Dissector Answers - Thoracic Wall, Pleura, & Pericardium

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Describe the framework of the thorax, including the sternum and its parts.
2. Diagram a typical intercostal space, including muscles, nerves, and vessels.
3. Describe the make up and surface projections of the pleural cavity. Identify its recesses.
4. Distinguish between parietal and visceral pleura and between parietal and visceral pericardium. Identify the various divisions of the parietal pleura.
5. Identify and describe the mediastinum, including its boundaries and subdivisions.
6. Identify the contents of the anterior mediastinum.
7. Describe the pericardium and its parts.
8. Identify the sternocostal projections of the heart, in addition to its borders, surfaces, and sulci.

Learning Objectives and Explanations:

1. Describe the framework of the thorax, including the sternum and its parts. (W&B 357-358, N185, TG4-04A, TG4-04B)

The thorax is the part of the trunk, including a cavity, between the neck and the abdomen. The cavity contains the heart and lungs. These viscera are protected by the "bony" thorax, which includes the three parts of the sternum anteriorly, the 12 thoracic vertebrae posteriorly, and the ribs in between.

2. Diagram a typical intercostal space, including muscles, nerves, and vessels. (W&B 363-369, N188, N191, N192, N254, TG4-07, TG4-08, TG4-09, TG4-10)

Muscles: There are three primary layers of muscle in intercostal spaces. Superficial to deep they are the external intercostal, the internal intercostal, and the innermost layer of intercostal muscles.

• The external intercostal muscle fibers originate on the inferior border of the rib above the intercostal space, travel inferiorly and medially, and insert on the superior border of the rib below. Medial to the costochondral joints on the anterior aspect of the thorax, the external intercostal muscle fibers are replaced with external intercostal membrane. (The direction of the external fibers is like putting your hands in your pockets. These descriptions also apply to the external and internal abdominus muscles, so they are handy to remember.)
• The internal intercostal muscle fibers originate on the superior border of the rib below the intercostal space, travel superiorly and medially, and insert on the inferior border of the rib above. On the posterior aspect of the thorax, medial to the angles of the ribs, the internal intercostal muscle fibers are replaced with the internal intercostal membrane.
• The innermost intercostal muscle layer roughly follows the internal fibers. (Anteriorly, this layer is represented by the transversus thoracis muscle. Posteriorly, the small subcostal muscles are found lateral to the articular tubercles of the ribs. Between is found the innermost intercostal muscle layer per se.)

  Nerves and vessels: Within each intercostal space there are two neurovascular bundles. The larger of the two travels along the inferior border of the rib, with the vein most superior, then the artery, and finally the nerve most inferior (VAN). The smaller, collateral branches of all three travel along the superior border of the rib below. The orientation is just the opposite, with the nerve most superior, then the artery, then the vein. (You could just remember that the vein is always closest to the rib and the nerve farthest away.)

3. Describe the make up and surface projections of the pleural cavity. Identify its recesses. (Greek, pleura = rib, side) (W&B 371-373, N194, N196, N197, N211, N230, N231, N237, TG4-13A, TG4-13B, TG4-15, TG4-16)

Make up: The pleural "cavity" is the potential space between the visceral pleura on the surface of the lung and the parietal pleura lining the back of the ribs, the intercostal muscles, the diaphragm, and the mediastinum. The parietal pleura is divided into four sections (see also below): cervical, costal, diaphragmatic, and mediastinal. The names of the section indicate what the section contacts or borders. (The cervical pleura is the exception. It and the apex of the lung extend out of the thoracic cavity via the superior thoracic aperture.)

Surface projections: As a general rule, the parietal pleura extends the space of two ribs inferior to the inferior border of the lung at the same point.

Landmark	Inferior Border of Lung	Reflection of Parietal Pleura
Midclavicular line	6th rib	8th rib
Midaxillary line	8th rib	10th rib
Scapular line	10th rib	12th rib

Recesses: These are two spaces where the lung does not occupy the pleura totally, especially during expiration. The costodiaphragmatic recess inferior to the lung is where the costal pleura and the diaphragmatic pleura touch one another. There is a potential space between them. The costomediastinal recess is a smaller potential space between the layer of costal pleura and mediastinal pleura anteriorly.

4. Distinguish between parietal and visceral pleura and between parietal and visceral pericardium. Identify the various divisions of the parietal pleura. (Latin, pariet- = wall (of a cavity), viscus = internal organ) (W&B 371-373, N196, N211, N241, TG4-17, TG4-18)

The visceral pleura is all of the serous membrane that is in direct contact with the lung. The parietal pleura is everything else. The same is true of the pericardium: if it directly touches the heart, it is visceral, but if it only contacts other tissue (including visceral pleura), it is parietal.

The divisions of the parietal pleura were mentioned above.

• Costal pleura: contacts the inner aspects of the ribs and intercostal muscles.
• Mediastinal pleura: forms the lateral walls of the mediastinum, contacting the structures within it, including the pericardium.
• Diaphragmatic pleura: contacts the superior aspect of the diaphragm.
• Cervical pleura (pleural cupula): forms a dome over the apex of the lung.

The mediastinum is the space behind the sternum, between the two pleural sacs. It is divided into four sections:

Subdivision	Boundaries	Contents
Superior*	Superior: plane of the first rib Inferior: plane b/w sternal angle and T4/T5 Anterior: manubrium Posterior: upper spinal column	arch of aorta, other great vessels, thymus, and some important nerves

Anterior	Superior: plane b/w sternal angle and T4/T5 Inferior: diaphragm Anterior: sternum and transverse thoracis muscles Posterior: pericardium (middle mediastinum)	areolar tissue, sternopericardial ligaments, lymph vessels and nodes
Middle	Superior: plane b/w sternal angle and T4/T5 Inferior: diaphragm Anterior: anterior mediastinum Posterior: posterior mediastinum	pericardium, heart, great vessels, bronchi (at roots of lungs)
Posterior	Superior: plane b/w sternal angle and T4/T5 Inferior: diaphragm Anterior: pericardium (middle mediastinum) Posterior: spinal column	descending thoracic aorta, azygos system, esophagus, thoracic duct

*The inferior mediastinum is referred to by Moore, and contains the anterior, middle, and posterior subdivisions.

See above.

The pericardium, like the pleurae around the lungs, is a serous sac into which the heart grows during development. The pericardium differs, however, in that the parietal portion is invested by a strong fibrous coat. (This has important clinical ramifications, such as with cardiac tamponade.) The pericardium protects the heart and allows its movement during beating. It also encloses the roots of the great vessels, with the fibrous layer becoming continuous with the outer fibrous layers of the aorta, the superior vena cava, and the pulmonary arteries.

Sternocostal projections: According to Woodburne and Burkel, we can define the sternocostal projections based on three points: the apex (left inferior), the base (most superior, where the vessels emerge), and the right end of the diaphragmatic surface. The apex sits about 8cm from midline in the fifth intercostal space. The base sits at about the third intercostal space. Finally, the third point lies under the chondrosternal junction at the sixth or seventh rib.

Borders: The structures outlined by the borders of the heart are very important, especially in terms of radiology. (Learn this. Learn it well.) There are four borders:

• Right border: right atrium between superior and inferior vena cavas
• Inferior border: mostly the right ventricle, a small part of the left ventricle
• Left border: mostly the left ventricle
• Superior border: right and left atria

Surfaces: The heart can be thought of as having three surfaces, which are also very important:

• Anterior (sternocostal) surface: mostly right ventricle
• Diaphragmatic (inferior) surface: mainly left ventricle, a little bit of the right ventricle
• Pulmonary (left) surface: mostly left ventricle

Sulci: There are three sulci, which appear as physical indentations along the pericardial surface. They overlie the septa separating the chambers of the heart, and are usually the path taken by major cardiac veins and coronary arteries.

• Coronary* (atrioventricular) sulcus: separates atria from ventricles
• Anterior interventricular sulcus: separates right and left ventricles, continuous with posterior interventricular sulcus
• Posterior interventricular sulcus: separates right and left ventricles, continuous with anterior interventricular sulcus

* "Coronary", for future reference, refers to something that is "crown-like", i.e., goes around a loop. Although often associated with the heart, you will also see coronary vessels in the abdomen. "Cardiac" refers to the heart specifically.

The Thorax - The Muscles of the Thorax - The Pericardium - The Thoracic Aorta - The Thoracic Nerves - Surface Anatomy of the Thorax - Surface Markings of the Thorax

Questions and Answers:

Nerves: An intercostal space is typically supplied by an intercostal nerve. Intercostal nerves are continuations of the ventral primary rami of spinal nerves that travel around the trunk in a groove under each rib. Just lateral to the costovertebral joint, the nerves (and arteries and veins) give off collateral branches that then travel on top of each rib. Other important branches of the intercostal nerves include the lateral cutaneous branches, coming off at about the midaxillary line, and the anterior cutaneous branches, emerging just lateral to the sternum.

Blood supply:

• The thoracic aorta gives off 9 pairs of posterior intercostal and 1 pair of subcostal arteries. (The first two intercostal spaces are supplied by branches of the costocervical trunk to be studied later.)
• The subclavian artery gives off the internal thoracic artery from which arise the anterior intercostal arteries
• The axillary arteries give rise to the highest and most lateral thoracic arteries.

Each intercostal space is supplied by three arteries, a large posterior intercostal artery and a small pair of anterior intercostal arteries.

This is an observation question. Remnants of the thymus, if present, should be in the anterior mediastinum. Although thymic nodules are rare, the remaining fat is in lobes that are shaped like the thymus that used to be there. (Greek, thymus = warty excrescence)

Boundary	Structure
Superior	superior thoracic aperture
Inferior	plane defined by sternal angle and T4/T5 IV disc
Lateral	mediastinal pleura
Anterior	manubrium of the sternum
Posterior	spinal column

In pericardiocentesis, a needle is inserted upward and backward at the upper end of the left xiphochondral junction. Thus, the needle should miss the left pleura and the lung and reach the cavity of the pericardium at its most inferior portion.