150 THE MUSCLES. 



Action. — Assists the serratus posterior superior. 



M. transversus costarum, or steinocostalis externus 

 (Fig. 7'iyj)- — This is a thin muscle appHed to the cranial part 

 of the side of the thorax, covering the cranial end of the rectus 

 abdominis (/'). 



Origi)i 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, /'"). the 

 rectus abdominis (Fig. 73, k), and the intercostales externi 

 (Fig. 73, w). At its insertion it is luiitcd with a portion of the 

 scalenus (Fig. 73,/'")- 



Action. — Draws the sternum forward. 



Mm. levatores costarum. — Small muscles having origin 

 on the transverse processes of the thoracic vertebrae, 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, /), and craniad with the iliocostal (Fig. 69, Ji). Inner 

 surface with the internal intercostals (Fig. 69, k). 



Action. — Pull the ribs dorsocraniad. 



Mm. intercostales externi (Fig. 73, m\ Fig. 69, /). — 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 intercos- 

 tals are more nearly craniocaudal in direction. 



Relations. — Outer surface with obliquus abdominis externus 



