Formation and Shape of Thorax

Topic Overview

Skeleton of Thorax

• The skeleton of the thorax, or thoracic cage, is an osseocartilaginous elastic cage designed to alter intrathoracic pressure during respiration.

• Function:

  • During inspiration, it expands to allow air entry into the lungs.

  • During expiration, it contracts to expel air.

• It provides protection to the heart, lungs, and great vessels and supports the pectoral girdle and upper limbs.

Formation of the Thoracic Cage

• Anteriorly: Sternum (composed of manubrium, body, and xiphoid process).

• Posteriorly: 12 thoracic vertebrae with their intervertebral discs.

• Laterally: 12 pairs of ribs with costal cartilages.

  • True (vertebrosternal) ribs: 1st–7th ribs—each directly articulates with the sternum.

  • False (vertebrochondral) ribs: 8th–10th ribs—join the cartilage of the rib above, forming the costal margin.

  • Floating (vertebral) ribs: 11th and 12th—anterior ends free, not attached to sternum.

• Joints involved:

  • Costovertebral, costotransverse, manubriosternal, and chondrosternal joints enable respiratory movements.

Shape of the Thorax

• The thorax resembles a truncated cone — narrow above, broad below.

  • Upper end: Continuous with the neck, partly separated by the suprapleural membrane (Sibson’s fascia).

  • Lower end: Separated from the abdomen by the diaphragm, concave downward.

• The thoracic cavity is smaller than it appears because:

  • The upper part appears wider due to the shoulders.

  • The lower part is encroached upon by the abdomen (due to the diaphragm’s dome).

• Transverse Section:

  • Adults: Oval/reniform (kidney-shaped); transverse diameter > anteroposterior.

  • Infants: Circular—ribs horizontal, leading to abdominal respiration.

  • Quadrupeds: Anteroposterior diameter > transverse.

Clinical Anatomy

• Respiration:

  • The diaphragm descends during inspiration, increasing the vertical diameter.

  • In adults, both thoracic and abdominal respiration occur; in infants, mainly abdominal.

• Rib Fractures:

  • The ribs are weakest at their angles, where fractures commonly occur due to indirect trauma.

  • The upper two ribs (protected by the clavicle) and lower two ribs (floating) are least likely to fracture.

  • In children, rib fractures are rare due to high elasticity of the chest wall.

• Hiccups:

  • Caused by spasmodic contraction of the diaphragm with sudden closure of the glottis.

  • May result from gastric irritation, phrenic nerve irritation, uraemia, or hysteria.

Summary Table