Next we need to look at the important layer of tissue which lines the thoracic wall on the inside - the pleura. To understand the pleura, we need to take a brief look at the way the main stuctures inside the thorax are arranged.
To see what’s inside the thorax, we’ll look at a dissection in which
ribs two through seven have been divided on each side along this line and removed, along with most of the sternum, leaving the costal arches intact.
Here are the divided ends of the ribs that were removed, here’s the intact eighth rib, here‘s the costal arch, here’s the divided lower end of the sternum, here’s the upper part of the manubrium, and here’s the intact first rib. Here are the bodies of the thoracic vertebrae.
This is our first look at the diaphragm, which forms an almost complete partition between the thorax and the abdomen. We’ll take a good look at the diaphragm when we look at the muscles of respiration.
With everything removed, the thoracic cavity looks like one continuous space. In reality it’s divided into two separate cavities by a partition, the mediastinum, which extends from the vertebral bodies behind, to the sternum in front. The heart, the great blood vessels, the esophagus, and the trachea are contained within the thickness of the mediastinum.
To see the mediastinum, we'll put the sternum back in place. Here’s the sternum. Here are the divided ends of the costal cartilages. This is the mediastinum. This is the hilum, or root, of the lung. This glistening layer of smooth lining tissue is the pleura, also called the pleural membrane. It forms a complete lining for this cavity, which is called the pleural cavity.
Here, the pleura is reflected off the vertebral bodies and onto the mediastinum. Here behind the sternum the pleura continues round, onto the front of the chest wall. Below, the pleura is reflected off the chest wall, and onto the diaphragm, and off the diaphragm, onto the mediastinum.
Above, the pleura fills in the gap that’s created by the curvature of the first rib. Here’s the first rib, seen from below. Here’s the highest part of the pleura, known as the dome or cupola of the pleura. Here it is, seen from the outside.
We’ll take a better look at the dome of the pleura from the outside, in a more intact dissection. Here’s the fist rib, here’s the manubrium. Here’s the divided end of the trachea, going down into the mediastinum. Here’s the dome of the pleura.
When seen from the side, the dome of the pleura is level with the proximal end of the first rib. When the pressure inside the chest is raised, the pleura rises well above the first rib.
So far we’ve been looking at the right side. The left pleural cavity is similar, except that the heart, enclosed here within the pericardium, projects into it. In the living body the pleural cavity on each side is completely filled by the lung , which we’ll add to the picture.
Here are the lungs, discolored by a moderate degree of smoke damage. The layer of smooth tissue which covers the outside of the lung is also pleura. All the way round the pleural cavity, the two layers of pleura touch, with nothing between them except a thin film of fluid.
The layer that covers the lung is called the visceral pleura , the layer that lines the cavity is called the parietal pleura. The two layers of pleura, the parietal, and the visceral, are continuous with each other here, around the hilum of the lung.
Each lung occupies a completely sealed space. Its volume can never be greater or less than the volume of the pleural cavity. When the volume of the cavitiy is increased, whether by downward movement of the diaphragm, or by forward and upward movement of the ribs, the parietal pleura exerts a pull on the visceral pleura, the lung expands, and we breathe in. When the volume of the cavity is decreased, the lung is compressed, and we breathe out.