Superior Aperture/Inlet of Thorax & Inferior Aperture/Outlet of Thorax

Topic Overview

Superior Aperture / Inlet of Thorax

Definition and Orientation

• The superior aperture is the narrow upper end of the thorax that communicates with the neck.

• It is kidney-shaped, with a transverse diameter of about 10–12.5 cm and an anteroposterior diameter of about 5 cm.

• The plane of the inlet slopes downward and forward, making an angle of roughly 45°.

  • The anterior border (manubrium sterni) lies 3.7 cm lower than the posterior border (body of T1 vertebra).

Boundaries

• Anteriorly: Upper border of the manubrium sterni.

• Posteriorly: Upper surface of the body of the first thoracic vertebra (T1).

• Laterally: First ribs and their cartilages.

Partition and Membrane

• Each half of the thoracic inlet is covered by a suprapleural membrane (Sibson’s fascia), a thick fascia that:

  • Provides rigidity to the inlet during respiration.

  • Prevents the root of the neck from expanding or contracting during breathing.

  • Is attached to the tip of the transverse process of C7 and the inner border of the first rib.

Structures Passing through the Inlet

Viscera:

• Trachea

• Oesophagus

• Apices of both lungs with pleura

• Remains of thymus

Large Vessels:

• Brachiocephalic artery (right)

• Left common carotid artery

• Left subclavian artery

• Right and left brachiocephalic veins

Smaller Vessels:

• Internal thoracic arteries

• Superior intercostal arteries

• First posterior intercostal veins

• Inferior thyroid veins

Nerves:

• Right and left phrenic and vagus nerves

• Sympathetic trunks

• First thoracic nerves (ascending over first rib to brachial plexus)

Muscles:

• Sternohyoid

• Sternothyroid

• Longus colli

Clinical Anatomy of Thoracic Inlet

• Cervical Rib:

  • An accessory rib arising from C7, seen in about 0.5 % of individuals.

  • May compress the lower trunk of the brachial plexus (C8–T1), producing pain, paresthesia, and wasting of hand muscles.

  • Can also compress the subclavian artery, causing vascular symptoms.

• Thoracic Inlet Syndrome (TOS):

  • Compression of the subclavian artery and first thoracic nerve as they pass over the first rib.

  • May result from a cervical rib or abnormal scalenus anterior muscle insertion.

  • Symptoms can be neural, vascular, or both.

• Coarctation of Aorta:

  • Collateral circulation through posterior intercostal arteries leads to notching of ribs on X-ray.

Inferior Aperture / Outlet of Thorax

Definition

• The broad lower opening of the thorax, surrounding the upper part of the abdominal cavity, separated from it by the diaphragm.

Boundaries

• Anteriorly: Infrasternal angle between the two costal margins.

• Posteriorly: Inferior surface of the body of the T12 vertebra.

• Laterally: Costal margins formed by the 7th – 12th costal cartilages.

Diaphragm at the Outlet

• The outlet is closed by the thoracoabdominal diaphragm, a musculotendinous partition that separates thoracic and abdominal cavities.

• It contains three major openings:

  1. Vena caval opening (T8) — for IVC and right phrenic nerve.

  2. Oesophageal opening (T10) — for oesophagus and vagal trunks.

  3. Aortic opening (T12) — for aorta, thoracic duct, and azygos vein.

Summary