_Origin_ by tendon from the side of the sternum between the attachments of the third and sixth ribs. The muscle passes dorsocraniad to its
_Insertion_ on the first rib and the lateral portion of its costal cartilage.
_Relations._--Outer surface with the pectoralis minor (Fig. 65, _o_). Inner surface with the scalenus (Fig. 73, _f‴_), the rectus abdominis (Fig. 73, _k_), and the intercostales externi (Fig. 73, _m_). At its insertion it is united with a portion of the scalenus (Fig. 73, _f‴_).
=Mm. levatores costarum.=--Small muscles having _origin_ on the transverse processes of the thoracic vertebræ, passing caudoventrad, and becoming _inserted_ on the angle of the rib lying immediately caudad of the origin. They are continuous with the external intercostals.
_Relations._--Outer surface with the longissimus dorsi (Fig. 69, _f_), and craniad with the iliocostal (Fig. 69, _h_). Inner surface with the internal intercostals (Fig. 69, _k_).
=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_).
=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.