00:01
Now, that we've seen the ribs,
we can talk about the space
between the ribs,
called the intercostal space.
00:07
There are a lot of muscles
and neurovascular structures
that exist in this
intercostal space.
00:12
The first thing we'll see are the
muscles that run between the ribs
appropriately called
that intercostals.
00:18
And there are three sets
stacked on top of each other
from superficial to deep.
00:24
The first set is the most
external set and called
the external intercostals.
00:29
Their fibers run in a
diagonal orientation,
such as, as they're going
from one rib to the rib below,
they travel somewhat medially,
at least from this
anterior point of view.
00:41
And what that really means is that
when these muscles contract,
they have the effect of elevating
the ribcage and expanding it
and causing the thoracic cavity
to expand helps with inhalation.
00:54
If we go a layer deep to that we
have the internal intercostals,
but their fibers run in
the opposite direction.
01:02
And so they're gonna
have the opposite effect.
01:04
They're going to
depress the ribcage,
decreasing the thoracic cavity,
and causing exhalation.
01:11
Finally, the deepest layer is
the innermost intercostals.
01:15
And their fibers have the same
orientation as the internal,
so they do the same thing.
01:21
Now, the vasculature of
the intercostal space
is very interesting.
01:26
And the first thing
we have to talk about
are these vessels called
the internal thoracic or
internal mammary arteries and veins
that are branches of the
subclavian arteries and veins.
01:38
You'll notice that
just below each rib,
there's a little vein
called an intercostal vein,
running along anteriorly until
it meets the internal thoracic.
01:49
Similarly, at each level,
there's a branch coming off
of the internal thoracic artery
and running just below
the intercostal vein
called the intercostal artery.
02:01
Finally, below that
is a spinal nerve branch
called the intercostal nerve.
02:08
And that's our
neurovascular bundle.
02:10
Let's take a closer look at this
bundle now in cross section.
02:14
So, in cross section,
we again see external intercostal
is the most external,
then we have the internal,
and finally the innermost
and it's between
the internal intercostal
and the innermost intercostal,
that this neurovascular
bundle travels.
02:31
And some people use
the mnemonic VAN, V-A-N
to remember the order of this
bundle from superior to inferior.
02:39
So superior,
we have the intercostal vein,
then we have
the intercostal artery,
and then we have
the intercostal nerve,
So, vein, artery, and nerve.
02:48
So, that's a good start to
talking about the vasculature.
02:51
But there's a little bit more
to the story.
02:53
So, let's zoom out a little bit
so we could see a fuller picture.
02:58
So we have the subclavian artery,
as we mentioned,
giving rise to the internal thoracic
or internal mammary artery.
03:04
And that's where we're
going to get those
anterior branches
of the intercostal arteries.
03:09
But it turns out,
they're going to announce the most
with some coming from posterior.
03:15
Most of those are going to
come directly off of the aorta,
except for the first two,
which tend to come off
something called
the cost of cervical trunk
of the subclavian.
03:25
But otherwise,
these posterior ones arise
directly from the aorta
and anastomoses
with the ones coming from
the internal thoracic artery.
03:34
So, if we put it all together,
we can see how that fits.
03:38
So again, from superficial to deep,
we have the external intercostal,
internal intercostals,
and the innermost intercostals.
03:47
Posterior leads where
we have the aorta,
giving rise to the
posterior intercostal arteries
that are traveling again between
internal and innermost intercostals.
03:59
Now, it's also going to
give some other branches.
04:01
So there's a dorsal branch that's
going to go off and supply the back.
04:06
And there are going to be branches
that pierce through the muscle
to provide supply
to the overlying skin.
04:12
Before the anastomoses with
the anterior intercostals
coming from the
internal thoracic artery,
which itself is going to give
some perforating branches
to supply the overriding skin.
04:23
And if we go back
to an anterior view,
we can see the intercostal
vessels and nerves
in their little bundle
in the intercostal space.
04:32
And again,
we see the internal thoracic artery
running along either side
of the sternum internally.
04:38
And if we look inferiorly,
we see that it's going to end.
04:41
It's going to bifurcate into
two arteries at its termination.
04:45
The first is going to head off,
sort of laterally, diagonally,
and it's going to supply
abdominal muscles and the diaphragm
and so it's appropriately called
the musculophrenic artery.
04:56
Musculo for the abdominal muscles
and phrenic because phrenic
refers to diaphragm
whenever you see that term.
05:02
The other branch basically
continues along the vertical path,
becoming the superior
epigastric artery.
05:10
And if you haven't
picked up by now,
the thing with anatomy is
if you hear superior so and so
there's probably an
inferior so and so.
05:18
And there is an
inferior epigastric artery
that will come from
pelvic vessels below,
anastomoses with the
superior epigastric artery,
right around the umbilicus,
which is our fancy
word for belly button.
05:32
Providing an important
source of collateral supply
in case you ever get an obstruction
in your abdominal aorta.
05:38
All right, so let's look at those
nerves a little bit greater detail.
05:43
So, we already mentioned
the intercostal nerves
run in that same groove
in between internal
and innermost intercostals.
05:51
And they're also going to give off
branches along their course too.
05:55
So they're also going to have
lateral cutaneous branches
and supply the skin that's
overlying the ribcage in that area.
06:02
And that's why dermatomes
look the way they do.
06:05
Dermatomes look like
rectangular strips
that are roughly about the size
and shape of an intercostal space.
06:12
There's going to be
cutaneous branches anteriorly.
06:16
And there's going to be some
stuff happening posteriorly
that is a little smaller, so we're
going to zoom in to look at it.
06:23
So here we see the spinal cord with
its anterior and posterior roots.
06:29
And then we have the
posterior ramus of this spinal nerve
that's coming out at this level,
immediately going to the back and
supplying structures in the back.
06:38
Whereas, the anterior ramus
is essentially what
the intercostal nerve is.
06:44
And running parallel to
the vertebra on either side
our sympathetic trunk
with its ganglia,
and they're going to communicate
with the spinal nerves at each level
via the gray and white
rami communicantes,
which is just a very long word
for communicating branches.
07:00
We're going to wrap up
with some minor muscles
in the intercostal space,
starting with subcostales muscles
or subcostales,
which are variable in tend to only
exists really on the lower ribs.
07:15
But they do attach to the
internal surface of one rib,
and then skip a couple ribs,
and they don't attach again
until two or three levels below.
07:27
And they have the
same fiber orientation
as the internal and
innermost intercostal.
07:31
So they're going to
have that same function
of assisting with exhalation.
07:37
And then the last one is
the transversus thoracis.
07:41
Also a very variable muscle
but generally is going to sit
along the internal surface
of the say 2 to 6 ribs,
whether it's at the bony part,
or the cartilaginous part
that varies as well.
07:58
And then they'll attach to
the lower half of the sternum
where the body and the
xiphoid process are.
08:06
Same deal. They have a
very minimal effect on breathing.
08:12
And it's the same as the other
muscles we just mentioned,
where they depress the ribs
and assist with exhalation.
08:19
Though it's important
to keep in mind
that all the muscles we mentioned
whether they help with
inhalation or exhalation
are really assistive or sensory.
08:27
They're not really the
primary drivers of breathing.
08:30
By far the primary driver of
inhalation is the diaphragm.
08:34
And for the most part,
passive exhalation is
just from elastin fibers
that exist in the lung.
08:39
So these are really
minor muscles of respiration.