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

Figure 1.10 Transverse sections through the ventral body wall to show the species variation in the sheath of the rectus abdominis. (a) The horse, (b) the ox and (c) the dog (caudal third of abdomen only).1 = parietal peritoneum; 2 = transverse abdominis muscle; 3 = interior oblique abdominis muscle; 4 = exterior oblique abdominis muscle; 5 = yellow abdominis tunic; 6 = skin; 7 = rectus abdominis muscle; 8 = linea alba; 9 = ventral sheath of rectus abdominis muscle

      The aponeuroses of the external, internal and transverse abdominal oblique muscles together form a sheath that encloses the rectus abdominis muscle either side of the midline of the abdominal wall. There are species differences and, in the dog, variations in the craniocaudal location.

      Species variations: In the caudal third of the abdomen of the dog the tendon of the transverse abdominal muscle lies ventral to the rectus abdominis muscle. In the middle third of the abdomen the transverse abdominal tendon lies dorsal, and that of the internal abdominal oblique muscle passes ventral to the rectus abdominis (as in the horse). In the cranial third of the abdomen the tendon of the transverse abdominal muscle lies dorsal to the rectus muscle. In addition, the internal oblique tendon divides into a ventral and dorsal portion (as in the ox).

      In the horse the aponeurosis of the internal oblique muscle lies ventral to the rectus abdominis. In addition, in this species, the yellow abdominal tunic is present.

      In the ox the aponeurosis of the internal oblique divides to pass on both sides of the rectus abdominis, and a yellow abdominal tunic is again present. In this species the linea alba is particularly wide.

1.5 Clinical Importance of the Ventral Body Wall

      A surgical incision in the abdominal wall is called a laparotomy. It may be made in the midline, to either side of the midline or in the flank on either side. The choice of location of the laparotomy depends on a number of factors:

      1 The avascularity of the linea alba resulting in slow healing; this is a particular problem especially in cattle where the linea alba is extensive.

      2 The bulk and weight of the abdominal contents leading to slow healing and risk of herniation.

      3 In the dog the ventral sheath of the rectus abdominis is particularly strong, and failure to suture this may result in breakdown of a midline incision.

      4 In a midline incision contraction of the muscles of the abdominal wall tends to retract the wound edges laterally.

      5 Flank incisions should be parallel to the muscle fibres to minimise bleeding from the vascular muscular tissue.

      6 In the cow a further complication of a midline incision is that branches of the mammary vein may cross the midline to anastomose with the opposite mammary vein.

1.6 The Inguinal Canal (Figures 1.11 and 1.12)

Figure 1.11 Ventral view of inguinal canal of the pig. The left side of the diagram shows the superimposition of the superficial inguinal ring almost directly upon the vaginal ring in this species.

Figure 1.12 Lateral view of the inguinal canal of the horse. A window has been cut from the pelvic part of the tendon of the external abdominal oblique muscle immediately adjoining the superficial inguinal ring. This exposes that part of the internal oblique abdominal muscle which forms the medial (deep) wall of the inguinal canal.

      The inguinal canal is a potential space extending between the superficial and deep inguinal rings. The canal does not have a surrounding wall. The external opening (superficial inguinal ring) is a slit in the aponeurosis thereby dividing it into two parts, an abdominal part (cranially) and a pelvic part (caudally).

Species variations: The deep inguinal ring is different in the pig from that in the horse due to the different extent to which the internal oblique abdominal muscle inserts caudally. In the horse the deep inguinal ring is small, being bordered caudally by the inguinal ligament and cranially by the caudal edge of the internal oblique muscle (Figure 1.12) In the pig the deep inguinal ring is larger, bordered caudally by the inguinal ligament, cranially by the caudal edge of the internal abdominal oblique muscle and medially by the lateral border of the rectus abdominal muscle and the prepubic tendon (Figure 1.11). In the other domestic animals, the anatomy of the inguinal ring is between the horse and the pig but tends to be closer to the latter.

In the male foetus of all species an outpouching of parietal peritoneum, the vaginal process (Figure 16.6), enters the inguinal canal. The gubernaculum (see Section 16.9) develops from mesenchyme partly within the inguinal canal and joins the testes to the scrotum. In the adult male the inguinal canal contains the vaginal tunic, the cremaster muscle and the spermatic cord in addition to the external pudendal artery and vein, the inguinal lymph vessels and nerves. In the adult female it is only in the bitch that a rudimentary vaginal process extends through the inguinal canal.

1.7 Hernias

      A hernia occurs when an organ or mesentery pushes through an opening in the muscle or tissue that normally holds it in place. Hernias occur most commonly in the abdomen when there is a deficit or weakness in the abdominal wall, but they may also occur at the diaphragm or the perineum. The several sites where hernias may occur are as follows:

      1 Inguinal

      2 Umbilical

      3 Perineal

      4 Diaphragmatic

      5 Post‐operative.

      1.7.1 Inguinal hernia

      The vaginal process develops in the embryo as an extension of the parietal peritoneum. Therefore the cavity of the vaginal process is continuous with the peritoneal cavity via the vaginal ring. In the male of all species and the bitch it is possible for abdominal contents (e.g. small intestine or great omentum) to protrude through the vaginal ring and enter the vaginal process. Within the vaginal process the herniated organ or tissue passes through the inguinal canal and may enter the scrotum. An inguinal hernia may or may not be reducible; an irreducible hernia may become strangulated if the blood supply becomes interrupted.

      Congenital inguinal hernias are common in pigs, but in sheep they are thought to be a result of trauma. In the horse inheritance has not been proven, but they are more common in certain breeds.

      1.7.2 Umbilical hernia

      Normally, at birth, the umbilical ring closes and the umbilical blood vessels, the vitelline duct and the allantoic stalk begin to degenerate. If contraction of the umbilical ring does not occur completely it is possible for abdominal contents to enter the aperture and appear as a soft swelling beneath the umbilical scar.

      1.7.3 Perineal hernia

The