The external intercostal muscles are the most superficial of all the intercostals. Within each intercostal space, they arise from the inferior border of the rib that lies superiorly. The muscle fibres pass obliquely down and inferiorly to insert into the superior border of the immediately inferior rib.
Each intercostal is muscular from the costal tubercle of each rib posteriorly but becomes membranous at the each respective costochondral junction anteriorly.
The external intercostals are supplied by collateral branches of the first to eleventh intercostal nerves.
The external intercostals act to:
- elevate the ribs during inspiration
- fix the ribs during respiration