288 FASCI# AND MUSCLES OF THE HORSE 
In this fold are the prefemoral lymph-glands. Medially it blends with the linea 
alba. It contains the abdominal cutaneous muscle (described on p. 254). 
The deep fascia is represented chiefly by the abdominal tunic (Tunica flava 
abdominis). This is a sheet of elastic tissue which assists the muscles in supporting 
the great weight of the abdominal viscera. It is practically coextensive with the 
obliquus externus, which it covers. Ventrally it is thick, and is intimately ad- 
herent to the aponeurosis of the muscle. Laterally it becomes thinner and is more 
sasily separated, although fibers from it dip in between the muscle-bundles. It is 
continued for some distance upon the intercostals and serratus ventralis. Traced 
forward, it passes as a thin layer beneath the posterior deep pectoral muscle. Pos- 
teriorly it is attached to the tuber coxze. In the inguinal region it forms the deep 
fascia of the prepuce or of the mammary glands. 
The linea alba is a median fibrous raphé which extends from the xiphoid car- 
tilage to the prepubic tendon. It is formed chiefly by the junction of the aponeu- 
roses of the oblique and transverse muscles, but partly by longitudinal fibers. A 
little behind its middle (about in a transverse plane tangent to the last pair of ribs) 
is a cicatrix, the umbilicus, which indicates the position of the umbilical opening 
of the foetus. 
1. Obliquus abdominis externus.—This is the most extensive of the abdominal 
muscles. It is a broad sheet, irregularly triangular in shape, widest behind. Its 
fibers are directed chiefly downward and backward. 
Origin.—(1) The lateral surfaces of the last fourteen ribs, and the fascia over 
the external intercostal muscles; (2) the lambo-dorsal fascia. 
Insertion.—(1) The linea alba and the prepubic tendon; (2) the tuber coxe and 
shaft of the ilium; (3) the medial femoral fascia. 
Action.—(1) To compress the abdominal viscera, as in defecation, micturition, 
parturition, and expiration; (2) to flex the trunk (arch the back); (3) acting singly, 
to flex the trunk laterally. 
Structure-—The muscle is composed of a muscular part and an aponeurosis. 
The muscular part lies on the lateral wall of the thorax and abdomen. It arises 
by a series of digitations, the anterior four of which alternate with those of the ser- 
ratus ventralis. The origin may be indicated by a slightly curved line (concave 
dorsally) drawn from the lower part of the fifth rib to the tuber cox. The fibers 
are directed downward and backward and terminate on the aponeurosis, except in 
the upper part of the flank, where they are less oblique in direction and end on the 
tuber coxee. The line of junction is a curve (concave dorsally) extending from the 
upper edge of the posterior deep pectoral muscle toward the point of the hip. The 
aponeurosis is intimately attached to the abdominal tunic, and its fibers are largely 
interwoven ventrally with those of the aponeurosis of the internal oblique. By 
this fusion is formed the outer sheath of the rectus abdominis, which blends at the 
linea alba with that of the opposite side. In the inguinal region the aponeurosis 
divides into two chief layers; one of these curves dorsally and backward and is in- 
serted into the tuber coxze and the prepubie tendon. Between these points the 
aponeurosis is much strengthened and is called the inguinal ligament (Ligamentum 
inguinale).!. This curves upward and somewhat forward, becomes thin, and blends 
with the iliac fascia. It forms the posterior wall of the inguinal canal. About an 
inch (ca. 2 to 3 em.) in front of the pubis and about two inches (ca. 4 to 5 em.) 
from the median plane the aponeurosis is pierced by a slit-like opening, the 
subcutaneous or external inguinal ring (Annulus inguinalis subeutaneus).? This is 
1 Also commonly known as Poupart’s ligament—based on a false historical allusion. It is 
in no proper sense a ligament, but is the inguinal part of the aponeurosis of the obliquus externus; 
it might therefore well be termed the lamina inguinalis. 
2 Tt is narrow and slit-like in the natural condition, but may appear oval in the dissecting- 
room, especially if the hind limb is drawn back and abducted. 
