246 FASCIA AND MUSCLES OF THE HORSE 



Traced forward, it passes as a thin layer beneath the posterior deep pectoral 

 muscle. Posteriorly it is attached to the external angle of the ilium. In the in- 

 guinal region it forms the deep fascia of the prepuce or of the mammary glands. 



The linea alba is a median fil^rous raphe which extends from the xiphoid 

 cartilage to the symphysis pubis. It is formed chiefly by the junction of the apo- 

 neuroses of the oblique and transverse muscles, but partly by longitudinal fibers. A 

 little behind its middle is a cicatrix which indicates the position of the umbilical 

 opening of the foetus. 



1. Obliquus abdominis extemus (great oblique; external ol^lique of the 

 abdomen). — This is the most extensive of the al)dominal muscles. It is a broad 

 sheet, irregularly triangular in shape, widest behind. Its fillers are directed chiefly 

 downward and backward. 



Origin. — (1) The outer surfaces of the last fourteen ribs, and the fascia 

 over the external intercostal muscles; (2) the lumbo-dorsal fascia. 



Insertion. — (1) The linea alba and the i^repubic tendon; (2) the external 

 angle and shaft of the ilium ; (3) the internal 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. 



Structnre. — The muscle is composed of a fleshy portion and an aponeurosis. 

 The muscular portion 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 

 serratus magnus. The origin may l)e indicated by a slightly curved line (concave 

 above) drawn from the lower part of the fifth rib to the external angle of the ilium. 

 The fibers are directed downward and backward and terminate on the aponeurosis, 

 except in the flank, where they are almost horizontal in direction. The line of 

 junction is a curve (concave above) extending from the upper edge of the posterior 

 deep pectoral muscle toward the external angle of the ilium. 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 upward and l)ackward and is inserted into 

 the external angle of the ilium and the prepubic tendon. Between these points 

 the aponeurosis is much strengthened and is called the inguinal (Poupart's) liga- 

 ment (Ligamentum inguinale). This curves upward and somewhat forward, lie- 

 comes thin, and blends with the iliac fascia. It forms the posterior wall of the 

 inguinal canal. About an inch (ca. 2 to 3 cm.) in front of the pubis and about two 

 inches (ca. 4 to 5 cm.) from the median plane the aponeurosis is pierced by a slit-like 

 opening,^ the external inguinal ring (Annulus inguinalis subcutaneus). This is the 

 external orifice of the inguinal canal. Its long axis is directed outward and forward, 

 and is about four inches (ca. 10 cm.) in length. The inner angle is rounded and is 

 well defined by the junction of the inguinal ligament with the prepubic tendon, but 

 the outer angle is not so sharply defined. The borders or pillars are constituted by 

 arciform fillers of the aponeurosis of the external oblique (Crus mediale, laterale). 

 The femoral layer of the aponeurosis (Lamina femoralis) passes on to the inner sur- 

 face of the thigh, where it blends with the femoral fascia. A thin iliac layer (Lamina 

 iliaca) passes over the outer margin of the iliacus to the external border of the ilium. 



Relations. — Superficially, the skin, the panniculus carnosus, the abdominal 

 tunic, and the posterior deep pectoral muscle; deeply, the ril)s and their cartilages, 

 the intercostal muscles, the internal oblique, the contents of the inguinal canal, 

 and the sartorius and gracilis. 



1 It is narrow and slit-liko in the natural condition, but may appear oval in the dissecting- 

 room, especially if the hind limb is drawn back and abducted. 



