Now, we want to take a brief look at each
of the mediastinal areas and the purpose of
taking a look or a stop at each one is to
understand the major or key viscera that are
occupants of these mediastinal divisions and
So, the superior mediastinum will have the
trachea that we see here and then running
posterior to it in the superior mediastinum
will be the esophagus.
In the superior and anterior mediastinum,
up until puberty and a little bit beyond,
your major occupant that will reside in both
of these mediastina collectively will be the
thymus gland. The thymus gland reaches its
maximum mass at around puberty and that mass
is in the neighborhood of 35 gms. Thereafter,
it involutes. And so, we have a degeneration
of the thymic tissue, so very little remains
in an elderly person, and what does remain
is scattered within adipose tissue that would
be located within these mediastinal compartments.
The major function of the thymus gland is
to educate: educate our T lymphocytes so that,
then, they are able to recognise self-antigens.
And if this educational process goes awry,
that can lead to autoimmune disorders.
Our middle mediastinum is depicted here.
The pericardium is shown, so that would be an
occupant of the middle mediastinum. Lying
deep would be the heart and the initial segments
of the great vessels would also lie within
the middle mediastinum.
When we think about the heart within the middle
mediastinum and we think about cardiac valves,
the four cardiac valves, a very important
part of the physical examination is to listen
to the cardiac valves in your patients. There
are topographic relationships that you need
to bear in mind to best auscultate the cardiac
valves. And so, if we take a look here, we
have the aortic valve being depicted here
and note the vector of blood ejection going
up into the right side of the body. This is,
then, the optimal location to pick up the
sounds that emanate from the aortic valve
and this area is in the right second intercostal space.
We also have, right in through here, the pulmonic
or pulmonary semilunar valve. And again, if
you follow the vector of blood flow out through
this valve, you will see that the best area
or the optimal area to detect closure of this
valve is going to be in the second intercostal
space as well, but in the left intercostal space.
If we take a look at our atrioventricular
valves, which lie here and here, here’s your
tricuspid. And the best location to pick up
auscultation from the right atrioventricular
valve is going to be in the right fourth intercostal
space. And then, here’s the vector that’s
associated, where we can best hear the closure
of your left AV valve and that is at the level
of the apex of the heart. So, that’s going
to be in the left fifth intercostal space.
The posterior mediastinum.
This region will house the esophagus. And
so, if we take a look here, we have the esophagus
extending from the superior mediastinum into
the posterior mediastinum. We can also see
the divisions of the trachea within the posterior
mediastinum. Here, in this right view, we
see the right bronchus, and on the left side,
we would have the left bronchus. Some authorities,
some anatomists, will assign the primary bronchi
into the middle mediastinum, but others will
assign these, as I am, into the posterior
mediastinum. I think it’s simpler to keep
that in mind because anything posterior to
the posterior pericardium is, then, in the
And now, we can do a brief quizzing session
or moment with you to see if you kind of understand
where we are within the thoracic cavity. And
so, we could take a look here at this particular
structure and note its relationship to other
structures within the thoracic cavity, such
as the heart. And then we have our lungs.
And so, where are we within the mediastinum?
Since we are posterior to the heart, this
is the esophagus within the posterior mediastinum.
If we take look over here, where are we within
the thoracic cavity? Left side of the image,
but again, we’re viewing this as if we’re
standing at the feet of a patient, looking
up toward their head, so this would be the
right pleural cavity.
Now, let’s summarise the key take-home messages.
The thoracic cavity is divided into three
major compartments. We have two pleural cavities
and we have that area standing between them
in the middle of the thoracic cavity called
The apex of each lung extends above the clavicle
to the level of C7 for the vertebral level
and inferior limit of the lung will correspond
to ribs 6, 8, 10.
The mediastinum is divided into superior and
inferior divisions by a plane that runs from
the sternal angle to an area that is located
between T4 and T5.
The middle mediastinum is best defined by
its major occupant, the heart and pericardium,
with the other sub-divisions of the inferior
mediastinum located anterior to the heart
and then posterior to the heart.
The points of auscultation of the cardiac
valves are located in the right and left second
intercostal spaces and the right fourth and
left fifth intercostal spaces.
The trachea is located in the superior mediastinum,
as is the esophagus, and the esophagus will
extend into your posterior mediastinum.
And the thymus functions to educate T lymphocytes
against self-antigens and is located, at its
maximum development, in the anterior and superior
Thank you for joining me on this lecture on
an overview of the thoracic viscera.