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 (2) on the dorsal part 
of the thorax and neck. 
Origin by fleshy slips 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 (2), the 
latissimus dorsi (Fig. 68, #), and the rhomboideus. Inner 
surface with the external intercostals (Fig. 73, 2), the iliocos- 
tal (Fig. 69, 2), the longissimus dorsi (Fig. 69, /), the spinalis 
dorsi (Fig. 69, g), and the splenius (Fig. 73, 4). 
Action.—Draws the ribs craniad. 
M. serratus posterior inferior (Fig. 73, 2).—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 (/). 
Insertion 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, m). Inner surface with the intercostales interni (Fig. 69, 4) 
at the origin and with the intercostales externi (Fig. 69, 7), 
iliocostal (Fig. 69, /), longissimus dorsi (Fig. 69, 7), and the 
caudal end of the spinalis dorsi (Fig. 69, g). 
