DEV2022 – Lecture 7
Thorax in vertebrates find support in vertebral column posteriorly, the ribs and
sternum and the muscles to which all of these structures are associated. There is
an inlet through which these structures travel N and S, but the outlet is closed
almost completely by the skeletal muscles structure (diaphragm).
Thoracic contents has 2 types of compartments:
There are 2 compartments. In the midline is a tall, long column of space or
partition. In that partition are organs and big blood vessels and nerves. To the
lateral side (blue), we find the lungs. The two compartments are separate, and
they do not communicate.
General principle of cavities and spaces inside the thorax:
The inlet is open at the top. The general partition is cylinder running down the
middle, where this is connective tissue space. It is called the mediastinum. On
either side are the cavities for the lungs. The serous cavity is the watery type
fluid, is completely enclosed, so the L lung does not communicate with the R lung.
It is lined by 2 layers of the serous membrane (wet membrane). The serous
membrane, which is stuck to the surface of the lung, is the visceral layer. The
layer, which is attached to the inner aspect of the thorax and the domes of the
diaphragm, is the parietal layer.
General principle of cavities and spaces inside the thorax 2:
The double wall serous cavity contains the lungs. So the lungs are contained in
their own cavities. In the midline, the mediastinum contains the heart chiefly and
its massive vessels, and nerves.
The central partition is the mediastinum. In the mediastinum, we find a range of
structures – major organ (heart). In this region, posterior to the heart, we find
the trachea. We find the esophagus travelling inferiorly towards the stomach. We
find the major lymph vessel in the thoracic duct. We also find nerves and the
The pleural cavity do not communicate with the mediastinum.
Mediastinum has 2 main subdivisions:
There is an imaginary line which divides the mediastinum into 2 regions – upper
and lower regions.
The inferior mediastinum is itself self-divided – interior, middle and posterior.
The lungs are located lateral to the mediastinum:
The lungs are located lateral to the midline partition. They are in their own sac.
The apex of the lung extends up into the neck and extends to and slightly above
the first rib. The lungs inside their own cavity do not fill their own cavity entirely.
If they don't, then what does? Fluid. There is a small amount of fluid contained
within the cavity. A lung is located in each pleural sac:
Lung tissue = pink
Serous membrane which is stuck to the surface of the lung is the visceral pleura
(thin membrane, can be peeled off). The serous membrane stuck inside the
thoracic cavity is wallpapered with the parietal pleura. These two pleuras are
separated by a gap – pleural cavity. That cavity is called a potential space (it isn’t
actually a gap, but its filled with pleural cavity).
When you put ur hand against a balloon, the part of the balloon that shapes is the
visceral cavity. The part of the balloon, which is opposite to this, is parietal. The
air inside the balloon is the pleural cavity.
If you take 2 glass of blank histology slides, and moisten them with water and
put them over the top of each other. Use your finger nails, try to pry apart these
two parts because they are drawn together by forces of the fluid and glass. But if
u use your fingers and slide them off, they come apart. The pleura will never
come apart, and if they do, the lung collapses.
Add in the heart which is centrally placed in mediastinum:
The space occurred by the heart is the middle mediastinum.
The heart itself sits in its own sac. You don't see the heart immediately, it sits
inside a fibrous sac (pericardium). The pericardium is thick and tough and itself,
it contains two other membranes inside of it. It has a pericardial cavity which is
filled with fluid. The function of the pericardium is to support the heart.
The heart is a very robust, active organ, it is contained by the fibrous
pericardium. But because it is mobile and constantly moving, it needs a sac
The pericardium sits on top of the diaphragm. Above the pericardium blends
with and disappears where the major vessels are emerging in the heart. Inside
the pericardium, we find 2 other membranous layers – parietal and visceral. The
gap between the two is the pericardial fluid.
L + R pleural cavities:
The two pleural membranes must encounter the root or hilum of each lung,
which is where the bronchi goes into the lung, blood vessels and nerves go into
the lungs. At that point, the parietal and visceral pleura come close to the root of
the lung, and there’s a dead end, where they are reflected, so they turn back
Two hollow tubes travel inferiorly – trachea and esophagus.
The vagus nerve is a cranial nerve. It is on its way to the abdominal cavity. It
gives off branches, one of which is doing a u-turn to go back up where it came
from. The vagus nerve surrounds the esophagus to hitch hike to the abdomen.
No thymus = no T lymphocytes.
Thymus in the infant is very large, and it lies in the superior and inferior
mediastinum. As we go through puberty and beyond, it shrinks, so its located in
the superior mediastinum. It is replaced slowly with a fibro-fatty tissue. Thymus gland:
It is developed very early in humans. Its job is to produce T lymphocytes.
Even in week 6, it can be recognized as part of the parathyroid, where th