Anatomy of the Pelvis Hello, in this episode of Proko we’re going
to learn the anatomy of the Pelvis. We’ll explore the structure of the parts, the difference
between a male and female pelvis, and how to simplify the structure to make it manageable
to draw. Let’s take a look at the Parts of the Pelvis.
The Parts of the Pelvis The 2 hip bones, along with the sacrum and
coccyx of the spine form what's known as the pelvis. Each hip bone consists of 3 parts.
These 3 parts actually start out as 3 separate bones at birth and fuse together throughout
puberty. On the top half stands a wing-like wall called the ilium. Under the ilium, in
the front is the pubis. And behind the pubis is the ischium. You can remember that there’s
all sorts of “icky” stuff that happens in the ischium.
These 3 bones fuse together right in the middle of the acetabulum - a deep socket of the ball
and socket joint of the hip. The femur sits right in there and rotates in all directions
to move the leg.
Ilium The Ilium is the largest of the 3 parts. The
2 curved walls converge in the back, connected by the sacrum. The part of the ilium that
is visible in the surface is the top edge known as the iliac crest, so let’s take
a closer look at it. The iliac crest starts in the front at the
ASIS, that’s Anterior Superior Iliac Spine . And it ends in the back at the PSIS or Posterior
Superior Iliac Spine. Dont miss this. I mention these two landmarks a lot. And I use them
a lot when drawing the figure. From top view, the crest curves outward from
the ASIS for a short distance to the tubercle. Then curves back for a longer distance, and
then almost straight backward for a short distance to the PSIS.
From side view, the crest creates a half circle arc. This arc shape will vary, from a smooth
curve in some people, to a sharp corner at the top point in other people. This high point
is somewhat back from center. Men tend to have a more angular high corner, while women
have a smoother curve.
Only the front 1/4 of the crest is actually visible on surface, starting from this tubercle.
The rest is covered by the external oblique muscle on the sides and flank fat in the back.
Here’s a good example where you can clearly see the ASIS in the front and the external
oblique covering the side portion. On a more muscular or overweight body type, it appears
as a sharp edge, indicating the bottom border of the external oblique. If the torso bends
laterally, the external oblique is stretched up, sometimes exposing the middle section
of the crest. On a very lean person, the ASIS pokes out as a visible protrusion. It pokes
out even farther when the spine is extended, and the muscles around the ASIS are stretched.
The visible portion curves downward and inward.
Try to feel it on yourself.. In the back the Posterior Superior Iliac Spines
are surrounded by muscles and flank fat. They are usually seen as two dimples where connective
tissue attached to the spines pull the skin inward. You might see a subtle indication
of the crest in men, though mostly softened by the flank fat. In women it can be so softened
that the hips appear to continue all the way to the top of the waist. In some poses when
a lean model stretches her back muscles, the bones push out and appear as protrusions instead
of a dimple.
Pubis Ok, let’s move on to the pubis. The two
sides are joined by a cartilaginous disc at the pubic symphysis. This is a commonly used
landmark, but you won’t actually see a bony point because it’s covered by pubic fat.
You can find it by following the trail of the inguinal ligament. A thin string-like
ligament stretching from the ASIS to the pubic tubercle just lateral of the pubic symphysis.
It defines the lower border of the abdominal mass called the "furrows of the groin" and
appears deeper in men. The pubic symphysis aligns with the level
of the tailbone, although this varies. In men, it’s also on the same vertical plane
with the ASIS. Another useful alignment, is the pubic symphysis tends to be the vertical
center of the whole body. In the back, the vertical center is between
the bottom of the sacral triangle and the bottom of the buttocks, right in the middle
of where the glutes meet.
Gender Differences The pelvis has the most divergence between
male and female of the entire skeleton. Mainly because a female pelvis is built to allow
a woman to give birth. Skelly, I’d like you to meet, Skella!
What makes Skelly so attracted to Skella’s pelvis? Well, the female pelvis has lighter
and thinner parts. It’s about 2 finger widths wider and 2 finger widths shorter than a male
pelvis. The walls of the ilium are more vertical, but not as tall. The pelvis is tilted forward
more on a female. The ASIS protrude forward more and don't align with the pubic symphysis.
On a man, the ASIS and pubic symphysis align on the same vertical plane.
You might also notice that the pubic arch is much wider on a female pelvis. About 120
degrees in women, but only 90 degrees in men. This means that women’s ischia are farther
apart. Creating a wider gap between the leg muscles because the insertion points of the
adductor muscles are farther. On a woman, you might see a gap even if the legs are together.
On a male, the gap is much smaller. Sometimes there is no gap, even if the legs are separated.
The widening of all these parts on a female pelvis pushes the acetabula laterally, which
in turn pushes the greater trochanters. These differences, along with more fat distribution,
account for why women have more prominent hips. An average woman will have a higher
body fat percentage than an average man, so the baby doesn’t starve during hard times.
Like the pelvis, the sacrum is also shorter, wider, and curves posteriorly more. On the
surface, this plane of the lower back is more angled on a woman and more vertical on a man.
The main reason for the wider female pelvis is to create a larger inlet and larger outlet
for a baby to pass through. The inlet is this round shape here. Very round
and wide on a female. On a male pelvis the inlet shape is more like this.
The outlet is the space between the bottoms of the ischia and tailbone. You can see how
everything is more spaced out on the female pelvis, perfect for childbirth. Allow me to
demonstrate. As you can see, the baby passes through Skella's pelvis quite easily. But
with Skelly. Well... To review, a male pelvis is characterized
by height, weight, more vertical angle, thickness, sharper angles, and a smaller cavity. A female
pelvis is characterized by width, lightness, forward tilt, thinness, and a larger cavity.
Simplified Structure As you can probably tell, the forms of the
pelvis are quite complex. Trying to imagine these forms from various angles let alone
drawing them is very challenging. Let’s take a look at a few ways we can simplify
the pelvis to make it easier to construct and pose the body.
The relationship between the top half and bottom half of the pelvis simplifies to 2
perpendicular planes. The Ilium generally angles inward to the sacrum. The ischium and
pubis together angle perpendicularly to that, meeting at the pubic symphysis. From the side,
It's kinda like a figure 8. A twisted figure 8..
Bucket Now, the method I’ll use a lot in this Anatomy
course, is the bucket.. I like it because starting the construction with a bucket allows
us to get very accurate placements for the landmarks.
A bucket is basically a tapered cylinder. This cylinder, in a neutral standing pose,
is tilted forward, like the pelvis. It's wider at the top and narrower at the bottom. Also,
the width is longer than the depth. So the top and bottom caps are not perfect circles.
This ellipse of the top cap is the first thing I draw. It determines a lot of things. The
width, the forward tilt, and the side to side leaning. It's also not too hard to find on
the surface of a model. From the front, use the ASIS points to determine the side to side
leaning angle. Now, from the top you'll notice that this ellipse doesn't really follow the
shape of the iliac crest all the way. It does for this middle portion but then the crest
takes a sharp turn inward, leaving this extra space.. Well, if you consider the muscles
and flank fat in this area, this ellipse actually follows the surface curve nicely! And in the
front, the curve follows the form of the abs. Of course, on an obese body, this won't work
as well. So, we’re looking just at this portion of the iliac crest when drawing the
top ellipse. After I've established the ellipse, I'll add
the side planes. The length depends on how much foreshortening there is caused by the
tilt and our point of view. Longer lines when there’s no foreshortening. And shorter lines
when there’s a lot of foreshortening. You also have to think about the gender differences.
Remember, a female pelvis is wider and shorter than a male pelvis.
Finally, I'll add the bottom cap and that gives me a simplified form for the pelvis.
There we have a simple representation of the pelvis as a bucket. In the next lesson, I’ll
show you how to break it down further to construct the minor forms of the pelvis. In the meantime,
fill a page with buckets from various angles. Premium Anatomy students, you can login to
your account and use the 3d model of the bucket and pelvis as reference. Spin it around and
draw the bucket! If you’d like to sign up for the Premium Anatomy Course, go to proko.com/anatomy