 They are at first situated between the pleurae and the intercostales interni, and then between the intercostales interni and innermost.They supply the intercostal muscles and, by branches which perforate the intercostales externi, the pectoral muscles and the mamma.The musculophrenic artery arises from the internal thoracic artery, directed obliquely downward and laterally, behind the cartilages of the false ribs; it perforates the diaphragm at the eighth or ninth costal cartilage, and ends, considerably reduced in size, opposite the last intercostal space.