» The Abdominal Wall (see also pp. 47-49)

Abdominal surgery is frequently performed in dogs and cats. Therefore, it is necessary to be familiar with the details even though the ventrolateral abdominal wall is constructed in a common pattern with only a few distinct features mainly concerning the linea alba and rectus sheath; these along with the inguinal canal are more fully described here.

The linea alba is the fibrous seam in which the aponeuroses of the right and left oblique and transverse abdominal muscles come together. It extends from the xiphoid process to the pubis and includes the umbilicus at about the level of the third lumbar vertebra. The linea alba is about 1 cm* wide cranial to the umbilicus but gradually narrows behind this point and is reduced to a barely visible line in its caudal third (see Fig. 2.26). Incisions through the linea alba spare the muscles, vessels, and nerves and avoids retraction of the parietal peritoneum from incision edges, which occurs with a median incision. The falciform ligament (see later) and the median ligament of the bladder attach to the dorsal surface of the linea alba, cranial and caudal to the umbilicus, respectively. Umbilical hernias, often associated with an over-wide linea alba and hypoplastic rectus muscles, are common.

The rectus sheath is formed by the aponeuroses of the oblique and transverse abdominal muscles. In the dog and cat the aponeuroses of the external and internal oblique muscles pass ventrally (externally) to the rectus muscle over the length of the linea alba. However, the most cranial portion of the internal oblique muscle also detaches an additional lamina that passes dorsally (internally) to the rectus (see Fig. 2.26A). The aponeurosis of the transverse abdominal muscle passes dorsally (internally) to the rectus muscle in the cranial half of the abdomen, but it changes position to the ventral surface in the caudal portion, leaving the dorsal surface of the rectus covered only by fascia and peritoneum (see Fig.

» TABLE 14.1

Blood supply and lymph drainage of mammary glands

The inguinal canal is a potential space between the external and internal abdominal oblique muscles that extends between deep and superficial openings (rings). The deep ring leads from the canal into the abdominal cavity, and the superficial ring leads from the canal to the subcutaneous tissues of the groin. The canal conveys the external pudendal vessels and the genitofemoral nerve in both the sexes, the spermatic cord in the dog and tom, and the vaginal process in the bitch and queen. These all emerge at the superficial inguinal ring, a nearly sagittal slit in the external abdominal oblique aponeurosis about 3 cm lateral to the linea alba, close to where it attaches on the pubis (see Fig. 2.27A/4'). Only the caudal end of the ring is palpable. The narrow strip of aponeurosis (see Fig. 2.27A between 4' and 6) lateral to the ring forms the only barrier between the structures emerging from the canal and the large femoral vessels and saphenous nerve as they enter the thigh through the vascular lacuna (see Fig.

The deep inguinal ring is visible only from within the abdomen. It is bounded caudolaterally by the caudal border of the external abdominal oblique aponeurosis (inguinal ligament), cranially by the unattached border (caudal edge) of the internal abdominal oblique muscle, and medially by the rectus muscle (see Fig. 2.27B). None of these boundaries is palpable in the intact animal. The parietal peritoneum that covers the ring evaginates through the inguinal canal and, then named the vaginal tunic, accompanies the spermatic cord into the scrotum. In the bitch and queen it envelops the round ligament of the uterus and is known as the vaginal process, which is not present in females of other domestic species and is the occasional recipient of herniated abdominal organs (see Chapter 15).

Blood Supply

The ventral abdominal wall is supplied by four paired arteries that come from the sternal and pelvic region. The cranial superficial epigastric artery branches off the internal thoracic artery and runs between the abdominal muscles and the skin (Table 14.1). It supplies the region cranial to the level of the umbilicus (it is enlarged in the lactating bitch). The cranial epigastric artery runs deep to the rectus, between it and its sheath. The caudal superficial epigastric artery, a branch of the external pudendal, is distributed subcutaneously and also supplies the prepuce. The caudal epigastric artery arises from the pudendoepigastric trunk and passes forward, first along the lateral border and then on the deep surface of the rectus muscle (see Figs. 14.3 and 2.26). Cranial and caudal sets of vessels anastomose (Fig. 14.2).

Paracentesis of the abdominal wall is most safely performed a short distance caudolateral to the umbilicus to avoid both the fat-filled falciform ligament and risk of injury to a full bladder. The falciform ligament, which carries the round ligament of the liver in its free border, is the remnant of the ventral mesogastrium that conveyed the umbilical vein from the umbilicus to the liver in the fetus. The part adjacent to the liver survives, if at all, as a simple peritoneal fold. The blood supply of the falciform ligament arises from along the length of the linea alba. The ligament commonly serves as a major fat storage depot and may become so thickened and enlarged that it complicates the opening and closure of a midline abdominal incision (Fig. 14.13), especially in dogs. Part or all of this obstruction may be excised; care must be taken to place a ligature at the cranial end, before the ligament is totally removed.