=M. obliquus abdominis internus= (Fig. 73, _o_).--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 vertebræ 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, _k_). At its cranial end the fibres divide into two sheets or laminæ, one of which passes outside of the rectus and the other inside. The outer lamina unites with the aponeurosis of the external oblique, while the inner lamina unites with that of the transversus. There is thus formed a sheath for the cranial part of the rectus muscle.
_Relations._--Outer surface with the obliquus externus (Fig. 68, _p_). Inner surface with the transversus abdominis (Fig. 69, _l_), and by its ventral tendon with the rectus.
=M. transversus abdominis= (Fig. 69, _l_).--A thin sheet covering the whole surface of the abdomen and lying beneath the internal oblique. Its fibres are nearly transverse.