290 FASCIA AND MUSCLES OF THE HORSE 



laterale). The femoral lamina of the aponeurosis (Lamina femoralis) passes on 

 to the medial surface of the thigh, where it blends with the femoral fascia. A 

 thin iliac lamina (Lamina iliaca) passes over the lateral margin of the ihacus and 

 is attached to the lateral border of the ilium. 



Relations. — Superficially, the skin, the abdominal cutaneus, the abdominal 

 tunic, and the posterior deep pectoral muscle; deeply, the ribs and their cartilages, 

 the intercostal muscles, the obliquus abdominis internus, the contents of the inguinal 

 canal, and the sartorius and gracilis. 



Blood-supply. — Intercostal and lumbar arteries. 



Nerve-supply. — Intercostal and lumbar nerves. 



2. Obliquus abdominis internus. — 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. 



Origin. — The tuber coxee and the adjacent part of the inguinal 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 muscular part is fan-shaped, and is situated chiefly in the 

 flank. At its iliac origin it is covered by a glistening aponeurosis. Traced medially 

 and ventrally 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. This medial part of the muscle forms the anterior wall of the inguinal 

 canal. The abdominal orifice of the canal, the abdominal or internal inguinal 

 ringi (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 in- 

 guinal ligament. Near the last rib the muscle divides into two parts. The small 

 dorsal part is inserted by four or five thin tendinous strips to the medial surface of 

 the last four or five costal cartilages. The aponeurosis of the large ventral part 

 is to a great extent blended with that of the external oblique, being, indeed, consider- 

 ably interwoven with it ventrally. Where it covers the rectus abdominis it is at- 

 tached to the tendinous inscriptions of that muscle. It may be noted that the dor- 

 sal margin of the aponeurosis varies in different subjects in the fact that it may 

 cover the costal arch or lie ventral to it. 



Relations. — Superficially, the obliquus externus; 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 ab- 

 dominal wall ; it extends from the sternal region to the pubis. 



Origin. — The cartilages of the fourth or 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 especially adapted to 

 flex the lumbo-sacral joints and the lumbar and thoracic parts of the spine. 



Structure. — The fibers of the muscle are directed longitudinally. Nine to 

 eleven transverse bands of fibrous tissue extend in an irregular maimer across the 



1 It must be admitted that the term "ring" is rather misleading as applied to the abdomi- 

 nal opening of the canal, since normally it is a mere dilatable sHt. The ring-Uke constriction 

 which exists here in the male is constituted by the peritoneum, which descends into the canal 

 to form the tunica vaginahs. This peritoneal ring is termed the vaginal ring (Annulus vagi- 

 nalis), and must not be confused with the subperitoneal ring, i. e., the abdominal or internal 

 inguinal ring. The internal inguinal ring is six or seven inches (ca. 16 cm.) in length. Its 

 direction corresponds approximately to a line from the lateral margin of the prepubic tendon to the 

 ventral part of the tuber coxae. 



