Final answer:
The two main posterior thoracic muscles involved in respiration are the diaphragm and the external intercostal muscles, which help change the thoracic cavity size and internal pressure to facilitate breathing.
Step-by-step explanation:
The two posterior thoracic muscles primarily involved in the process of respiration are the diaphragm and the external intercostal muscles. The diaphragm is a dome-shaped muscle that plays a crucial role in breathing. When the diaphragm contracts,
it moves inferiorly towards the abdominal cavity, which increases the volume inside the pleural cavities, decreasing the pressure and causing air to flow into the lungs.
The external intercostal muscles span the space between the ribs, aiding in changing the shape of the rib cage during inspiration and expiration by moving the ribs upward and outward during inhalation to expand the thoracic cavity.
These actions facilitate the pulmonary ventilation process, comprising the major steps of inspiration and expiration.