248 FASCIA AND MUSCLES OF THE HORSE 



Origin. — The external angle of the ilium and the adjacent }:)art 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. A])out four or five inches (ca. 10 to 12 cm.) from the 

 linea all)a the muscle separates from the ligament and forms the antf^'ior wall 

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

 ring^ (Annulus inguinalis abdominalis), is found here. It is normally a narrow 

 slit, liounded in front by the edge of the internal oblique, and behind by the in- 

 guinal ligament. The aponeurosis is to a great extent blended with that of 

 the external oblique, l^eing, 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 ol)lique; 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 adja- 

 cent 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. 



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

 eleven transverse bands of fibrous tissue extend in an irregular manner across 

 the muscle. These are termed inscriptiones tendineae. They strengthen the 

 muscle and serve to prevent separation of its fibers. The width of the muscle is 

 greatest about its middle. 



Relations. — Superficially, the aponeuroses of the olilique muscles (which 

 constitute the external rectus sheath), and the posterior deep pectoral; deeply, 

 the transversus, intercostals, the cartilages of the ribs, and the sternum. The 

 posterior abdominal artery runs along the outer edge of the muscle posteriorly, 

 and the anterior abdominal artery on or in its anterior part. 



Blood-supply. — Anterior and posterior abdominal arteries. 



Nerve-supply. — Intercostal and lumbar nerves. 



4. Transversus abdominis. — This muscle, named from the general direction 

 of its fibers, is a triangular curved sheet. Its lateral part is muscular, its ventral 

 aponeurotic. 



Origin. — (1) The inner surfaces of the distal ends or the cartilages of the 

 asternal ribs, meeting the costal attachment of the diaphragm; (2) the transverse 

 processes of the lumbar vertebrae, by means of the deep layer of the lumbo-dorsal 

 fascia. 



Insertion. — The xiphoid cartilage and the linea alba. 



1 It must 1)0 ailmitted that the term "ring" is ratlier misleading as applied to the abdomi- 

 nal opening of the canal, sinec normally it is a more dilatable slit. _ The ring-like constriction 

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

 to form the tunica vaginalis. This peritoneal ring is termed t lie vaginal ring (Annulus vaginalis), 

 and must not be confused with the subperitoneal ring, i. c, the internal inguinal ring. 



