=Mm. intercostales externi= (Fig. 73, _m_; Fig. 69, _i_).--The external intercostals are placed in the outer portion of the intercostal spaces. They are composed of bundles of fibres attached by their ends to the adjacent borders of the ribs and having in general the direction of the external oblique muscle, i.e., they pass from their cranial ends caudoventrad. They occupy the intercostal spaces between the true ribs and extend even caudad into the spaces between the false ribs. They are lacking between the ventral ends of the costal cartilages of the first six to eight ribs, so that the internal intercostals (Fig. 69, _k_) are here exposed. The more caudal external intercostals are more nearly craniocaudal in direction. _Relations._--Outer surface with obliquus abdominis externus (Fig. 68, _p_), latissimus dorsi (Fig. 68, _m_), serratus posterior inferior (Fig. 73, _n_) and superior (Fig. 73, _l_), serratus anterior (Fig. 73, _i_), scalenus (Fig. 73, _f_), and iliocostal (Fig. 69, _h_). Inner surface with the internal intercostals (Fig. 69, _k_). _Action._--Protractors of the ribs. =Mm. intercostales interni= (Fig. 69, _k_).--The internal intercostals are similar to the external intercostals, beneath which they lie. Their fibres pass between the ribs at nearly right angles to those of the external intercostals and have nearly the direction of the fibres of the internal oblique. They occupy all the intercostal spaces from the first to the thirteenth ribs. _Relations._--Outer surface with the external intercostals (Fig. 69, _i_), and ventrad with the scalenus (Fig. 73, _f_), transversus costarum (Fig. 73, _j_), and rectus abdominis (Fig. 73, _k_). Inner surface with the pleura and the transversus thoracis. _Action._--Retractors of the ribs. =M. transversus thoracis= (triangularis sterni; sternocostalis internus).--This represents a thoracic portion of the transversus abdominis. It consists of five or six flat muscular bands lying on the inner surface of the thoracic wall. _Origin_ on the lateral borders of the dorsal face of the sternum, opposite the attachments of the cartilages of the third to the eighth ribs. The six bands thus formed, each about one centimeter wide, pass laterad and are _inserted_ into the cartilages of the ribs near their junction with the ribs, and into the fascia which covers the inner surface of the internal intercostals in this region. _Relations._--Outer surface with the internal intercostals and the cartilages of the ribs. Inner surface with the pleura. =Diaphragma= (Fig. 74).--The diaphragm consists of a central so-called =semilunar= tendon (_e_) and of muscular fibres which pass radially from the body wall to the tendon. It forms a complete oblique partition between the abdominal and thoracic cavities. The dorsal end is farther caudad than the ventral.