The lower extremity begins at the pelvis and then travels through the thigh, the leg, into
the foot.
So the pelvis is the junction between the torso and the lower extremity.
So we need to take some time and look at the pelvis separately for a second, and then venture
into the lower extremity.
And then when we do the axial skeleton we will come back to how the pelvis fits in with
the vertebral column at that time.
So I want to take some time to look at just the pelvis, and then break the pelvis down
into the individual bones.
Here we see a pelvis with the right and left os coxae, also known as the innominate bones.
And in the middle of the pelvis we see the lower part of the lumbar vertebra which is
not actually part of the pelvis, and the sacrum and the coccyx.
Sacrum and coccyx form part of the pelvis.
They are also part of the vertebral column.
Now the pelvis itself is not a portion of the lower extremity, but the innominate bones,
or the os coxae, are considered portions of the lower extremity.
Also before moving on, I want to point out that most anatomy students would place the
pelvis like this in the human body.
And that is not correct.
The pelvis has an anterior tilt so it is not actually like a basket, as most of us tend
to view it.
As we go through the os coxa we will learn the tuberosity of the pubic bone and the anterior
superior iliac spine of the ilium.
Those are in the same vertical plane.
So if I turn this sideways, this spot and this spot should be in the same vertical plane.
So that shows you the tilt of the pelvis anatomically.
As I say most students in the lab tend to put the pelvis in this position.
It should be turned in a tilted anterior position.
This is a portion of the pelvis that over the years has had many different names.
Some people call it the hip bone.
Anatomically that is not correct, but we often use the term.
It was named the innominate bone.
Strange thing about that is that the word in innominate means no name, so it was named
the no-name bone.
Its correct name is the os coxa.
Now the os coxa is actually three bones that have fused.
The upper portion of this os coxa is a bone known as the ilium.
The anterior portion is a bone known as the pubis.
And the posterior inferior portion is a bone knows as the ischium.
Now in order to tell a left os coxa from a right os coxa it pretty much helps to visualize
the entire pelvis.
Now remember when we saw the pelvis just a moment ago, we had a whole pelvis with the
sacrum in the middle.
There is an articulating spot on the ilium called the sacral articulation, also known
as the auricular fossa.
That is for articulation with the sacrum.
So if you imagine putting the sacrum here we begin to form that someone basket shape.
Anteriorly we have the pubic bone.
That gives us a left os coxa.
Now if we look carefully at this os coxa, on the lateral aspect we have a large cup
shape, or socket, forming the ball and socket for the hip joint.
As we can see here on this articulated skeleton.
That socket is known as the acetabulum.
So to tell a left from right on an os coxa you should know that the auricular surface
is posterior and medial, and then obviously the acetabulum must be lateral.
It would be very strange to have a leg right in the middle.
So acetabulum lateral, auricular surface medial and posterior and we have ourselves a left
os coxa.
Now there are many many features that we need to know on this particular bone.
If we go across the top of the pelvis, this again is the ilium, this is called the iliac
crest.
And the iliac crest comes to a point at the front, and you can feel that point on yourself
here, and a lot of people say they banged their hip bone, well that is really the anterior
surface of this ilium and it is known as the anterior superior iliac spine.
And about an inch below that is an anterior inferior iliac spine.
We have the crest of the ilium.
And then within the pelvis we have what is called the iliac fossa.
On the external surface of the ilium, if we imagine somebody were to draw a Y shape in
the middle of our acetabulum, above that Y is the ilium.
Anterior to the Y is the pubis.
And posterior inferior to that is the ischium.
If we look at the iliac crest as it goes along from a lateral view of the pelvis, it will
end is a large tubercle known as the posterior superior iliac spine.
And a smaller area known as the posterior inferior iliac spine.
And then we see a very large notch.
That notch is called the greater sciatic notch.
From there down we are no longer ilium.
So if I hold my hands in this position I should be hiding pretty much everything of the innominate
bone that is not ilium.
So what you can see remaining in this picture is ilium.
If we look at the pubic bone, the pubic bone we said is toward the front.
The pubic bone has a body, and then it has two arms.
The anatomical word for arm is ramus.
It has a superior ramus and it has an inferior ramus.
So two arms reaching away from the body of the pubis.
The medial portion of the body of the pubis has an area for the pubic symphysis, a fibrocartilaginous
joint between the two pubic bones.
If we look at the articulated skeleton we can see the left and right pubic bones and
a fibrocartilaginous joint in the middle, known as the pubic symphysis.
So we can clearly see the articulating spot for the pubic symphysis.
Across the top of the pubic bone we have what is known as the pubic crest.
And at the very end of the pubic crest there's a small rounded lump known as the pubic tubercle.
If I cover this region and this region, what you see remaining is pubic bone.
So the only remaining bone we need to talk about from this os coxa is the ischium.
Remember the ischium was the posterior inferior portion after we drew a Y through the acetabulum.
The ischium has a small projection pointing into the pelvis, on this bone it is been chipped
away a little bit, on this bone you can see here and here, that is called the ischial
spine.
Immediately below the ischial spine is a small notch called the lesser sciatic notch.
Remember we had a greater sciatic notch on the ilium.
We have a lesser sciatic notch on the ischium.
Then the ischium has a very large tubercle.
Only in this case is called ischial tuberosity.
The ischial tuberosity is what many of you in street slang may refer to as your butt
bone.
If you ever fallen and slipped on the ice and landed rather hard, that is the bone that
smashed against the ground and probably caused a great deal of pain.
The ischial tuberosity.
Now the ischium also has an arm, leading forward from it, and the arm of the ischium meets
with the inferior arm of the pubis.
Remember the anatomical word for arm is ramus.
So the ischial ramus meets with the inferior pubic ramus.
Together that is often called the ischiopubic ramus.
So when we fuse all three of these bones, the ilium, the ischium, and pubis together
we see that the ischium and the pubis are fused and that leaves a very large opening
in this os coxa known as the obturator foramen.
Notice the ilium is not a portion of the obturator foramen.
We’ll go back laterally and we will look at our Y shape and imagine the top of the
Y and the bottom of the Y.
Above my fingers is ilium.
So the upper portion of the acetabulum and above is ilium.
Anterior, pubis.
Posterior and inferior, ischium.
So the obturator foramen is made up by the fusion of the ilium or excuse me the ischium
and the pubis, but not the ilium.
So again you should be able to pick this up and you should be able to see the crest of
the ilium, and as we come to the back of the crest of the ilium, a large tubercle type
structure, and then an auricular surface for the articulation with the sacrum.
That goes medial and to the posterior.
The acetabulum for the thigh goes laterally.
So this would be a left os coxa.