📝 STANDARD DESCRIPTION:
Type: Expiratory muscles of thoracic wall
Origin: Cranial border of each rib
Insertion: Caudal border of preceding (cranial) rib
Fibre Direction: Cranio-VENTRAL (opposite to external — at right angles)
Function: Pull ribs backward and inward → REDUCE thorax → EXPIRATION.
Extent: From sternum (intersternal portions) up to angle of rib.
📚 DETAILED DESCRIPTION:
THE BREATHING-OUT MUSCLES OF THE WALL
The internal intercostal is the deep layer, lying just beneath the external intercostal. Its fibres cross those of the external at a right angle.
FIBRE DIRECTION: Fibres run cranio-ventrally — opposite to the external intercostals. Together the two layers form a strong cross-hatched wall that resists the suction of breathing.
HOW IT BREATHES OUT: When it contracts, it pulls the upper rib down toward the lower rib. The chest narrows and the lung air is pushed out (expiration). Quiet breathing-out is mostly passive, but during exercise/heavy breathing, internal intercostals are strongly active.
EXTENT: Reaches further forward than the external intercostal — its intersternal part lies between the cartilages of the true ribs, attaching to the sternum.
NERVE SUPPLY: Intercostal nerves.
// COMPARATIVE ANALYSIS
| HORSE |
Strong development for forced breathing during running. |
| DOG |
Important during panting (active expiration through small bursts). |
CLINICAL RELEVANCE
Flail Chest: If 2 or more adjacent ribs fracture in 2 places, that segment moves opposite to the rest of the chest during breathing — paradoxical breathing — and intercostal action is lost.
