_Nerve-supply._—Intercostal nerves. 3. =Internal intercostals= (Mm. intercostales interni).—These occupy the entire length of the intercostal spaces, including their interchondral portion. _Origin._—The anterior borders of the ribs and their cartilages. _Insertion._—The posterior borders of the preceding ribs and cartilages. _Action._—To draw the ribs backward in expiration. It seems probable, however, that the intercartilaginous portion is inspiratory.[47] _Structure._—The direction of the fibers is oblique downward and forward. There is a smaller amount of tendinous tissue than in the external set, and the thickness diminishes from below upward. In the upper part of the spaces fibers sometimes cross a rib in a fashion similar to the subcostals of man. A thin aponeurosis separates the internal from the external intercostal muscle in each space. _Relations._—Superficially, the levatores costarum and the external intercostals; deeply, the endothoracic fascia and pleura, the transversus thoracis, diaphragm, transversus abdominis, and the internal thoracic and asternal vessels. In the upper part of the intercostal spaces the intercostal vessels and nerves lie between the internal and external intercostal muscle, but below they lie chiefly on the deep face of the inner muscle. _Blood-supply._—Intercostal and internal thoracic arteries. _Nerve-supply._—Intercostal nerves. 4. =Retractor costæ.=—This is a small triangular muscle which lies behind the last rib, chiefly under cover of the serratus posticus. _Origin._—The transverse processes of the first three or four lumbar vertebræ by means of the lumbar fascia. _Insertion._—The posterior border of the last rib. _Action._—To retract the last rib. _Structure._—The muscle arises by a thin aponeurosis. Its fibers are parallel to those of the adjacent internal oblique. _Relations._—Superficially, the serratus posticus and external oblique; deeply, the transversus abdominis. _Blood-supply._—Lumbar arteries. _Nerve-supply._—Lumbar nerves. 5. =Rectus thoracis= (M. transversus costarum; lateralis sterni).—This is a thin muscle which lies under cover of the deep pectoral muscles. It is directed obliquely backward and downward, and crosses the lower part of the first three intercostal spaces. _Origin._—The outer surface of the first rib, below the scalenus. _Insertion._—The cartilage of the third or fourth rib. The aponeurosis usually joins the rectus abdominis. It may reach the sternum. _Action._—It may assist in inspiration or concur with the rectus abdominis. _Relations._—Superficially, the deep pectoral muscles; deeply, the intercostal muscles and the ribs. 6. =Transversus thoracis= (Triangularis sterni).—This is a flat muscle situated on the thoracic surface of the sternum and the cartilages of the sternal ribs. _Origin._—The sternal ligament. _Insertion._—The cartilages of the ribs, from the second to the eighth inclusive.