_Insertion_ into the median raphe ventrad of the sternum from the insertion of the seventh costal cartilage to the xiphoid process, into the linea alba from the sternum to the pubic tubercle, and into the tubercle and the cranial border of the pubis. Caudad of the xiphoid process the aponeurosis is closely united to the superficial layer of the internal oblique, where it forms the outer layer of the sheath of the rectus abdominis. Laterad of the pubic tubercle the tendon is perforated by the inguinal canal. In the cat neither the caudal part of the muscle nor its tendon is attached to the ilium, as it is in man and the dog, so that no Poupart’s ligament, or inguinal ligament, is formed. _Relations._--Outer surface with the cutaneus maximus, the integument, and near the origin with the latissimus dorsi (Fig. 68, _m_). Inner surface with the obliquus internus (Fig. 73, _o_), the rectus abdominis (Fig. 73, _k_), the intercostales externi (Fig. 73, _m_), a small part of the serratus posterior inferior (Fig. 73, _n_), and by its dorsal tendon with the longissimus dorsi (Fig. 69, _f_). _Action._--Constrictor of the abdomen. =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.