Bony Thorax Anatomy

Bony Thorax

• The bony thorax protects the organs of the thorax and aids in respiration.
• It consists of the thoracic vertebrae, sternum, ribs, and costal cartilages.
• The 12 thoracic vertebrae make up the posterior boundary of the thoracic cage.
• The anterior boundary is created by the sternum, located midline.
• The sternum has three components: manubrium, body, and xiphoid process.

Sternal Components

• The triangular-shaped manubrium is the most superior portion and articulates with the first two pairs of ribs and the clavicles.
• A common landmark, the jugular notch, is located on the superior border of the manubrium at approximately the level of T2–T3.
• The manubrium and body of the sternum come together at an angle to form a ridge known as the sternal angle, which is located at approximately the level of T4–T5.
• The slender body of the sternum has several indentations along its sides where it articulates with the cartilage of the third through seventh ribs.
• The small xiphoid process is located on the inferior border of the sternum and is a site for muscle attachment.

Ribs

• Forming the lateral borders of the thoracic cage are the 12 pairs of ribs.
• The spaces between adjacent ribs are referred to as the intercostal spaces.
• All 12 pairs of ribs articulate posteriorly with the thoracic spine.
• The ribs consist of a head, neck, tubercle, and body.
• The facets of the head of the rib articulate with the vertebral bodies at the costovertebral joints.
• The facets of the tubercles articulate with the transverse processes of the vertebrae to form the costotransverse joints.
• The first seven pairs of ribs (true ribs) articulate anteriorly with the sternum via costal cartilage.
• The lower five pairs of ribs are considered false ribs because they do not attach directly to the sternum.
• The costal cartilage of the 8th, 9th, and 10th ribs attach to the costal cartilage of the 7th rib.
• The 11th and 12th ribs are considered floating because they attach only to the thoracic vertebrae and contain no neck or tubercle, only vertebral and sternal ends.

Thoracic Apertures

• There are two openings, or apertures, associated with the bony thorax.
• The superior thoracic aperture (thoracic inlet) is formed by the first thoracic vertebra, the first pair of ribs and their costal cartilages, and the manubrium.
• The inferior thoracic aperture (thoracic outlet) is much larger and is made up of the 12th thoracic vertebra, 12th pair of ribs and costal margins, and xiphoid sternal junction.

Pleural Cavities

• Each lung lies within a single pleural cavity that is lined by a serous membrane, or pleura.
• The pleura can be divided into two layers: the parietal pleura and the visceral pleura.
• The parietal pleura is the outer layer that is continuous with the thoracic wall and diaphragm and moves with these structures during respiration.
• The visceral pleura is the inner layer that closely covers the outer surface of the lung and continues into the fissures to cover the individual lobes as well.
• Both membranes secrete a small amount of pleural fluid that provides lubrication between the surfaces during breathing.

Pleural Cavity Recesses

• Deep pockets or recesses of the pleural cavities are the costomediastinal and costodiaphragmatic recesses.
• The costomediastinal recesses are located at the point where the mediastinum and costal cartilages meet anteriorly.
• The costodiaphragmatic recesses are located where the diaphragm and ribs connect inferiorly.
• These recesses serve as expansions to provide additional pleural space where parts of the lung can glide during inspiration.

Lungs

• The lungs are the organs of respiration.
• They are composed of a spongelike material, the parenchyma, and are surrounded by the visceral pleura.
• The lungs extend up to or slightly above the level of the first rib at their apex and down to the dome of the diaphragm at their wide bases or diaphragmatic surfaces.
• Each lung has a mediastinal or medial surface that is apposed to the mediastinum and a costal surface that is apposed to the inner surface of the rib cage.
• Each lung also has inferior, anterior, and posterior borders.

Lung Features

• The inferior border extends into the costodiaphragmatic recess of the pleural cavity.
• The anterior border of each lung extends into the costomediastinal recess of the pleural cavity.
• Two prominent angles can be identified at the medial and lateral edges of the lung bases: the cardiophrenic sulcus and the costophrenic sulcus.
• The lungs are divided into lobes by fissures that are lined by pleura.
• The right lung has three lobes (superior, middle, and inferior), whereas the left lung has just superior and inferior lobes.
• The inferior lobe of the right lung is separated from the middle and superior lobes by the oblique (major) fissure.
• Separating the middle lobe from the superior lobe is the horizontal (minor) fissure.
• An oblique fissure also separates the superior and inferior lobes of the left lung.
• The left lung has a large notch on the medial surface of its superior lobe called the cardiac notch and a tonguelike projection off its inferoanterior surface termed the lingula.
• Each lung has an opening on the medial surface termed the hilum, which acts as a passage for the mainstem bronchi, blood vessels, lymph vessels, and nerves to enter or leave the lung and is commonly referred to as the root of the lung.

Bronchi

• The trachea bifurcates into the left and right mainstem (primary) bronchi at approximately the level of T5.
• At the hilum, the mainstem bronchi enter the lungs and divide into secondary or lobar bronchi.
• Secondary bronchi correspond to the lobes of the lungs, with three divisions on the right (superior, middle, inferior) and two divisions on the left (superior and inferior).
• There is further division of the secondary bronchi into tertiary or segmental bronchi, which extend into each segment of the lobes (bronchopulmonary segments).

Mediastinum

• The mediastinum is the midline region of the thoracic cavity located between the two pleural cavities of the lungs.
• It extends from the superior thoracic aperture to the diaphragm and is bordered anteriorly by the sternum and posteriorly by thoracic vertebrae.
• The mediastinum can be subdivided into compartments for descriptive purposes.
• The superior and inferior compartments are made by drawing an imaginary line between the sternal angle and the intervertebral disk of T4–T5.
• The superior compartment constitutes the upper portion of the mediastinum.
• The inferior compartment can be further divided into anterior, middle, and posterior compartments.
• Structures located within the mediastinum include the thymus gland, trachea, esophagus, lymph nodes, thoracic duct, heart and great vessels, and various nerves.

Thymus Gland

• The thymus gland is a triangular-shaped bilobed gland of lymph tissue located in the superior portion of the mediastinum just behind the manubrium.
• It is considered the primary lymphatic organ responsible for the development of cellular immunity.