The external and internal intercostal muscles play distinct roles in breathing, differentiated by their actions and the phase of respiration they facilitate:

### 1. External Intercostal Muscles
- Function: Contract during inhalation (both quiet and forced).
- Action: Their fibers run *downward and forward*. When they contract, they lift the ribs up and outward, expanding the thoracic cavity.
- Result: Increases volume in the chest, lowering air pressure and drawing air into the lungs.
- Mnemonic: "Ex"ternal = "Ex"pand (for inhalation).

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### 2. Internal Intercostal Muscles
- Function: Primarily active during forced exhalation (e.g., coughing, singing, exercising).
- Action: Their fibers run *upward and backward*. When they contract, they pull the ribs down and inward, compressing the thoracic cavity.
- Result: Decreases chest volume, increasing air pressure to push air out forcefully.
- Mnemonic: "In"ternal = "In"ward/downward (for forced exhalation).

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### Key Clarifications
- Quiet Breathing:
- Inhalation: Active (diaphragm + external intercostals contract).
- Exhalation: Passive (muscles relax; lungs recoil).
- Forced Breathing:
- Exhalation: Internal intercostals and abdominal muscles actively contract.

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### Summary
- External intercostals = INhalation (ribs up/out).
- Internal intercostals = FORCED EXhalation (ribs down/in).
- Quiet exhalation requires no muscle contraction (passive process).