154 THE MUSCLES. 
73, 0), the rectus abdominis (Fig. 73, #), the intercostales ex- 
terni (Fig. 73, #), a small part of the serratus posterior inferior 
(Fig. 73, 2), and by its dorsal tendon with the longissimus 
dorsi (Fig. 69, /). 
Action.—Constrictor of the abdomen. 
M. obliquus abdominis internus (Fig. 73, 0).—A thin 
sheet similar to the preceding but of less extent. Its fibres 
cross those of the external oblique nearly at right angles and 
lie beneath them. 
Origin.—(1) Between the fourth and seventh lumbar verte- 
brz from the lumbar aponeurosis which is common to it and the 
external oblique. The lumbar aponeurosis takes origin from 
the lumbar spinous processes and the interspinous ligaments, 
and is continuous craniad with the aponeurosis of the serratus 
posterior inferior. Laterad it splits into three sheets, two for 
the above-named muscles and a third which passes to the 
vertebral transverse processes and forms the fascia covering the 
supraspinous muscles of the lumbar region. (2) By a similar. 
aponeurosis from “the ventral half of the iliac crest. (3) By 
fleshy fibres from the three crural arches. These are three 
delicate ligamentous arches which stretch from the crest of the 
ilium to the pubic spine. The dorsal one gives exit to the 
iliopsoas muscle, the middle to the femoral vessels, and the 
ventral to the spermatic cord. In the female the middle and 
ventral arches may fuse. The pillar between the dorsal and 
middle arches is attached to the iliopectineal eminence. 
Insertion.—The fibres pass cranioventrad and end along a 
longitudinal line in a thin aponeurosis of insertion, which is 
united in the linea alba to those of the external oblique and 
transversus. At the caudal end of the aponeurosis all its fibres 
pass outside of the rectus abdominis (Fig. 73, %). At its 
cranial end the fibres divide into two sheets or lamine, one of 
which passes outside of the rectus and the other inside. The 
outer lamina uhites with the aponeurosis of the external 
oblique, while the inner lamina unites with that of the trans- 
versus. There is thus formed a sheath for the cranial part of 
the rectus muscle. 
Relations. —Outer surface with the obliquus externus (Fig. 
