📝 STANDARD DESCRIPTION:
Type: Inspiratory muscles of thoracic wall
Origin: Caudal border of each rib
Insertion: Cranial border of next (caudal) rib
Fibre Direction: Caudo-VENTRAL (downwards & backwards — like fingers in pocket)
Function: Pull ribs forward and outward → ENLARGE thorax → INSPIRATION.
Extent: Fill the intercostal spaces from vertebrae to costochondral junction.
📚 DETAILED DESCRIPTION:
THE BREATHING-IN MUSCLES OF THE WALL
Between every two ribs there are two layers of small muscles — the intercostals. The outer (more superficial) layer is the external intercostal.
FIBRE DIRECTION: Easy memory aid — fibres run as if you put your hand in your trouser pocket: from cranio-dorsal to caudo-ventral.
HOW IT BREATHES IN: When the muscle contracts, it pulls the lower rib upward and forward toward the rib above. Because of the way ribs hinge at the spine, this lifts and rotates the ribs out, increasing the side-to-side width of the chest. Air rushes in (inspiration).
EXTENT: Present from the spine down to the level where bone becomes cartilage (costochondral junction). It does not reach the sternum.
NERVE SUPPLY: Intercostal nerves (ventral branches of thoracic spinal nerves T1-T13).
// COMPARATIVE ANALYSIS
| HORSE |
Same arrangement; very important during exercise breathing. |
| DOG |
Less developed; diaphragm does most of the breathing work. |
CLINICAL RELEVANCE
Intercostal nerve block: Local anaesthetic placed at the caudal border of the rib (where vessels & nerve run) blocks pain from rib fractures or from chest wall surgery.
