n pl :  a series of 12 muscles on each side that arise from the transverse processes of the seventh cervical and upper 11 thoracic vertebrae, that pass obliquely downward and laterally to insert into the outer surface of the rib immediately below or in the case of the lowest four muscles of the series divide into two fasciculi of which one inserts as described and the other inserts into the second rib below, and that raise the ribs increasing the volume of the thoracic cavity and extend, bend, and rotate the spinal column .
 
   
 
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