148 THE MUSCLES. 



Action. — Assists the pectoralis minor. 



B. Muscles of the Wall of the Thorax. — M. ser- 

 ratus posterior superior (Fig. 73, /). — A thin sheet of muscle 

 and tendon beneath the serratus anterior (/) on the dorsal part 

 of the thorax and neck. 



Origin by fleshy shps from the outer surfaces of the first 

 nine ribs just ventrad of their angles. The origin may extend 

 as far as the tenth or eleventh ribs. The first slip is usually 

 delicate. The fibres are directed dorsad and craniad and unite 

 to form a continuous sheet. This ends along a longitudinal 

 line opposite the vertebral transverse processes in the aponeuro- 

 sis which covers the longissimus dorsi and other muscles in this 

 region. Its insertion is into the median dorsal raphe between 

 the axial spinous process and the tenth thoracic spinous 

 process. 



Relations. — Outer surface with the serratus anterior (/), the 

 latissimus dorsi (Fig. 68, in), and the rhomboideus. Inner 

 surface with the external intercostals (Fig. 73, ;//), the iliocos- 

 tal (Fig. 69, h), the longissimus dorsi (Fig. 69,/), the spinalis 

 dorsi (Fig. 69, g), and the splenius (Fig. 73, b). 



Action. — Draws the ribs craniad. 



M. serratus posterior inferior (Fig. 73, n). — The serratus 

 posterior inferior is a thin muscle lying caudad of the serratus 

 posterior superior (/) and sometimes overlying the caudal end 

 of the latter. 



Origin by four or five heads from the last four or five ribs. 

 In some specimens only three heads may be present. The 

 separate bundles pass dorsocaudad and unite to form a contin- 

 uous sheet which ends in a continuation of the aponeurosis of 

 the serratus posterior superior (/). 



Lisertion into the lumbar spinous processes and the inter- 

 vening interspinous ligaments. 



Relations. — Outer surface with the latissimus dorsi (Fig. 

 68, /), and at the origins with the intercostales externi (Fig. 

 73, ;;/). Inner surface with the intercostales interni (Fig. 69, k) 

 at the origin and with the intercostales externi (Fig. 69, /), 

 iliocostal (Fig. 69, h), longissimus dorsi (Fig. 69, /), and the 

 caudal end of the spinalis dorsi (Fig. 69, g). 



