290 FASCL® 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 ilacus and 
is attached to the lateral border of the illum. 
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 cox 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 prepubie 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 1s 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 
ring! (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 manner across the 
‘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 slit. The ring-like constriction 
which exists here in the male is constituted by the peritoneum, which descends into the canal 
to form the tunica vaginalis. This peritoneal ring is termed the vaginal ring (Annulus vagi- 
nalis), and must not be confused with the subperitoneal ring, 7. e., the abdominal or internal 
inguinal ring. The internal inguinal ring is six or seven inches (ca. 16 em.) in length. — Its 
direction corresponds approximately to a line from the lateral margin of the prepubic tendon to the 
ventral part of the tuber cox. 
