internal intercostal (muscle)

in·ter·nal in·ter·cos·tal (mus·cle)

flat muscle of thorax arising from lower border of rib and passes obliquely downward and backward to be inserted into upper border of rib below; action, contract during expiration, also maintain tension in the intercostal spaces to resist mediolateral movement; nerve supply, intercostal.
Farlex Partner Medical Dictionary © Farlex 2012
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