it’s not. The rib cage is more like an egg because the top is narrower than the bottom. And more specifically, the rib cage is an egg with planes. It has clear front, side, and back planes. The front plane is composed of the sternum and costal cartilage. The front plane transitions to side plane right where the costal cartilage connects to the ribs. From there the ribs continue backward in a subtle convex curve. Then they reach  the angle of the ribs where they take a sudden turn medially for the back plane. The back plane actually has a concave wedge where the ribs curve forward to connect to the spine.
The ribcage is shaped like an egg

Planes of the ribcage

This is unique to humans, allowing us to lie on our backs comfortably. Quadrupeds, like dogs and horses, have pointed spines – the spinous processes of the vertebrae extend way up.

Now don’t miss this. The top plane actually slants forward. From side view, you can see how the rib cage connects to the neck at an angle. The neck curves back to hold up the head vertically. Ignoring this will result in the infamous ‘lollipop neck’.

The bottom plane is complicated. It curves. In the front, from the bottom of the sternum, the costal cartilage angles outward creating the upside down V shape called the thoracic arch. The cartilage of the 10th rib has a sharp “corner of the ribcage”, which you can see and feel  on the surface. From that corner, the bottom plane curves around to the back and then up to the 12th thoracic vertebra.

Thoracic arch and cartilage landmarksOutline of thoracic arch on a model

Parts of the rib cage

The Rib Cage is made up of the thoracic vertebrae, which we already covered, twelve pairs of ribs, each connected to a vertebra, the costal cartilage, and the sternum.


“Do we really need to know how many ribs there are?”

It may seem like overkill, but when you invent poses or sculpt the figure, you may want to know where to attach the muscles of the torso. The rib cage is an origin and insertion area for many muscles. So, let’s learn the ribs so we can attach the muscles in the right place.

“But there’s so many of them!”

Well, not really. Only 12. But don’t worry, if you understand the simple structure of the planar egg and the general pattern of the ribs, you’ll find it easy to place the ribs in that egg structure.

12 pairs of ribs

From the back, the ribs angle down slightly. As they reach the side plane, they dive diagonally at about 45 degrees and stay at that angle until they reach the costal cartilage in the front. The costal cartilage of the top half stays close to horizontal. The bottom half curve upward toward the sternum.

“Do women have an extra rib?”

Nope! There is an abnormality in a very small percentage of people who have an additional cervical rib (that’s in the neck). This happens more often in females, but also occurs in males.

So what parts of the rib cage show up on the surface? On a muscular person when the muscles stretch, we see some of the lower ribs in the front and also in the back.

On a lean person it doesn’t take much of a stretch to reveal the ribs in the front and back, and they’re much more obvious.

Now, how do we figure out where the rib cage is in poses where we can’t see the ribs? Well, we can use our knowledge of where the muscles attach. Rib number 5 is an important one. The bottom of the pec aligns with the level of 5th rib. The first digit of the external oblique originates at the 5th rib. And the first visible digit of the serratus originates at the 5th rib. The 3 most prominent serratus digits originate at the 6th, 7th, and 8th ribs.

Muscle attachments on rib cage

Costal Cartilage

The Costal Cartilages connect the ribs to the sternum. They also make the ribcage more flexible and elastic.

On a very lean person with thin pec muscles, you’ll see the first few costal cartilages connecting to the sternum in the front. A bit lower and to the side, the muscle and breast tissue (on a female) will cover the ribs even on a very lean person.

The lower front edge of the rib cage is the Thoracic Arch. It’s made up of the cartilages from the 7th to 10th ribs. Starting from where the Costal Cartilages of the 7th ribs attach to the Sternum, down to the corner of the 10th rib. This corner often stands out in the figure, especially when the model is inhaling or leaning back.

Because the thoracic arch is made of cartilage, it will have more variation than bone. Artists tend to idealize this shape to be 90 degrees on males and a narrower 60 degrees on females. And, a more masculine arch will curve outward and a feminine arch will curve inward.

Visible costal cartilage

But the shape of the thoracic arch is more of an idealization than a rule.

Idealized variations of thoracic arch


Also known as the Breastbone, the Sternum is made up of three pieces and looks like a downward facing dagger ornecktie. You can think of these pieces like the sword of a Roman Gladiator. The top piece is the Manubrium, which means ‘handle’, the Body is the Gladiolus which means blade, and the tip of the sternum is called the Xiphoid process. Xiphos, means sword.

The top corners of the Manubrium are where the clavicles attach, forming the pit of the neck.

The body of the sternum is the longer bone, thinner at the top and thicker at the bottom. It’s about twice the height of  the Manubrium. On a male, the lower end of the Body is usually located below the nipples and above the level of the lower border of the Pecs. On a female, the placement of the nipple and bottom of the breast varies greatly.

Parts of the sternum- Manubrium, Gladiolus, Xiphoid Process

At the lower tip of the Sternum is the Xiphoid Process, or the dagger! It’s about the size of the tip of your thumb and is made of cartilage. Sometimes the Xiphoid Process sticks out from the surface, and sometimes it digs in and makes a depression, surrounded by thick costal cartilage.

Assignment: Draw the Rib Cage

Construct a Robo Skelly rib cage and the pelvis using the bucket method. Try to be as accurate as you can with them. Don’t just draw a generic rib cage shape in there. Look for clues from landmarks and muscle attachments that will tell you exactly where the rib cage is. Post your work in the Anatomy for Artists Facebook group.

joint assignment

Download Assignment Photos

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Comments (17)

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  1. RafnLuis says:

    hi, Proko! very good work!
    i have a question for you: when you publish the anatomy videos of hands and feet?


    • I will go through muscles of the torso first. Then arms and legs (those will include hands and feet).

      • Tom Bolt says:

        Hi Stan,
        I am a 73 year old VietNam Era Veteran. Your educational material is simply mind blowing, jus incrediable. I have viewed a number of course offerings and yours is by far the most professional. I haven’t enough appropriate words to adulate with. Wish I could afford your courses but being a Veteran and on a limited retirement income it is simply impossible for me. Please do not mistake this as an attempt to niggle some kind of a deal from you, it isn’t. I merely wanted to share my feelings regarding the quality of your professionalism in the material. I learn a little from your stuff but never, it seems, quiet enough to be complete. Thank you ever so much for producing these materials.

  2. Jason says:

    Thanks for another great video. I’ve always had trouble orienting the ribs correctly under the skin. This helps!

  3. janey says:

    Hi proko, very good videos! Love them.
    Are you going to cover the whole skeleton before going to the muscles of the course?
    Or are you going to focus on the bones from the current course (torso) and then the muscles?


  4. Roberta Remy says:

    yesterday I purchased the package of arms/legs/torso. I cannot access it? what happened? Please help? How do I get these programs?

    • Filip Miucin says:

      Roberta, I checked your account and looks like the Anatomy 3 pack has been added. You just need to login to your account and you will see it on your dashboard. If you forgot your username, please email I don’t want to provide that info here in public comments.

  5. Jacques says:

    I just bought Skelly for mobile. You’re a damn genius. When can the world start calling you Maestro?

  6. Steven says:

    Are you planning on doing any videos that focus on the gender-specific body parts?

    • Yes, in my lesson on the chest, I will teach about both male and female chest. I also described gender differences in lessons on spine, pelvis and rib cage. I wont be covering the parts ‘down there’.

  7. Pilares says:

    Hello, proko, I’m from Brazil and my English is the google translator.
    I want to thank and congratulate your work. I am a self-taught designer, and all the books and tutorials I’ve seen, his is the most complete, perfect. It changes a lot with their classes.
    Keep it up! Best wishes to you and your family.

  8. Becky says:

    I actually found this more entnrtainieg than James Joyce.

  9. Justin says:

    Hi Stan,

    Your channel is incredibly helpful. The information you jam (in a good way) into these short lessons is amazing. It covers the science to the everyday visual.

    I do have a question: when the torso is twisting or bending, what happens to the rib cage? As I’m learning more about the structure of the core, I tend to lose a little rhythm. I figured you would have some helpful tips!

    Have a great week,


  10. Natália says:

    Hi Proko, I’d like to ask you what would you recommend to continue with after your lessons for beginners, the anatomy or figure drawing lessons?
    Thank you and great job!

  11. None says:

    I need extended information regarding the ribs flexibility and the vertebrae movement.

    First of all, We did cover that the range of motions in the vertebrae and while limited, they add up to a greater range, But here i lack in information, regarding the PROCESS of the movement itself.

    Let’s consider the following no visual example, That i want to bend with my body towards my legs [touching your toes exercise] BUT i only want to slightly bend my torso, let’s say, by 10 degrees,

    do all the thoracic vertebrae activate?
    or is it that one vertebra reaches its limit, it drags another?

    Where does the motion starts exactly?
    Is it possible to have a situation in which slight bending will only cause T-12 to T-5 to move and T-4 to T-1 to remain static or do they all move?

    Now for the ribs : We have also learned that the ribs are connected to a corresponding vertebrae and the sternum, So during bending or any other spinal movement, What happens to the ribs? Do they stretch?

    *[Consider that each vertebra got slight different range of motion, the spinal movement won’t be uniform, some vertebra can rotate more than others] so what happens to the ribs in relation to the vertebra they connect to?

    Is there a way to show the ribs in a skeletal form during spinal stretching?

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