The external intercostal muscles (mm. intercostales externi) fill the intercostal spaces from the spine to the costal cartilages. They originate from the inferior border of each rib, descend posteroanteriorly, and attach to the superior border of the contiguous rib above. Between the costal carti­lages the muscles are replaced by a fibrous band with fibres lying in the same direction (membrana intercostalis externa). Innervation: Thj.n. Intercostal nerves. The internal intercostal muscles (mm. intercostales interni) are under the external muscles, and the direction of their fibres is opposite to that of the fibres of the latter, which they intersect at an angle. Originating from the superior border of the contiguous rib below, they ascend anteriorly and attach to the contiguous rib above. In contrast to the external muscles, the internal intercostal muscles stretch between the costal cartilages and reach the ster­num. Posteriorly, the internal intercostal muscles reach only the costal an­gles and are further replaced by the posterior intercostal membrane (membrana intercostalis interna) stretching between the posterior ends of the ribs. Inner­vation: Th!_n. Intercostal nerves. The subcostal muscles (mm. subcostales) are thin muscular bundles found on the inner surface of the lower part of the thoracic cage in the region of the costal angles. Their fibres are directed like the fibres of the internal intercostal muscles, but the muscles overlap one or two ribs. Innervation: Th8_n. Intercostal nerves. 4. The transversus thoracis muscle (m. transversus thoracis) is also on the inner surface of the thoracic cage, on its anterior surface, and is a continua­tion of the transverse muscle of the abdomen. Innervation: Th3_6. Inter­costal nerves. Action. The external costal muscles raise the ribs and expand the thora­cic cage in the anteroposterior and transverse directions and are, consequent­ly, muscles of inspiration during normal quiet breathing. Other muscles also take part in forced respiration; these are muscles capable of raising the ribs (the scalene, sternocleidomastoid, the major and minor pectoral muscles, etc.) provided the mobile points of their attachment in other places are held firmly fixed, as occurs, for instance, instinctively in patients suffering from dyspnoea. Diminution in the capacity of the thorax in expiration is mainly due to the elasticity of the lungs and the thoracic cage itself. Some authors claim that the internal intercostal muscles also contribute to quiet expiration. The subcostal muscles, the transverse muscle of the thorax, and other muscles which draw the ribs downward (the abdominal muscles) also act in forced expiration.