When the diaphragm and external intercostal muscles contract, e) the volume of the thorax increases. This contraction leads to an increase in the volume of the thoracic cavity, allowing the lungs to expand as air is drawn into them.
To elaborate, the diaphragm is a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. When it contracts, it flattens downwards, which increases the vertical dimension of the thorax. Simultaneously, the external intercostal muscles, which are located between the ribs, also contract to lift the ribcage upwards and outwards. This combined action results in a larger thoracic volume, creating a decrease in internal pressure relative to the atmospheric pressure, causing air to flow into the lungs.
So, in summary, the correct answer highlights that the contraction of these muscles results in an increase in the volume of the thorax, which is crucial for the process of inhalation.