King's Applied Anatomy of the Abdomen and Pelvis of Domestic Mammals. Geoff Skerritt

Figure 1.4 Ventral view of inguinal region of the horse showing the left and right external abdominal oblique muscles. The arrows indicate the left and right femoral canals, providing exit for the femoral arteries and veins.

Figure 1.5 Lateral view of the abdomen of the ox showing the left external abdominal oblique muscle.

      Origin: The lateral surfaces of the ribs caudal to the fourth rib and the lumbodorsal fascia.

      Insertion: The linea alba and prepubic tendon.

Structure: Most of the muscle fibres run caudoventrally. At its origin it consists of muscle fibres but towards its insertion caudoventrally it becomes a tendinous aponeurosis. Towards its insertion in the prepubic tendon there is a slit in the aponeurosis; this is the superficial inguinal ring. The slit divides the tendon into an abdominal part cranially and a pelvic part caudally. The caudal edge of the pelvic part of the tendon is the inguinal ligament.

Species variations: The external abdominal oblique muscle of the dog and pig is mainly muscular almost to the dorsal edge of the rectus abdominis muscle. In ruminants there is no origin from the lumbodorsal fascia, but there is an insertion on the tuber coxae. In the ox the aponeurosis of this muscle is extensive. In the horse the external abdominal oblique muscle is very large and inserts onto the femoral fascia, linea alba, tuber coxae and the prepubic tendon.

1.3.5 Internal abdominal oblique muscle (Figures 1.6–1.8)

Figure 1.6 Lateral view of inguinal area of horse showing the internal abdominal oblique muscle. The left external abdominal oblique muscle has been removed although the position of the left superficial inguinal ring is shown. The mid‐section of the left cremaster muscle has been excised to expose the vaginal tunic.

Figure 1.7 Ventral view of inguinal region showing internal abdominal oblique muscle.

Figure 1.8 Lateral view of abdomen of ox showing left abdominal oblique muscle. The external abdominal oblique has been removed.

      Origin: Tuber coxae and lumbodorsal fascia.

      Insertion: Linea alba (except for the most caudal part), last rib and cartilages of the caudal ribs.

      Structure: This is a sheet of muscle and tendon with the fibres running cranioventrally. It is muscular at its origin and becomes tendinous ventrally. In the male a slip of the internal abdominal oblique muscle passes through the inguinal canal on the lateral aspect of the vaginal process and becomes the cremaster muscle (see Section 16.4).

Species variations: The fibres of this muscle run almost ventrally in the dog. In carnivores the tendinous portion divides to pass dorsally and ventrally to the rectus abdominis muscle in the cranial third of the abdomen; it passes only ventrally in the caudal two‐thirds of the abdomen. In the ox the internal abdominal oblique is quite substantial, being the largest flank muscle in this species; its tendon passes both ventrally and dorsally to the rectus abdominis. In the horse the internal abdominal oblique muscle originates only from the tuber coxae, and its tendon passes ventrally to the rectus abdominis. See Figures 1.10a–c for a summary of the species variation of the sheath of the rectus abdominis.

1.3.6 Transverse abdominal muscle (Figure 1.9)

Figure 1.9 Medial view of abdomen of ox showing the transverse abdominal muscle.

      Origin: The medial surfaces of the ventral parts of the caudal ribs and the deep layers of the lumbodorsal fascia.

      Insertion: The linea alba.

      Structure: Again this muscle is sheet‐like, although its fibres run ventrally and transversely to the longitudinal axis. Caudally the muscle thins out to only a fascial layer.

Species variations: In the dog the cranial two‐thirds of the tendon pass dorsally to the rectus abdominis with the caudal third passing ventrally.

      1.3.7 Retroperitoneal fascia

This tissue layer is equivalent to the superficial fascia but less defined. Its significance is due to its large fat content in the adult pig, fat ponies and beef breeds of cattle. Where the fascia is minimal the peritoneum is closely applied to the transverse abdominal muscle. The falciform ligament (see Sections 3.3 and 3.4) is a fold of peritoneum attached to the liver. It is a remnant of the peritoneum that contained the umbilical vein of the foetus; it attaches to the abdominal wall at the umbilicus.

      1.3.8 Parietal peritoneum

      This peritoneal layer lines the whole abdominal wall. It is a largely transparent and delicate layer that is reflected as mesenteries that are continuous with the visceral peritoneum that covers the abdominal viscera. The peritoneum comprises an outer layer of simple squamous epithelium called the mesothelium and is supported by a layer of loose connective tissue.

1.4 The Sheath of the Rectus Abdominis Muscle (Figures 1.10a–c)