(_b_) The =sternocostal part= (_c_, _c′_) takes origin from the xiphoid process and the last five ribs, by fleshy bundles which interdigitate with those of the transversus abdominis (_i_). The fibres converge to the central tendon (_e_). Between the crus of each side and the most dorsal of the costal fibres is the membranous interval (_d_) mentioned above.
4. =Abdominal Muscles.= =M. obliquus abdominis externus= (Fig. 68, _p_).--A large, thin sheet of muscle covering the whole abdomen and part of the thorax ventrally.
_Origin._--(_a_) From the last nine or ten ribs by means of as many tendons, which are interconnected to form arches that span the slips of the serratus anterior. The muscle-fibres arise from these tendons and from their intervening arches. (_b_) From the lumbodorsal aponeurosis common to it and the internal oblique. The cranial fibres pass nearly ventrad, the caudal fibres caudad, and the intervening fibres take an intermediate course. The fibres end in a thin aponeurosis of insertion along a curved line which passes at first caudad and then laterodorsad. The aponeurosis fibres continue in the direction of the muscle-fibres to the
_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_).