The Thoracic Cage

Thoracic Cage

Bony and cartilaginous structure that surrounds and protects the heart and lungs.

Components of the Thoracic Cage

Sternum, 12 pairs of ribs, and thoracic vertebrae.

Function of Thoracic Cage

Protects vital organs and assists in respiration.

Superior Thoracic Aperture

Opening between the neck and thorax allowing passage of the trachea, esophagus, blood vessels, and nerves.

Inferior Thoracic Aperture

Opening between the thorax and abdomen that is closed by the diaphragm.

Sternum

Flat bone in the anterior thorax providing protection and rib attachment.

Manubrium

Superior portion of the sternum.

Body of Sternum

Middle and largest portion of the sternum.

Xiphoid Process

Inferior portion of the sternum; cartilaginous early in life and ossifies with age.

Function of Sternum

Protects thoracic organs and provides attachment for ribs and muscles.

Number of Rib Pairs

12 pairs of ribs in the human body.

True Ribs

Ribs 1–7 that attach directly to the sternum via their own costal cartilage.

False Ribs

Ribs 8–10 that attach indirectly to the sternum through shared costal cartilage.

Floating Ribs

Ribs 11–12 that have no anterior attachment and connect only to the vertebrae posteriorly.

Iliac Crest

Superior border of the ilium serving as an attachment site for abdominal muscles.

Anterior Superior Iliac Spine (ASIS)

Prominent anterior projection of the ilium; attachment site for ligaments and muscles.

Pubic Tubercle

Bony prominence on the pubis; medial attachment of the inguinal ligament.

Inguinal Ligament

Ligament formed by the inferior border of the external oblique aponeurosis extending from the ASIS to the pubic tubercle.

Inspiration

Active process where thoracic volume increases, drawing air into the lungs.

Expiration

Usually passive process where thoracic volume decreases, pushing air out of the lungs.

Primary Muscle of Breathing

The diaphragm.

Diaphragm

Dome-shaped skeletal muscle separating the thoracic and abdominal cavities and serving as the primary muscle of quiet breathing.

Central Tendon

Aponeurotic insertion where diaphragm muscle fibers converge.

Origin of Diaphragm

Inferior thoracic aperture, including lower ribs, costal cartilages, and lumbar vertebrae.

Insertion of Diaphragm

Central tendon.

Mechanism of Inspiration

Contraction flattens the dome, increasing thoracic volume and creating negative pressure.

Mechanism of Expiration

Relaxation allows abdominal organs to push the diaphragm upward, decreasing thoracic volume.

Right Dome of Diaphragm

Higher due to the presence of the liver.

Left Dome of Diaphragm

Slightly lower because of the stomach.

Caval Hiatus

Opening at vertebral level T8 for passage of the inferior vena cava.

Esophageal Hiatus

Opening at vertebral level T10 for passage of the esophagus.

Aortic Hiatus

Opening at vertebral level T12 for passage of the aorta.

Mnemonic for Diaphragm Openings

“I ate 10 eggs at 12” (IVC–T8, Esophagus–T10, Aorta–T12).

Phrenic Nerve

Nerve providing motor and sensory innervation to the diaphragm.

Origin of Phrenic Nerve

Cervical spinal nerves C3, C4, and C5.

Function of Phrenic Nerve

Motor control of the diaphragm and sensory input from the diaphragm and pleura.

Mnemonic for Phrenic Nerve

“C3, 4, 5 keep the diaphragm alive.”

Referred Pain of Diaphragm

Pain perceived in the shoulder or neck due to shared spinal cord segments (C3–C5).

Effect of Unilateral Phrenic Nerve Injury

Paralysis of one hemidiaphragm causing paradoxical breathing.

Paradoxical Breathing

Upward movement of the paralyzed diaphragm during inspiration instead of downward.

Pump-Handle Motion

Movement where the sternum elevates and moves anteriorly, increasing the anteroposterior diameter of the thorax.

Bucket-Handle Motion

Lateral movement of ribs increasing the transverse diameter of the thorax.

Intercostal Muscles

Muscles located between the ribs that assist with respiration.

Layers of Intercostal Muscles

External, internal, and innermost intercostals.

External Intercostals

Muscles with fibers running down and inward; elevate ribs during inspiration.

Mnemonic for External Intercostals

“Hands in pockets.”

Internal Intercostals

Muscles with fibers running up and inward; depress ribs during forced expiration.

Innermost Intercostals

Deepest layer assisting internal intercostals in forced expiration.

Subcostal Muscles

Muscles spanning multiple ribs that assist expiration.

Transversus Thoracis

Muscle extending from the sternum to ribs that pulls ribs downward during expiration.

Accessory Respiratory Muscles

Muscles that assist breathing during exertion or respiratory distress.

Accessory Muscles of Inspiration

External intercostals, scalenes, pectoralis minor, serratus posterior superior.

Accessory Muscles of Expiration

Internal intercostals, innermost intercostals, subcostals, transversus thoracis, serratus posterior inferior, and abdominal muscles.

Intercostal Neurovascular Bundle

Structure within each intercostal space containing a vein, artery, and nerve.

Arrangement of Bundle

Ordered from superior to inferior as Vein, Artery, Nerve (VAN).

Location of Bundle

Between the internal and innermost intercostal muscles.

Intercostal Nerves

Anterior rami of thoracic spinal nerves T1–T11 supplying thoracic wall structures.

Subcostal Nerve

The anterior ramus of T12.

Nipple Level

Corresponds to vertebral level T4.

Xiphoid Process Level

Corresponds to vertebral level T6.

Umbilicus Level

Corresponds to vertebral level T10.

Sternal Angle

Junction between the manubrium and body of the sternum.

Rib Associated with Sternal Angle

Rib 2.

Clinical Importance of Sternal Angle

Used to count ribs and marks the tracheal bifurcation and transition of the aortic arch.

Dermatome

Area of skin supplied by a single spinal nerve.

Thoracic Dermatome Pattern

Horizontal bands around the trunk.

T4 Dermatome

Level of the nipple.

T6 Dermatome

Level of the xiphoid process.

T10 Dermatome

Level of the umbilicus.

Shingles

Reactivation of the varicella-zoster virus causing a unilateral rash along a dermatome.

Skin

Most superficial layer of the abdominal wall.

Subcutaneous Tissue

Layer containing fat and superficial fascia for insulation and protection.

Scarpa’s Fascia

Deep membranous layer of superficial fascia continuous with the fascia lata of the thigh.

Deep Fascia

Connective tissue layer surrounding muscles.

Peritoneum

Serous membrane lining the abdominal cavity and covering abdominal organs.

Rectus Abdominis

Vertical “six-pack” muscle responsible for trunk flexion.

Origin of Rectus Abdominis

Pubic symphysis.

Insertion of Rectus Abdominis

Costal cartilages of ribs and xiphoid process.

External Oblique

Most superficial lateral abdominal muscle responsible for contralateral rotation.

Fiber Direction of External Oblique

Downward and inward (“hands in pockets”).

External Oblique Attachments

Ribs to linea alba, ASIS, and pubic tubercle.

Inguinal Ligament Formation

Inferior border of the external oblique aponeurosis from ASIS to pubic tubercle.

Internal Oblique

Middle abdominal muscle responsible for ipsilateral rotation.

Fiber Direction of Internal Oblique

Fan-shaped (upward, horizontal, and downward).

Origin of Internal Oblique

Thoracolumbar fascia and ASIS.

Insertion of Internal Oblique

Linea alba.

Rectus Sheath Contribution of Internal Oblique

Splits into anterior and posterior layers above the arcuate line.

Transversus Abdominis

Deepest abdominal muscle that compresses abdominal contents.

Fiber Direction of Transversus Abdominis

Horizontal.

Function of Transversus Abdominis

Compression and stabilization of the abdomen.

Aponeurosis

Flat sheet-like tendon connecting muscles to other structures.

Rectus Sheath

Fibrous compartment formed by the aponeuroses of the abdominal muscles surrounding the rectus abdominis.

Linea Alba

Midline fibrous structure where abdominal muscle aponeuroses merge.

Arcuate Line

Transition point in the abdominal wall where the posterior rectus sheath ends.

Structures Above Arcuate Line

External oblique anterior; internal oblique splits; transversus abdominis posterior.

Structures Below Arcuate Line

All aponeuroses pass anterior to the rectus abdominis; no posterior sheath.

Clinical Importance of Arcuate Line

Represents a weak point associated with hernias and is important in surgical procedures such as C-sections.

Costal Margin

Inferior border of the rib cage.

Semilunar Lines

Curved lateral borders of the rectus abdominis.

Transverse Intersections

Fibrous bands dividing the rectus abdominis to create the “six-pack.”