There are three types of intercostal muscle:

• (Fig 1a) External

The external intercostal muscles slope downward and forward.

They contract on inspiration to pull the ribs upward and outward, increasing the anteroposterior and lateral diameter of the thorax.

• (Fig 1b) Internal
• (Fig 1c) Innermost

There are three types of intercostal muscle:

• (Fig 1a) External
• (Fig 1b) Internal

The internal intercostal muscles slope downward and backward.

They are only used in active expiration and pull the ribs downward and inward to decrease the volume of the thoracic cavity. They stiffen the intercostal spaces to prevent them from bulging during straining.

• (Fig 1c) Innermost

There are three types of intercostal muscle:

• (Fig 1a) External
• (Fig 1b) Internal
• (Fig 1c) Innermost

The innermost intercostals consist of a thin layer of muscle linking the ribs together to stabilize the chest wall.