A Divisions of the thoracic cavity and mediastinum
Transverse section, superior view.
The thoracic cavity is divided into three large spaces:
• The mediastinum, in the midline, is divided into an upper, smaller superior mediastinum and a lower, larger inferior mediastinum (see B). The inferior mediastinum is further subdivided, from front to back, into the anterior, middle, and posterior mediastinum. The anterior mediastinum is an extremely narrow space between the sternum and pericardium, containing only small vascular components (see table, C).
• The paired pleural cavities on the left and right sides of the mediastinum are lined by serosa (parietal pleura) and contain the left and right lungs. They are completely separated from each other by the mediastinum. The mediastinum extends further to the left than to the right owing to the asymmetrical position of the heart and pericardium. Because of this, the pleural cavity (and lung) is smaller on the left side than on the right. The pleural cavities terminate blindly at their upper end, but the mediastinum is continuous with the connective tissue of the neck.
B Principal neurovascular structures that enter and leave the mediastinum
Superior mediastinum (borders the neck, yellow):
• The vagus and phrenic nerves, veins (tributaries of the superior vena cava), esophagus, and trachea enter the superior mediastinum from the neck.
• Arterial branches from the aortic arch and the cervical part of the sympathetic trunk leave the superior mediastinum to enter the neck.
Inferior mediastinum (borders the abdomen and pleural cavities, red):
• The thoracic duct and ascending abdominal lumbar veins (the azygos vein on the right side, the hemiazygos vein on the left side) pass through the diaphragm to enter the inferior mediastinum.
• The vagus and phrenic nerves, portions of the sympathetic nervous system, the aorta, and the esophagus descend from the inferior mediastinum and pass through the diaphragm to enter the abdomen.
Pulmonary arteries and veins, lymphatic vessels, autonomic nerves (pulmonary plexus), and the main bronchi connect the mediastinum to the lungs (and visa versa).
C Contents of the mediastinum (for divisions see A)
D Subdivisions of the mediastinum
Midsagittal sections viewed from the right side.
a Detailed view: simplified drawing of the pericardium, heart, trachea, and esophagus in midsagittal section. This lateral view demonstrates how the left atrium of the heart narrows the posterior mediastinum and abuts the anterior wall of the esophagus. Because of this proximity, abnormal enlargement of the left atrium may cause narrowing of the esophageal lumen that is detectable by radiographic examination with oral contrast medium. Radiologists call the area between the images of the heart and vertebral column the retrocardiac space.
b Schematic view: subdivisions of the mediastinum (described in A, with contents listed in C).
Note: Single diagrams cannot adequately show the components and configuration of the mediastinum, because of its asymmetry and extensions in all three axes. The anatomical relations in this space are best appreciated when viewed from multiple directions, at different planes (see p. 190ff).
A Projection of the diaphragm onto the trunk
Anterior view. The positions of the diaphragm in expiration (blue) and inspiration (red) are shown. The right hemidiaphragm rises as high as the fourth rib during expiration, and the diaphragm may fall almost to the level of the seventh rib at full inspiration.
• The exact position of the diaphragm depends on body type, sex, and age.
• The left diaphragm leaflet is lower than the right due to the asymmetrical position of the heart.
• Inspiration is marked by an overall depression of the diaphragm and also by a flattening of the diaphragm leaflets.
• The diaphragm is higher in the supine position (pressure from the intra-abdominal organs) than in the standing position.
• The degree of diaphragmatic movement during inspiration can be assessed by noting the movement of the hepatic border, which is easily palpated.
• The diaphragm in a cadaver occupies a higher level than the expiratory position in vivo due to the loss of muscular tone.