External Intercostals
Thorax · Trunk
The external intercostals are the outermost of the three layers of muscle that fill the intercostal spaces between adjacent ribs. Each muscle arises from the inferior border of a rib and runs obliquely downward and forward to insert on the superior border of the rib below, a direction often described as the same as sliding your hands into your front pockets. This fibre orientation allows them to elevate the ribs during inspiration, expanding the thoracic cage, and they also support and stabilise the intercostal spaces so the chest wall does not get drawn in during breathing. They are supplied by the intercostal nerves (T1 to T11). Anteriorly they are replaced by the external intercostal membrane over the costal cartilages.
Origin, Insertion, Action & Nerve
Origin
Inferior border of each rib
Insertion
Superior border of the rib below
Action
Elevate ribs during inspiration; support and stabilize intercostal spaces
Nerve
Intercostal nerves (T1–T11)
Attachments explained
The External Intercostals is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Inferior border of each rib
Insertion
Superior border of the rib below
Action & function
When the External Intercostals contracts, it produces the following movements: Elevate ribs during inspiration; support and stabilize intercostal spaces.
As part of the Thorax group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The External Intercostals receives its nerve supply from the Intercostal nerves (T1–T11). This nerve carries fibres from spinal segment(s) T1, T11.
Because a muscle can only contract when its nerve is intact, injury to the Intercostal nerves or to its spinal roots (T1, T11) can weaken or paralyse the External Intercostals, impairing the movements it normally produces (elevate ribs during inspiration). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the External Intercostals (the OIANS method)
Learn the external intercostals by their fibre direction, downward and forward like hands slipping into front trouser pockets, and let that geometry tell you the action: pulling the lower rib up toward the one above elevates the whole cage for inspiration. Grouping them as the primary muscles that increase chest volume when you breathe in makes the function memorable.
Keep the three intercostal layers straight by contrasting fibre directions and roles: the external layer runs down and forward and aids inspiration, whereas the internal layer beneath it runs the opposite way. Remembering the shared segmental supply by intercostal nerves (T1 to T11) links all the intercostal muscles to the classic neurovascular bundle that runs in the costal groove.
Frequently asked questions
Where is the External Intercostals located?
The External Intercostals is a muscle of the Thorax group, located in the Trunk.
What is the origin of the External Intercostals?
Inferior border of each rib
What is the insertion of the External Intercostals?
Superior border of the rib below
What movements does the External Intercostals produce?
Elevate ribs during inspiration; support and stabilize intercostal spaces
What nerve supplies the External Intercostals?
Intercostal nerves (T1–T11)
Is the External Intercostals free to study in OIANS?
The External Intercostals is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Trunk is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
Study the External Intercostals
Test yourself on Trunk muscles with interactive quizzes and flashcards in the free OIANS app.
Quiz yourself → Flashcards Open the OIANS app
Browsing is always free. A one-time Lifetime unlock adds every region's quizzes, flashcards, progress tracking and explore tools.