Ribcage Hello, welcome to Proko my name is Stan Prokopenko.
So, we’re done with the pelvis and now we’re going to move our way up to the rib cage.
In this episode we’ll learn about the simple structure of the rib cage. Then look at some
of the detailed anatomical parts. 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 more like 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. 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. That might explain why Skelly’s
always being so lazy! 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 this
angle. The neck curves back to hold up the head vertically. Ignoring this will result
in the infamous ‘lollipop neck’. It tastes good. But it doesn’t look good. Oh, the bottom plane. It’s 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. 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. 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. I’ve invented this x-ray pad to help us.
With some x-ray paint and a simple canvas I was able to jerry-rig one of these bad boys!
Cool huh? 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. 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. But the shape of the thoracic arch is more
of an idealization than a rule. In this example we can see a wide arch on a female. Sternum Also known as the Breastbone, the Sternum
is made up of three pieces and looks like a downward facing dagger or necktie. 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. 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. Sometimes the xiphoid
process sticks out from the surface, and sometimes it digs in and makes a depression, surrounded
by thick costal cartilage. Check out proko.com/anatomy for extended anatomy
lessons and more examples of the assignments. That’s proko.com/anatomy Assignments For our assignment this week, let’s build
on the assignment from the Spine lesson. Now that we know more about the structure of the
pelvis and rib cage, we can do a more precise version. 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. I’ve uploaded a bunch of model photos,
which you can find in the description below. That’s it, thanks for watching! If you’re
posting your drawings, use hashtag #proko and don’t forget to follow me on Facebook
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