Shape and Outline of the Thoracic Cage Including Inlet and Outlet
The thoracic cage, also known as the rib cage, is a bony and cartilaginous structure that forms the skeleton of the thorax. It has a conical shape, being narrower superiorly at the thoracic inlet and wider inferiorly at the thoracic outlet. The cage is somewhat flattened anteroposteriorly.
- Thoracic Inlet (Superior Thoracic Aperture):
- The thoracic inlet is kidney-shaped and smaller compared to the outlet.
- Boundaries:
- Posterior: Body of T1 vertebra.
- Lateral: First pair of ribs and their costal cartilages.
- Anterior: Superior border of the manubrium.
- Structures passing through include the trachea, esophagus, major blood vessels (e.g., subclavian arteries/veins), and nerves (e.g., vagus nerve).
- Thoracic Outlet (Inferior Thoracic Aperture):
- The thoracic outlet is larger than the inlet and irregular in shape.
- Boundaries:
- Posterior: Body of T12 vertebra.
- Lateral: 11th and 12th ribs (floating ribs) and costal margins formed by ribs 7–10.
- Anterior: Xiphisternal joint.
- It is closed off by the diaphragm, which separates it from the abdominal cavity. Structures such as the aorta, inferior vena cava, and esophagus pass through openings in the diaphragm.
The thoracic cage provides protection for vital organs like the heart, lungs, thymus, and great vessels while allowing flexibility for respiration.
Anatomical Landmarks of the Anterior Chest Wall
The anterior chest wall contains several important anatomical landmarks:
- Jugular Notch:
- Located at the superior border of the manubrium between clavicular notches.
- Sternal Angle (Angle of Louis):
- Formed by articulation between manubrium and body of sternum at T4-T5 level.
- Marks bifurcation of trachea into bronchi and beginning/end of aortic arch.
- Xiphisternal Joint:
- Junction between xiphoid process and body of sternum at T9 level.
- Midclavicular Line:
- Vertical line passing through midpoint of each clavicle.
- Costal Margins:
- Formed by cartilages of ribs 7–10 on each side.
- Intercostal Spaces:
- Spaces between adjacent ribs numbered according to rib above them (e.g., first intercostal space lies below first rib).
These landmarks are crucial for clinical procedures like auscultation or locating structures during surgery.
Structures Making Up the Thoracic Wall
The thoracic wall consists of several components that provide structural support, protection, and aid in respiration:
- Bones:
- Sternum (manubrium, body, xiphoid process).
- 12 pairs of ribs with costal cartilages.
- 12 thoracic vertebrae.
- Muscles:
- Intercostal muscles (external, internal, innermost).
- Subcostals.
- Transversus thoracis.
- Diaphragm.
- Fascia:
- Endothoracic fascia lining inner surface.
- Neurovascular Structures:
- Intercostal nerves, arteries, veins running along costal grooves.
- Skin & Subcutaneous Tissue.
Muscles of the Thoracic Wall
Below is a list detailing muscles associated with the thoracic wall:
1. External Intercostals
- Attachments: Lower border of one rib to upper border of rib below.
- Action: Elevates ribs during inspiration; increases thoracic volume.
- Nerve Supply: Intercostal nerves (T1-T11).
- Blood Supply: Intercostal arteries.
2. Internal Intercostals
- Attachments: Costal groove to superior surface of rib below.
- Action: Depresses ribs during forced expiration; decreases thoracic volume.
- Nerve Supply: Intercostal nerves (T1-T11).
- Blood Supply: Intercostal arteries.
3. Innermost Intercostals
- Attachments: Medial edge of costal groove to superior surface of rib below.
- Action: Similar to internal intercostals; stabilizes ribs during respiration.
- Nerve Supply: Intercostal nerves (T1-T11).
- Blood Supply: Intercostal arteries.
4. Subcostals
- Attachments: Inner surface near angle of one rib to inner surface two or three ribs below it.
- Action: Depresses ribs during expiration; stabilizes ribcage.
- Nerve Supply: Intercostal nerves.
5. Transversus Thoracis
- Attachments: Posterior sternum to internal surfaces of costal cartilages 2–6.
- Action: Weakly depresses ribs during expiration.
- Nerve Supply: Intercostal nerves (T2-T6).
- Blood Supply: Internal thoracic artery branches.
Accessory Muscles
Includes pectoralis major/minor, serratus anterior/posterior muscles aiding in forced respiration.
Parts & Characteristic Features of Thoracic Vertebrae
Thoracic vertebrae are unique due to their articulation with ribs:
Parts
- Vertebral Body – Heart-shaped; bears facets for rib articulation on sides (demifacets).
- Vertebral Arch – Includes pedicles and laminae forming vertebral foramen for spinal cord passage.
- Spinous Process – Long and angled downward; overlaps inferior vertebrae slightly (“shingled” appearance).
- Transverse Processes – Bear costotransverse facets for tubercle articulation with corresponding rib (except T11/T12).
- Superior/Inferior Articular Processes – Allow articulation with adjacent vertebrae via synovial joints.
Characteristic Features
- Presence of costovertebral joints where head/tubercle articulate with demifacets/transverse processes respectively.
- Smaller circular vertebral foramen compared to cervical/lumbar regions.
Sternum and Its Joints
The sternum, or breastbone, is a flat, elongated bone located in the anterior midline of the thoracic wall. It serves as an important site for rib attachment and provides protection to vital organs such as the heart and lungs. The sternum consists of three main parts:
- Manubrium: The superior portion of the sternum, which articulates with:
- The clavicles at the sternoclavicular joints.
- The first pair of ribs via costal cartilage.
- The second pair of ribs at the manubriosternal joint.
- Body (Corpus Sterni): The largest part of the sternum, which articulates with:
- Costal cartilages of ribs 2 through 7.
- Manubrium superiorly at the manubriosternal joint (forming the sternal angle or Angle of Louis).
- Xiphoid Process: The smallest, inferior portion that remains cartilaginous until adulthood but ossifies later in life. It provides attachment for abdominal muscles.
Joints of the Sternum
- Manubriosternal Joint: A secondary cartilaginous joint (symphysis) between the manubrium and body of the sternum. It allows slight movement during respiration.
- Xiphisternal Joint: A primary cartilaginous joint (synchondrosis) between the xiphoid process and body of the sternum. This joint typically ossifies with age.
- Sternocostal Joints:
- Ribs 1–7 articulate with the sternum via costal cartilages.
- Rib 1 forms a synchondrosis (primary cartilaginous joint).
- Ribs 2–7 form synovial plane joints.
Classification and Parts of Ribs
Classification of Ribs
The ribs are classified based on their attachment to the sternum and their anatomical features. They are divided into three main categories:
- True Ribs (Vertebrosternal Ribs):
- These include rib pairs 1 through 7.
- They attach directly to the sternum via their own costal cartilages.
- These ribs provide structural support and protection for thoracic organs.
- False Ribs (Vertebrochondral Ribs):
- These include rib pairs 8 through 10.
- They do not attach directly to the sternum but instead connect to the costal cartilage of the rib above them.
- Their indirect connection provides flexibility and aids in respiration.
- Floating Ribs (Vertebral or Free Ribs):
- These include rib pairs 11 and 12.
- They do not attach to the sternum at all, either directly or indirectly.
- Instead, they end within the abdominal musculature, providing some degree of protection for posterior organs like the kidneys.
Additionally, ribs can be categorized as typical or atypical based on their anatomical structure.
Parts of a Rib
Each rib has distinct anatomical components that contribute to its function:
- Head:
- The head is wedge-shaped and contains articular facets.
- Typical ribs have two facets: one articulates with the vertebra of the same number, while the other articulates with the vertebra above it.
- Neck:
- The neck connects the head to the body (shaft) of the rib.
- It is generally unremarkable in terms of bony landmarks.
- Tubercle:
- Found at the junction between the neck and body.
- It has two parts:
- A smooth articular part that articulates with the transverse process of its corresponding vertebra.
- A rough non-articular part where ligaments attach.
- Body (Shaft):
- The shaft is flat, thin, and curved.
- Its internal surface contains a groove called the costal groove, which protects intercostal nerves and vessels.
- Costal Cartilage:
- This cartilage extends from each rib anteriorly, allowing for articulation with either other costal cartilages or directly with the sternum.
- Costal Angle:
- The point where a rib curves most prominently anterolaterally.
- Serves as an attachment site for deep back muscles.
Comparison Between Typical and Atypical Ribs
Typical Ribs
- Include ribs 3 through 9.
- Have all standard features: head with two facets, neck, tubercle, shaft with costal groove, and costal angle.
- Provide structural support for respiration by moving during inhalation/exhalation.
Atypical Ribs
- Include ribs 1, 2, 10, 11, and 12 due to unique features that deviate from typical anatomy:
- Rib 1:
- Shortest and widest rib; highly curved.
- Only one facet on its head (articulates solely with T1 vertebra).
- Superior surface has grooves for subclavian vessels separated by a scalene tubercle for muscle attachment.
- Rib 2:
- Thinner and longer than Rib 1 but still atypical due to its roughened area on its superior surface where serratus anterior muscle originates.
- Rib 10:
- Has only one facet on its head (articulates solely with T10 vertebra).
- Ribs 11 & 12:
- Shorter than other ribs; lack necks or tubercles entirely.
- Have only one facet on their heads (articulate solely with corresponding vertebrae).
- Do not articulate anteriorly with any structure (floating ribs).
Intercostal Spaces
Definition
Intercostal spaces are anatomical spaces located between adjacent ribs. There are eleven intercostal spaces on each side, numbered according to the rib above them (e.g., space below rib 1 is called “first intercostal space”).
Components
Each intercostal space contains:
- Intercostal Muscles:
- External Intercostals: Elevate ribs during inspiration; fibers run inferoanteriorly (“hands in pockets”).
- Internal Intercostals: Depress ribs during forced expiration; fibers run inferoposteriorly opposite to external intercostals.
- Innermost Intercostals: Assist internal intercostals; separated from them by neurovascular bundle.
- Neurovascular Bundle: Located within each space along its inferior border inside the costal groove, arranged as VAN (Vein-Artery-Nerve from superior to inferior):
- Intercostal vein drains into azygos or hemiazygos system.
- Intercostal artery arises from thoracic aorta or internal thoracic artery.
- Intercostal nerve originates from anterior rami of spinal nerves T1–T11.
- Other Structures: Includes lymphatic vessels and connective tissue supporting muscle layers.
Diaphragm
The diaphragm is a dome-shaped musculotendinous structure separating thoracic and abdominal cavities. It plays a critical role in respiration by altering thoracic cavity volume.
Origin
The diaphragm has three peripheral origins:
- Sternal Part: Xiphoid process.
- Costal Part: Lower six ribs and their costal cartilages.
- Lumbar Part:
- Right crus arises from L1–L3 vertebrae and discs.
- Left crus arises from L1–L2 vertebrae and discs.
Insertion
All muscle fibers converge onto a central tendon located near its apex.
Function
- Primary muscle for respiration:
- Contracts during inspiration, flattening downward to increase thoracic volume for lung expansion.
- Relaxes during expiration, returning to dome shape as air exits lungs.
Nerve Supply
Motor innervation is provided by phrenic nerves (C3–C5). Sensory innervation comes from phrenic nerves centrally and lower intercostal nerves peripherally.
Blood Supply
Arterial supply includes:
- Inferior phrenic arteries (from abdominal aorta).
- Superior phrenic arteries (from thoracic aorta).
- Pericardiacophrenic arteries and musculophrenic arteries (branches of internal thoracic artery).
Venous drainage mirrors arterial supply via accompanying veins.
Openings in Diaphragm
The diaphragm contains three major openings allowing passage between thoracic and abdominal cavities:
- Caval Opening (T8): Structures passing through include:
- Inferior vena cava
- Right phrenic nerve
- Esophageal Hiatus (T10): Structures passing through include:
- Esophagus
- Vagus nerves
- Aortic Hiatus (T12): Structures passing through include:
- Aorta
- Thoracic duct
- Azygos vein
