_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. _Action._--Compressor of the abdomen. =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. _Origin._--(1) By fleshy fibres or by a thin aponeurosis from the cartilages of all the false and floating ribs, by interdigitation with the fibres of the diaphragm. (2) From the tips of all the lumbar transverse processes. (3) From the ventral border of the ilium. (4) From the dorsal and middle of the three crural arches, where it may blend partly with the internal oblique. The muscle is continuous craniad with the transversus thoracis muscle. Near the lateral border of the rectus abdominis the muscle ends in a thin aponeurosis of insertion which is continued (its fibres having the direction of the muscle-fibres) to the _Insertion_ in the linea alba. _Relations._--Outer surface with the internal oblique (Fig. 73, _o_) and the rectus abdominis (Fig. 73, _k_), dorsad also with the longissimus dorsi (Fig. 69, _f_). Inner surface with a thin fascia covering the peritoneum and ventrocaudad with the rectus (Fig. 73, _k_). The dorsal edge of the muscle touches the iliopsoas and longissimus dorsi; the ventral edge touches the muscle of the opposite side. _Action._--Constrictor of the abdomen. =M. rectus abdominis= (Fig. 73, _k_).--A rather thick, flat muscle which lies near the median ventral line separated by the linea alba from its fellow of the opposite side and stretching from the pubis to the first costal cartilage. Opposite the first lumbar vertebra it is approximately four centimeters wide. It narrows at both ends. _Origin_ by a strong tendon from the tubercle of the pubis. The muscle passes craniad at first between the peritoneal fascia and the transversus aponeurosis, then in the sheath formed by the internal and external oblique and the transversus aponeurosis. It emerges from the sheath opposite the xiphoid process and passes ventrad of the costal cartilages. Between the third and fourth costal cartilages it ends in a thin tendon which passes beneath the transversus costarum and is