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