2. =Obliquus abdominis internus= (small oblique; internal oblique of the abdomen).—This muscle is situated under the preceding one. Its fibers are directed downward, forward, and inward. It forms a triangular curved sheet with the base behind. [Illustration: FIG. 186.—VENTRAL MUSCLES OF HORSE, AFTER REMOVAL OF GREATER PART OF PANNICULUS CARNOSUS AND ABDOMINAL TUNIC. SARTORIUS AND GRACILIS REMOVED FROM RIGHT THIGH. _c_, Mastoido-humeralis; _g_, anterior superficial pectoral; _g′_, posterior superficial pectoral; _h_, posterior deep pectoral; _l′_, aponeurosis of obliquus abdominis externus; _t_, pectineus; _u_, ilio-psoas; _v_, semimembranosus; _w_, gracilis; _x_, sartorius; _y_, vastus internus; _z_, _z′_, adductor; _14_, sternum; _14′_, cariniform cartilage. (After Ellenberger-Baum, Anat. f. Künstler.) ] _Origin._—The external angle of the ilium and the adjacent part of the inguinal (Poupart’s) ligament. _Insertion._—(1) The cartilages of the last four or five ribs; (2) the linea alba and the prepubic tendon. _Action._—Similar to that of the preceding muscle. _Structure._—Like the external oblique, it is composed of a fleshy portion and an aponeurosis. The fleshy portion is fan-shaped, and is situated chiefly in the flank. At its iliac origin it is covered by a glistening aponeurosis. Traced inward and downward along the abdominal surface of the inguinal ligament, the muscular origin is found to become much thinner, and also becomes loosely attached to the ligament. About four or five inches (ca. 10 to 12 cm.) from the linea alba the muscle separates from the ligament and forms the anterior wall of the inguinal canal. The abdominal orifice of the canal, the =internal inguinal ring=[51] (Annulus inguinalis abdominalis), is found here. It is normally a narrow slit, bounded in front by the edge of the internal oblique, and behind by the inguinal ligament. The =aponeurosis= is to a great extent blended with that of the external oblique, being, indeed, considerably interwoven with it ventrally. Where it covers the rectus abdominis it is attached to the tendinous inscriptions of that muscle. _Relations._—Superficially, the external oblique; deeply, the rectus abdominis, transversus abdominis, and the peritoneum. _Blood-supply._—Circumflex iliac, lumbar, and intercostal arteries. _Nerve-supply._—Ventral branches of the lumbar nerves. 3. =Rectus Abdominis.=—This muscle is confined to the ventral part of the abdominal wall; it extends from the lower part of the chest-wall to the pubis. _Origin._—The cartilages of the fifth to the ninth ribs inclusive, and the adjacent surface of the sternum. _Insertion._—The pubis, by means of the prepubic tendon. _Action._—Similar to that of the oblique muscles. It is specially adapted to flex the lumbo-sacral joints and the lumbar and thoracic parts of the spine.