00:01
Now that we've seen the lungs,
we can talk about the
space between the lungs,
which is called the mediastinum.
00:09
And as a space, it's sometimes
hard to conceptualize a space.
00:12
So we'll start by talking about
the borders of the mediastinum.
00:17
The first border superiorly is
the superior thoracic aperture.
00:22
Inferiorly, we have
the diaphragm.
00:26
Anteriorly, we have
the sternum.
00:29
And posteriorly, we have
the thoracic vertebra.
00:32
And of course, laterally, as
we've defined this on either side,
we have the parietal pleura,
on the right and left.
00:40
The mediastinum is divided
into more parts.
00:43
In order to talk about those parts,
we first have to come up with
an imaginary structure
called the transverse plane.
00:51
So this isn't a real anatomic
structure.
00:53
It's one we're going to imagine
starting from the sternal angle,
and going straight back
to the vertebra
till it hits around the T4-T5 area.
01:03
And that transverse plane is
going to divide
the superior mediastinum above
from the inferior mediastinum below.
01:14
Now the superior mediastinum
is just the superior mediastinum.
01:18
But the inferior mediastinum
gets further subdivided.
01:23
And ordered to describe
those subdivisions,
we first have to find
the middle mediastinum.
01:29
And the middle
mediastinum is pretty easy.
01:32
It's defined as everything within
the pericardium,
which is pretty much just the heart
and the origins of the great vessels
that attach to the heart.
01:40
Anterior to that is the
anterior mediastinum.
01:44
And posterior to that
is the posterior mediastinum.
01:49
If we zoom in on that
superior mediastinum
a little bit more,
we again see the boundaries,
same as before
the superior thoracic aperture,
and inferiorly is that
transverse plane
that we're imagining
starting at the sternal angle.
02:05
Anteriorly, we have the sternum.
02:07
Posteriorly, the vertebra.
02:09
And again, the parietal
pleura on either side laterally.
02:13
There's some really important
structures that go through here.
02:17
First of all, we have some
very important nerves.
02:20
We have the vagus nerves
coming through,
and we have the phrenic nerves.
02:25
We also have some
very prominent veins.
02:27
We have the
brachiocephalic veins,
coming down and joining
with the superior vena cava.
02:34
We have very prominent
arteries too.
02:36
We have the aortic arch
and its prominent branches,
the brachiocephalic trunk,
the left common carotid artery,
and the left subclavian artery.
02:46
All big stuff and all stuff we're
gonna see in greater detail
when we talk about these vessels.
02:51
But there's also a structure here
that we kind of see.
02:54
We see a little bit of the thymus.
02:57
We're gonna see the rest of it
when we go down
into the inferior mediastinum.
03:03
Starting with the middle mediastinum
because it's the easiest.
03:06
The middle mediastinum is
anything within the pericardium.
03:10
And there's not much
besides the heart and
the origins of the great vessels
that attached to it.
03:16
We're gonna have a separate
section on those don't worry.
03:19
Anterior to the pericardium
is the anterior mediastinum.
03:25
And it's pretty small.
03:27
Its boundaries are the
transverse plane superiorly.
03:31
The diaphragm, inferiorly.
Sternum, anteriorly.
03:36
And that pericardium, posteriorly.
03:38
And of course, on either side,
we have the parietal pleura.
03:42
But it's a pretty small space.
03:45
And essentially, the only thing
we have in here
that we have to worry about
is the rest of that thing
that we saw
in the superior mediastinum
a little bit called the thymus.
03:53
And how much we actually see
depends on the age of the person.
03:59
And I'll explain that
at the very end of this section.
04:03
First, let's cover
the posterior mediastinum.
04:06
So the posterior mediastinum
is everything posterior
to the middle mediastinum.
04:10
Therefore, everything
posterior to the pericardium.
04:14
And its superior border
is again the transverse plane,
inferior border is the diaphragm,
anterior is the pericardium,
posterior the thoracic vertebra,
and on either side
we have the parietal pleura.
04:28
Now, the posterior mediastinum
is quite a bit bigger.
04:30
So there's a lot more
traveling through here.
04:33
For example, we have
the bronchi going into the lungs.
04:37
We have the esophagus on its
way down to the abdomen.
04:40
We have both the aorta and
the Azygos system.
04:44
We have the sympathetic trunks
on either side of the vertebra.
04:49
Before we finish,
I promised I would explain
what I meant by how
much of the thymus you can see.
04:54
So, what is the thymus?
So the thymus is a lymphoid organ
that it's very important
for T cell development.
05:03
But it's really important more so
in the early stages of life.
05:07
It's very prominent
in the anterior mediastinum
in infancy and early childhood.
05:13
However, over time,
it starts to regress,
or what we say
involute and turn to fat.
05:20
So that by adulthood even
though there are some little bits
of functional thymic tissue
that are still in there,
it becomes indistinguishable
from just normal body fat.